###### Abstract

Stellar evolution theory is most uncertain for massive stars. For reliable
predictions of the evolution of massive stars and their final fate, solid
constraints on the physical parameters, and their changes along the evolution
and in different environments, are required. Massive stars evolve through a
variety of short transition phases, in which they can experience large mass-
loss either in the form of dense winds or via sudden eruptions. The B[e]
supergiants comprise one such group of massive transition objects. They are
characterized by dense, dusty disks of yet unknown origin. In the Milky Way,
identification and classification of B[e] supergiants is usually hampered by
their uncertain distances hence luminosities, and by the confusion of low-
luminosity candidates with massive pre-main sequence objects. The
extragalactic objects are often mistaken as quiescent or candidate luminous
blue variables, with whom B[e] supergiants share a number of spectroscopic
characteristics. In this review, proper criteria are provided, based on which
B[e] supergiants can be unambiguously classified and separated from other high
luminosity post-main sequence stars and pre-main sequence stars. Using these
criteria, the B[e] supergiant samples in diverse galaxies are critically
inspected, to achieve a reliable census of the current population.

###### keywords:

stars: massive; stars: emission line, Be; supergiants; stars: winds, outflows;
circumstellar matter

xx 1 5 Received: date; Accepted: date; Published: date yes A Census of B[e]
Supergiants Michaela Kraus 1 Michaela Kraus Correspondence:
<EMAIL_ADDRESS>
##  1 Introduction

Massive stars play a major role in the evolution of their host galaxies. Via
stellar winds, they strongly enrich the interstellar medium (ISM) with
chemically processed material and deposit large amounts of momentum and energy
into their surroundings during their entire lifetime, from the main-sequence
up to their final fate as spectacular supernova explosions (e.g. Groh et al.
(2013, 2013, 2013)). The released energy provides the ionizing radiation,
substantially supplies the global energy budget of the host galaxy and
significantly contributes to shaping the local ISM, whereas the released
material condenses into molecules and dust, providing the cradles for the next
generation of stars and planets (e.g., Pauldrach et al. (2012); Hopkins et al.
(2014)).

Despite their great importance, stellar evolution theory is most uncertain for
massive stars due to the often still poor understanding of some physical
processes in the stellar interiors (e.g., core convective overshooting,
chemical diffusion, internal differential rotation law and angular momentum
transport), the excitation and propagation of pulsation instabilities within
their atmospheres, the amount of mass loss via stellar (often asymmetric)
winds and (irregular) mass ejections, and the role of binarity for certain
phases.

From an observational point of view, the post-main sequence domain within the
Hertzsprung-Russell (HR) diagram is populated with various types of extreme
massive stars. These are found to be in transition phases, in which the stars
shed huge amounts of material into their environments, typically via episodic,
sometimes even eruptive events. These objects are luminous super- or
hypergiants populating the upper part of the HR diagram and spreading from
spectral type O to F or even later. The ejected material thereby accumulates
in either nebulae, shells, or even disk-like structures.

The mass-loss of massive stars not only critically depends on the physical
parameters, such as mass, effective temperature, and rotation speed, but also
on the chemical composition of the star. The amount of mass that is lost,
within each individual evolutionary phase, determines the fate of the object.
It is thus not surprising that relative numbers of various types of massive
stars can change drastically among galaxies with different metallicities (e.g.
Massey et al. (2016)). For reliable predictions of the evolutionary path of
massive stars in any environment, solid constraints on the physical
parameters, used in modern stellar evolution models, are indispensable. To
obtain such constraints, the properties of the members within each class of
objects need to be studied in great detail and within a variety of
environments. This requires statistically significant samples of stars in each
class of objects, suitable for a detailed analysis. The star-forming galaxies
within the local Universe, in which the metallicities spread over a factor of
about 25 between the most metal poor and the most metal rich representative,
are the most ideal sites to tackle this challenge.

This review is devoted to the B[e] supergiants which comprise one of the
various classes of extreme massive stars in transition. The article is
structured as follows: First an overview on the general properties of these
stars is given based mostly on the well-studied sample within the Magellanic
Clouds (Section 2), followed by a review on how these objects are searched for
in various environments (Section 3). A census of the currently known objects
in the Local Group galaxies and slightly beyond is presented in Section 4,
based on a critical inspection of the properties of the individual candidates.
The discussion of the B[e]SG samples and our conclusions are finally
summarized in Section 5.

Figure 1: HR diagram showing the positions of the classical MC B[e]SG sample
Zickgraf (2006). Also included are the stellar evolutionary tracks at SMC
metallicity for stars rotating initially with 40% of their critical velocity
(from Georgy et al. (2013)). The dotted square contains objects that display
CO band emission (except for S 89, see Section 2.3). For brevity and
readability, the identifiers LHA 120 and LHA 115 for objects within the LMC
and SMC, respectively, have been omitted.

##  2 B[e] supergiants

The early-type supergiants include a class of emission-line objects, whose
optical spectra display a peculiar character with strong Balmer emission along
with narrow emission lines from permitted and forbidden transitions (e.g.
Cannon (1924); Henize (1956); Smith (1957); Feast et al. (1960)). The latter
are indicative of a cool and slowly expanding medium. With the advent of
ultraviolet (UV) observations taken with the International Ultraviolet
Explorer (IUE), these stars were found to display very broad blueshifted
resonance lines of highly ionized elements in this spectral range. These
resonance lines originate from a hot and fast stellar line-driven wind which
is very typical for supergiants in this temperature and luminosity range.

Another peculiar property of these stars was discovered in the near-infrared,
in which these objects possess a pronounced excess emission pointing to hot
circumstellar dust (Allen, & Glass (1976); Glass (1977); Stahl et al. (1983,
1984); Zickgraf et al. (1985, 1986)). This dust was proposed to be most likely
produced within the slow and cool component and to possibly populate a ring or
disk-like region at far distances from the luminous central objects (Zickgraf
et al. (1985)).

Table 1: Names and parameters of the established B[e]SG sample within the Magellanic Clouds. Object | Other Common Identifiers |  log Teff |  log L/L⊙ | V | E(B-V) | Ref.
---|---|---|---|---|---|---
Large Magellanic Cloud B[e]SGs
LHA 120-S 12 | SK $-$67 23 | 4.36 | 5.41$\pm$0.04 | 12.6 | 0.2–0.25 | Zickgraf et al. (1986)
LHA 120-S 22 | HD 34664, SK $-$67 64 | 4.39$\pm$0.04 | 5.89$\pm$0.09 | 11.7 | 0.25–0.3 | Zickgraf et al. (1986)
LHA 120-S 35 | SK $-$66 97 | 4.34$\pm$0.06 | 5.21$\pm$0.10 | 12.5 | 0.07 | Gummersbach et al. (1995)
LHA 120-S 59 | … | 4.14$\pm$0.09 | 4.01$\pm$0.15 | 14.4 | 0.05 | Gummersbach et al. (1995)
LHA 120-S 73 | RMC 66, HD 268835 | 4.06 | 5.51$\pm$0.03 | 10.6 | 0.12–0.15 | Stahl et al. (1983)
LHA 120-S 89 | RMC 82, HD 269217 | 4.27 | 5.44$\pm$0.03 | 12.0 | 0.20 | Zickgraf et al. (1986)
LHA 120-S 93 | SK $-$68 66 | 4.00$\pm$0.06 | 4.61$\pm$0.10 | 12.7 | 0.22 | Gummersbach et al. (1995)
LHA 120-S 111a | HD 269599 | 4.19 | 6.06 | 10.3 | 0.28 | McGregor et al. (1988)
LHA 120-S 127 | RMC 126, HD 37974 | 4.35 | 6.12$\pm$0.02 | 10.9 | 0.25 | Zickgraf et al. (1985)
LHA 120-S 134b,c | HD 38489, SK $-$69 259 | 4.42 | 5.82$\pm$0.06 | 12.0 | 0.2–0.25 | Zickgraf et al. (1986)
LHA 120-S 137 | … | 4.11$\pm$0.10 | 4.24$\pm$0.16 | 14.0 | 0.17 | Gummersbach et al. (1995)
Small Magellanic Cloud B[e]SGs
LHA 115-S 6b | RMC 4, AzV 16 | 4.43 | 5.02 | 13.0 | 0.07 | Zickgraf et al. (1996)
LHA 115-S 18b,c | AzV 154 | 4.40 | 5.60 | 13.3 | 0.4 | Zickgraf et al. (1989)
LHA 115-S 23 | AzV 172 | 4.04 | 4.31 | 13.3 | 0.03–0.1 | Zickgraf et al. (1992)
LHA 115-S 65 | RMC 50 | 4.23 | 5.65$\pm$0.04 | 11.6 | 0.15–0.2 | Zickgraf et al. (1986)
Note: Former designations of some of the objects as Hen S # (see, e.g., Lamers
et al. (1998)) were omitted here, as these are not SIMBAD identifiers. Still,
the LHA 120-S respectively LHA 115-S and the former Hen S numbers refer to the
same objects.
a The star is also listed as RMC 105 in SIMBAD, but this designation should be
used for a neighboring, normal B-type star in this dense cluster (see
Appenzeller et al. (1984)).
bConfirmed or suspected binary.
cX-ray source (Clark et al. (2013b); Massey et al. (2014); Bartlett, & Clark
(2015)).

In the HR diagram these objects are all found beyond the main-sequence and
with luminosities spreading from about $\log L/L_{\odot}\sim$ 4 to about $\log
L/L_{\odot}\sim$ 6 implying that they are all evolved, massive stars. This
luminosity range was determined from the sample residing in the Magellanic
Clouds (MCs), for which the luminosity determination is unquestionable, due to
the low extinction towards the MCs and their well constraint distances. The
classification of Galactic objects as supergiants bears much higher
uncertainties due to their often poorly constraint distances hence
luminosities. We will come back to this issue in Section 4.4.

The position of the MC sample in the HR diagram is shown in Figure 1 for the
values of luminosity and effective temperature listed in Table
LABEL:tab:logTL. The stellar parameters (effective temperature $T_{\rm eff}$,
visual magnitude $V$, and color excess $E(B-V)$) of the sample have been taken
from the references listed in the last column of Table LABEL:tab:logTL. For
the calculations of the luminosities, distance moduli of 18.5 and 18.9 mag for
the Large respectively Small Magellanic Clouds have been utilized (see the
review paper by Humphreys, this volume) along with bolometric corrections from
Gray (2005).

The presence of dust around an early-type (typically of spectral type B)
supergiant, along with the often pure emission-line spectra with numerous
forbidden lines predominantly of [Fe ii] and [O i] finally resulted in the
designation of these objects as B[e] supergiants (B[e]SGs)111Note that these
objects have previously been abbreviated sgB[e] (Lamers et al. (1998)), to
separate them from other stars showing the B[e] phenomenon. We prefer the
designation B[e]SG, to be in line with the naming and abbreviation of other
type of supergiants such as blue supergiant (BSG) and red supergiant (RSG)..

###  2.1 General aspects of B[e]SG stars’ disks

There are compelling evidences that B[e]SGs are surrounded by gaseous and
dusty disks. The simultaneous presence of a hot and fast polar wind traced in
the UV, and the cool and slow equatorial wind traced at optical wavelengths
led to the assignment of a so-called hybrid or two-component wind model
(Zickgraf et al. (1985)). For this two-component wind a density contrast
between the equatorial and polar components of 100–1000 was proposed, meaning
that the equatorial wind might be assigned the character of an outflowing disk
(Zickgraf et al. (1989)).

The degree of non-sphericity of the envelopes, respectively the latitude
dependence of the wind density is pursued by the measured net intrinsic
polarization (Magalhaes (1992); Melgarejo et al. (2001)) and from
spectropolarimetric observations (Magalhães et al. (2006); Seriacopi et al.
(2017)). The often high degree of intrinsic polarization support the idea of a
combination of Thomson scattering by free electrons and Mie scattering by dust
in a circumstellar disk (Zickgraf, & Schulte-Ladbeck (1989)).

If the disks of B[e]SGs are supposed to form from a high-density equatorial
stellar outflow, there should be a transition zone between the atomic gas and
the location of the dust, in which molecules can form in substantial amounts,
because the high gas density can shield the material from the direct
irradiation with dissociating UV photons coming from the hot luminous star.
And in fact, molecular emission, in particular of the first-overtone bands of
carbon monoxide (CO), has been detected in the $K$-band spectra of a number of
B[e]SGs in the Galaxy and the MCs (e.g., McGregor et al. (1988, 1988, 1989);
Morris et al. (1996); Wheelwright et al. (2012); Muratore et al. (2012);
Oksala et al. (2012, 2013); Kraus et al. (2013, 2016); Torres et al. (2018),
see Table LABEL:tab:CO-O-Ca). To produce the characteristic observed emission
spectra with several individual band heads, temperatures of the CO gas higher
than $\sim$ 2000 K are required. These temperatures are in excess of the dust
sublimation temperature, which is on the order of $\sim$ 1500 K, placing the
CO emitting region closer to the star than the dust.

Additional hot molecular emission from silicon oxide (SiO) has been identified
in four Galactic B[e]SGs (Kraus et al. (2015)), and a feature arising in the
optical spectrum, that has been tentatively identified as emission from
titanium oxide (TiO), was reported from five MC B[e]SGs (Zickgraf et al.
(1989); Torres et al. (2012, 2018); Kraus et al. (2016)). However, to date no
systematic surveys for molecular emission has been performed, so that these
numbers are not representative for the existence or absence of molecules in
the environments of B[e]SGs. For instance, SiO emission has not been searched
for yet in any of the MC B[e]SGs, and only those Galactic B[e]SGs with the
most intense CO band emission have been observed in the wavelength range of
the first-overtone band of SiO arising in the $L$-band. Hence, one might
expect to find molecular emission from SiO in many more objects, but also
emission from other yet undiscovered molecules that might form in the
environments of B[e]SGs. What is interesting though is the fact that all MC
stars displaying TiO emission also have CO emission, whereas the opposite does
not hold. No detection of TiO from Galactic B[e]SGs has been reported so far.

Finally, the power of optical interferometry operating at near- and mid-
infrared wavelengths should be mentioned when talking about the disks of
B[e]SGs. Based on this technique, the disks of the closest and infrared
brightest Galactic objects could be spatially resolved, providing precise
measurements of the disk inclinations, disk sizes, and the distances of the
emitting material (dust, CO gas, and ionized gas traced by the Br $\gamma$
emission) from the central star (see Domiciano de Souza et al. (2007, 2011);
Millour et al. (2011); Cidale et al. (2012); Wang et al. (2012); Wheelwright
et al. (2012, 2012)).

Table 2: Presence of disk tracers in the optical and near-IR spectra of the Galactic and Magellanic Cloud B[e]SG samples. Object |  [Ca ii] |  [O i]a | Ref. | CO |  12C/13C | Ref. | TiO | SiO | Ref.
---|---|---|---|---|---|---|---|---|---
Large Magellanic Cloud B[e]SGs
LHA 120-S 12 | yes | no | Aret et al. (2012) | yes | 20$\pm$2 | McGregor et al. (1988); Oksala et al. (2013) | yes | … | Zickgraf et al. (1989)
LHA 120-S 22 | yes | yes | Aret et al. (2012) | no | … | McGregor et al. (1988); Oksala et al. (2013) | no | … | TWb
LHA 120-S 35 | yes | yes | Torres et al. (2018) | yes | 10$\pm$2 | Oksala et al. (2013); Torres et al. (2018) | yes | … | Torres et al. (2018)
LHA 120-S 59 | no | yes | Condori et al. (2019) | ? | … | Oksala et al. (2013) | no | … | TWb
LHA 120-S 73 | yes | yes | Aret et al. (2012); Kraus et al. (2016) | yes | 9$\pm$1 | McGregor et al. (1988); Oksala et al. (2013); Kraus et al. (2016) | yes | … | Kraus et al. (2017)
LHA 120-S 89 | no | no | TWb | no | … | McGregor et al. (1988); Oksala et al. (2013) | no | … | TWb
LHA 120-S 93 | yes | no | TWb | no | … | Oksala et al. (2013) | no | … | TWb
LHA 120-S 111 | yes | yes | Aret et al. (2012) | no | … | McGregor et al. (1988) | yes | … | Zickgraf et al. (1989)
LHA 120-S 127 | yes | yes | Kraus et al. (2007); Aret et al. (2012) | no | … | McGregor et al. (1988); Oksala et al. (2013) | no | … | TWb
LHA 120-S 134 | yes | yes | Aret et al. (2012) | yes | 15$\pm$2 | McGregor et al. (1988); Oksala et al. (2013) | yes | … | Zickgraf et al. (1989)
LHA 120-S 137 | no | yes | TWb | no | … | Oksala et al. (2013) | no | … | TWb
Small Magellanic Cloud B[e]SGs
LHA 115-S 6 | ? | yes | TWb | yes | 12$\pm$2 | McGregor et al. (1989); Oksala et al. (2013) | no | … | TWb
LHA 115-S 18 | yes | yes | Aret et al. (2012) | yes | 20$\pm$5 | Morris et al. (1996); Oksala et al. (2013) | yes | … | Zickgraf et al. (1989); Torres et al. (2012)
LHA 115-S 23c | ? | no | TWb | no | … | TWd | no | … | TWb
LHA 115-S 65 | yes | yes | Kraus et al. (2010); Aret et al. (2012) | yese | 20$\pm$5 | Oksala et al. (2012, 2013) | no | … | TWb
Galactic B[e]SGs/B[e]SG Candidates
MWC 137 | no | no | Kraus et al. (2017); Maravelias et al. (2018) | yes | 25$\pm$2 | Muratore et al. (2012, 2015); Oksala et al. (2013) | no | … | TWb
MWC 349 | yes | … | Aret et al. (2016) | yes | 4$\pm$1 | Kraus et al. (2000, 2019) | … | no | Kraus et al. (2019)
GG Car | yes | no | Maravelias et al. (2018) | yes | 15$\pm$5 | McGregor et al. (1988); Morris et al. (1996); Muratore et al. (2012); Kraus et al. (2013); Oksala et al. (2013); Maravelias et al. (2018) | no | … | TWb
Hen 3-298 | yes | yes | Miroshnichenko et al. (2005); Maravelias et al. (2018) | yes | 20$\pm$5 | Miroshnichenko et al. (2005); Muratore et al. (2012); Oksala et al. (2013); Maravelias et al. (2018) | no | … | TWb
CPD-52 9243 | yes | no | Maravelias et al. (2018) | yes | … | McGregor et al. (1988); Muratore et al. (2012); Maravelias et al. (2018) | no | yes | TWb,Kraus et al. (2015)
HD 327083 | yes | no | Maravelias et al. (2018) | yes | … | Muratore et al. (2012); Maravelias et al. (2018) | no | yes | TWb,Kraus et al. (2015)
MWC 300 | no | yes | TWb | no | … | Muratore et al. (2012); Liermann et al. (2014) | no | … | TWb
AS 381 | no | no | Miroshnichenko et al. (2002) | abs | ? | Miroshnichenko et al. (2002); Muratore et al. (2012); Liermann et al. (2014) | … | … | …
CPD-57 2874 | yes | no | Maravelias et al. (2018) | yes | … | McGregor et al. (1988); Muratore et al. (2012); Maravelias et al. (2018) | no | yes | TWb,Kraus et al. (2015)
Hen 3-938 | yes | yes | Condori et al. (2019) | … | … | … | no | … | TWb
MWC 342 | no | yes | Andrillat, & Jaschek (1999); Aret et al. (2016) | … | … | … | … | … | …
Hen 3-303 | no | no | TWb | no | … | Miroshnichenko et al. (2005) | no | … | TWb
CD-42 11721 | no | yes | Borges Fernandes et al. (2007) | no | … | Muratore et al. (2012) | no | … | TWb
HD 87643 | yes | no | Maravelias et al. (2018) | yese | … | Muratore et al. (2012); Maravelias et al. (2018) | no | … | TWb
HD 62623c | yes | yes | Maravelias et al. (2018) | yes | … | Muratore et al. (2012); Maravelias et al. (2018) | no | yes | TWb,Kraus et al. (2015)
Note: TW = This work; abs = in absorption; ? = uncertain detection/no value
available; …= no information.
a Refers to the presence of [O i]$\lambda$5577\. All sample stars show
emission of [O i]$\lambda\lambda$6300,6364.
b Based on (unpublished) high-resolution optical spectra taken between 2005
and 2017 with FEROS at the MPG 2.2m telescope.
c A[e]SG due to early-A spectral type assignment (Kraus et al. (2008);
Chentsov et al. (2010)).
d No indication of CO band features seen in a $K$-band spectrum (unpublished)
taken on 2013 October 20 with OSIRIS at the Southern Astrophysical Research
(SOAR) Telescope.
e No CO emission was detected during the observations taken between 1987 and
1989 (McGregor et al. (1988, 1989)).

###  2.2 Disk dynamics and structure

Determination of the kinematics within dense circumstellar environments
requires the use of reliable tracers. High-resolution near-infrared
spectroscopic observations have revealed that the band heads of the CO
emission from B[e]SGs typically display a characteristic shape, consisting of
a blue-shifted shoulder and a red-shifted maximum. For the generation of such
a band head profile, the individual CO rotation-vibration lines, superimposing
within the region of the band head, must display double-peaked profiles (see
Figure 2). Such line profiles can originate either from a circumstellar ring
of gas expanding with constant velocity (constant outflow), or from rotational
motion of a ring of gas around the central object. To discriminate between the
two scenarios, complementary tracers are needed.

The SiO band emission seen in four Galactic B[e]SGs displays a similar shape
of the band heads. Detailed modeling revealed that in each object the SiO
bands required a slightly lower value of the velocity (Kraus et al. (2015))
than the CO bands. The SiO molecule is less stable than CO, meaning that it
can form and persist only at lower temperatures. This fact naturally places
the region where SiO molecules are expected to form, and hence the SiO band
emitting region, at slower orbital velocities and greater distances from the
central object.

Figure 2: Sketch of the generation of the typical CO band head profile. (a)
Spectrum around the (2-0) band head of the CO first-overtone bands for a hot
gas with velocity dispersion of a few km s-1. (b) Profile of a single line
from a rotating gas ring with a velocity, projected to the line of sight, of
66 km s-1 as seen with a spectral resolution of 6 km s-1. (c) Total synthetic
CO band head spectrum resulting from the convolution of the band transitions
in (a) with the profile of the ring in (b). (d) CO band head observations of
the Galactic B[e]SG CPD-57 2874 (Maravelias et al. (2018)).

As CO is the most stable molecule, its formation and emission region marks the
inner edge of the molecular disk. Closer to the star, tracers for the
kinematics need to be found from line emission of the atomic gas. Here, the
lines from forbidden transitions are most suitable, because their emission is
optically thin, so that their profile shapes contain the full velocity
information of their formation region (Kraus et al. (2007, 2010)). Of
particular interest are hereby the [O i] lines, because they are one of the
defining characteristics of the B[e] phenomenon and hence observed in all
B[e]SGs. The ionization potential of O i is about the same as the one for H i,
which means that within the [O i] line forming regions, hydrogen should be
basically neutral as well, restricting the formation region of the [O i] line
emission to the neutral regions within the circumstellar disk. While
recombination in the equatorial region close to the star might be achieved,
e.g., with the model of a latitude dependent wind (Kraus, & Lamers (2003);
Kraus (2006); Zsargó et al. (2008)), the requirement of a hydrogen neutral
environment severely limits the number of free electrons that will be
available to collisionally excite the levels within O i from which the
forbidden transitions emerge. Consequently, the [O i] lines arise in regions
with high total density, but low electron density.

The profiles of the [O i] lines often display double-peaks, in line with their
formation in the disk. Typically, the [O i] $\lambda$5577 line, which arises
from a higher level than the $\lambda\lambda$6300,6364 lines, is broader,
indicating spatially distinct formation regions of the emissions, with the [O
i] $\lambda$5577 line being formed at higher velocities and higher densities
and hence closer to the star than the other two lines.

With the identification of the lines of [Ca ii] $\lambda\lambda$7291,7324 in
the spectra of numerous B[e]SGs, a further highly valuable tracer for the disk
kinematics has been found. These lines typically display double-peaked
profiles as well, with velocities comparable to or even greater than the one
traced by the [O i] $\lambda$5577 line (Kraus et al. (2010); Aret et al.
(2012); Maravelias et al. (2018)). This implies that they form in the same
region, or at least very close to each other, which is in agreement with their
comparably high critical density. Since the [O i] $\lambda$5577 line is not
always detectable, the [Ca ii] lines thus provide a suitable, complementary
benchmark for the dynamics within the disks of B[e]SGs.

In summary, the optical and near-infrared spectra provide emission features
from several species, which are suitable to pin down the kinematics within the
disks of B[e]SGs at various distances from the star, and Table LABEL:tab:CO-O-
Ca includes the information on the detection of the individual tracers in the
MC sample. Based on the physical constraints outlined above, the logical order
of the appearance of the divers tracers from inside out would be [Ca ii], [O
i], CO bands and SiO bands. The velocity information carried by these species
thereby implies a decrease with increasing distance from the star. While an
equatorial, outwards decelerating outflow might be able to explain some of the
observed line profiles (Kraus et al. (2007)), the velocity patterns seem to be
in better agreement with (quasi-)Keplerian rotation. In this respect it is
interesting to note that Keplerian rotation has been made directly discernible
by means of spectro-interferometric observations. The rotational motion of the
CO gas has been derived based on the differential phase spectrum (Wheelwright
et al. (2012, 2012)), and the rotational motion of the ionized gas based on
the spatially resolved Br$\gamma$ emission (Millour et al. (2011)).

While, in general, the rotational motion of the material within the
circumstellar disks of B[e]SGs seems now to be well established, maybe in
connection with a (very) slow outflow component (Kraus et al. (2010)), recent
investigations of the spatial distribution of the circumstellar gas revealed
that it is more likely accumulated in multiple rings, partial rings, and
possible spiral arm- like structures rather than in a smooth disk (Kraus et
al. (2016); Maravelias et al. (2018); Torres et al. (2018)). These rings might
result from multiple mass ejection phases caused by (pulsational)
instabilities acting in the outer layers of these luminous objects (e.g.,
Glatzel, & Kiriakidis (1993); Kiriakidis et al. (1993); Glatzel, & Mehren
(1996)), or from binary interaction in close systems as seems to be the case
for some of the Galactic objects (Maravelias et al. (2018)) in which the rings
are circumbinary rather than circumstellar. Other disk-forming mechanisms that
have been proposed in the literature over the years include equatorial mass-
loss from a critically rotating star (Kurfürst et al. (2014, 2018)), the
rotationally induced bi-stability mechanism (Pelupessy et al. (2000)), the
slow-wind solution (Curé (2004)), and the combination of the latter two (Curé
et al. (2005)). For an overview including a detailed description of the
various models and their limitations, see Kraus (2017).

The circumstellar material of many MC objects appears durable. This is
evidenced by their emission features and their infrared photometry that both
display no considerable variability over several decades, in combination with
chemically processed dust displaying emission from crystalline silicates
(Kastner et al. (2010)). It is tempting to imagine that in such an environment
even minor bodies might have formed from the long-lived disk material,
creating gaps within the disk in radial direction and hence leading to the
formation of the presumed ring structures (Kraus et al. (2016)). These minor
bodies or possible planets can also stabilize the neighboring rings, in
analogy to the shepherd moons in planetary systems. But so far, there is
insufficient observational evidence that might support the validity of such a
scenario.

###  2.3 Current evolutionary state of B[e]SGs

The formation mechanism of the observed gaseous and dusty rings or disk-like
structures around B[e]SGs is certainly one of the most important yet unsolved
issues. But equally important questions arise: What is the evolutionary phase
of B[e]SGs? What is their evolutionary connection to other evolved massive
stars? And exists such a connection at all? While the question on the relation
between B[e]SGs and other evolved objects is beyond the scope of this review,
we will briefly elucidate on the current knowledge about the evolutionary
status of B[e]SGs. Considering the MC sample, it is obvious from Fig. 1 that
all objects have evolved off the main sequence. Whether this occurred only
recently, or whether B[e]SGs might be on a blue loop or blueward evolution
after having passed through the turning point on the cool edge of their track
is still an open issue, in particular, since we lack clear methods for age
determinations of these emission-line objects, which only very rarely are
detected in clusters222Currently only four B[e]SGs are reported to be cluster
members. These are the two LMC objects LHA 120-S 111 in the compact cluster
NGC 1994 (Appenzeller et al. (1984)) and LHA 120-S 35 in SL482 (Torres et al.
(2018)), and the two Galactic sources MWC 137 in SH 2-266 (Mehner et al.
(2016)) and Wd1-9 in Westerlund 1 (Clark et al. (2005, 2013))..

In this regard, the detection of clear signs of the 13C isotope in form of
13CO band emission from a number of MC B[e]SGs (Liermann et al. (2010); Oksala
et al. (2013), see Table LABEL:tab:CO-O-Ca) was one major step forward. The
appearance of these bands has been predicted based on theoretical model
computations for a variation of the carbon isotope ratio 12C/13C (Kraus
(2009)). As surface abundance calculations have shown, this ratio will drop
during the evolution of massive stars from an initial, interstellar value of
$\sim$ 90 down to values $<$ 5, depending on the initial mass of the star and
its initial rotation speed. The surface material enriched in 13C is
transported via winds to the environments, where it will cool and condense
into 13CO molecules, whose emission can be observed in the $K$-band, together
with the emission from the main isotope, 12CO (see Figure 3). Hence, the
detected amount of 13CO is a measure for the stellar surface enrichment in 13C
at the time the material, that is currently traced in the molecular emission,
has been released from the stellar surface.

Figure 3: Synthetic spectra of the combined emission from 12CO and 13CO for
different values of the 12C/13C ratio. The computations have been performed
for the following physical parameters: a 12CO column density of $2\times
10^{21}$ cm-2, a gas temperature of 3000 K, a line-of-sight rotational
velocity of 66 km s-1, and a spectral resolution of 50 km s-1.

From the MC sample of 15 objects, 7 have been found to display CO band
emission, and all of them display clear indication of enrichment in 13C (see
Table LABEL:tab:CO-O-Ca). Interestingly, all these objects with CO emission
cluster in the same region of the HR diagram as indicated by the dotted black
square in Figure 1, i.e., in the luminosity range $\log L/L_{\odot}=5.0-5.8$.
None of the three most luminous stars (S 22, S 111, and S 127) or of the 4
low-luminosity objects (S 23, S 59, S 93, and S 137) displays clear signs of
CO emission. One outlier in the luminosity domain occupied by the CO emitting
B[e]SGs is the star S 89, which also has no detectable CO band emission
(Oksala et al. (2013)).

The absence of measurable CO band emission might have different reasons.
Either the intensity of the emission is too low to be detectable against the
strong near-IR continuum333This spectral region suffers from strong telluric
contamination, which is not always easy to remove, so that especially weak CO
emission features might be hidden within telluric remnants., or the density of
the molecular gas might be very high, resulting in optically thick emission
which has no characteristic band head structure anymore. Another possibility
would be that the CO emission from these stars might have variable CO band
emission and they have so far always been observed in phases of no emission.
In this context it is interesting to refer to the SMC object LHA 115-S 65, in
which CO band emission suddenly occurred, while observations taken about nine
months earlier did not detect any molecular features (Oksala et al. (2012)).
Alternatively, since we now know that the material is most probably
concentrated in rings, the conditions within the circumstellar environment in
terms of density and temperature might not be favorable for the excitation of
the first-overtone bands. For those stars, observations in the spectral region
of the fundamental bands might therefore be a possibility to search for cooler
CO gas.

Figure 4: Evolution of the 12C/13C isotope ratio along the solar metallicity
tracks of a star with initial mass of 32 M⊙ and initial rotation speeds
$v/v_{\rm crit}$ ranging from 0 to 0.4 (corresponding to $\Omega/\Omega_{\rm
crit}=0.0;0.1;0.2;0.3;0.4;0.5;0.568$). The individual tracks have been
obtained from the interpolation tool SCYCLIST provided by the Geneva group.
For clarity of the plot, we truncated the evolutionary tracks within the red
supergiant regions. Included are the positions of the MC B[e]SG sample from
Table LABEL:tab:CO-O-Ca with known 12C/13C ratio, following the same color
coding as for the tracks. The Galactic objects are excluded due to their
highly uncertain luminosities. Depending on the initial rotation speed of the
star, the observed ratio can be reached either in the pre-RSG (moderate
rotator) or post-RSG (slow rotator) phase.

The measured 12C/13C isotope ratios of the MC B[e]SGs are all very similar,
spreading from 9 to 20. This might point towards a similar formation history
of the circumstellar material, i.e. a similar phase in the evolution
(considering they are single stars or at least unaffected by a possible
companion) when the enriched material was ejected. Considering that stars are
typically born with some intrinsic rotation velocity, rotational mixing in
combination with enhanced mass-loss may drive the enrichment of the stellar
surface with 13C already in early stages of the evolution of massive stars.
This can be seen in Figure 4, where the evolutionary tracks of a 32 M⊙ star
with solar metallicity (Ekström et al. (2012)) and for a variety of initial
rotation velocities are shown. The covered rotation speeds spread from
$v/v_{\rm crit}=0$ to $v/v_{\rm crit}=0.4$, which correspond to values of
$\Omega/\Omega_{\rm crit}$ from 0 to 0.568. The interpolation of the tracks
has been performed with the SCYCLIST
tool444https://www.unige.ch/sciences/astro/evolution/en/database/syclist/
provided by the Geneva group.

The color coding along the tracks refers to the values of the 12C/13C isotope
ratio on the stellar surface. Also included in Figure 4 are the positions of
the seven MC B[e]SGs with known values of the 12CO/13CO isotope ratios. Their
colors correspond to the same color coding as the evolutionary tracks.
Obviously, the observed ratios for the sample stars might be reached either
along or after the main-sequence evolution for stars rotating initially with
rates $\Omega/\Omega_{\rm crit}\geq 0.3$. But they might also be reached
during or after the red supergiant stage for stars rotating initially with
rates smaller than $\Omega/\Omega_{\rm crit}\leq 0.3$. As we do not know the
initial rotation speeds of the progenitor stars of these B[e]SGs, the measured
values of the 12CO/13CO isotope ratio alone cannot solve the issue with the
current evolutionary state of the objects.

If B[e]SGs represent a specific phase in the evolution of massive stars, then
these objects should also exist in other environments with high content of
massive stars. Searching for representatives of B[e]SGs in other galaxies and
studying their properties and number statistics at various metallicities might
help to unveil their disk/ring formation mechanism, to pin down their
evolutionary phase (pre- versus post-RSG), and to set constraints on the
evolution of massive stars in general.

##  3 Identification of B[e] supergiants in the Local Universe

Identifying and classifying B[e]SGs is a tedious job, whether in the Milky
Way, where we face large amounts of foreground extinction and the issue with
the often unknown distances, or in other galaxies, where we have strong
contamination with foreground sources, often crowded regions, and the
faintness of the objects. While the first B[e]SGs have been found rather
accidentally, nowadays dedicated surveys can make use of the established
classification criteria. As mentioned in Section 2, there are basically four
characteristics a star should fulfill to be classified as B[e]SGs (Lamers et
al. (1998); Zickgraf (2006)). It should display:

  * •

A spectral-type B (with extensions to late-O and early-A types), as evidenced
from the hot underlying continuum or from photospheric absorption
features555We would like to stress that the high-luminosity B[e]SGs barely
display photospheric absorption lines whereas the low-luminosity B[e]SGs
typically do., and a luminosity higher than $\log L/L_{\odot}\sim 4$.

  * •

Very intense Balmer lines dominating the optical spectrum with an equivalent
width of H$\alpha$ reaching up to 1000 Å.

  * •

A spectrum indicating the hybrid character of the environment, consisting of a
hot, fast line-driven wind coexisting with a cool, slow component from which
the forbidden emission lines from neutral and low-ionized metals originate ([O
i], [Fe ii]).

  * •

Strong near- and mid-infrared excess emission, indicative of hot circumstellar
dust.

Despite these defining criteria, the identification of extragalactic B[e]SGs
is not as straightforward, because suitable candidates need first to be found
based on other means. These candidates can then be further investigated to
search for these, mostly spectroscopic, characteristics.

A suitable approach is to search for B[e]SG candidates among the luminous,
blue objects identified in imaging surveys that have been performed for
several galaxies over the past $\sim$30 years. For instance, the early
photographic and photometric surveys of M31 (Berkhuijsen et al. (1988); Massey
et al. (1986)) and M33 (Freedman (1984)) revealed (amongst many other objects)
the most luminous hot stars and the brightest blue supergiants, which could
then be studied spectroscopically to obtain indications for their possible
nature (e.g. Humphreys et al. (1990)). A milestone for the identification of
evolved massive stars was certainly the Local Group Galaxies Survey (LGGS)
project (Massey et al. (2006, 2007a)), which resulted in the discovery of
numerous putative emission-line stars in M31, M33, and seven more dwarf
galaxies. Spectroscopic follow-ups were used to sort out H ii regions, and to
match the remaining objects with the various known categories of evolved
massive stars. A major result of this survey was the identification of
numerous objects that were dubbed as luminous blue variable (LBV) candidates
(Massey et al. (2007b, 2016)), based on the appearance of their blue emission-
line or P Cygni-type spectra that resemble confirmed LBVs in quiescence. Since
the blue optical spectra of LBVs in their quiescence state display a number of
common characteristics with B[e]SGs (Massey et al. (2000, 2007b); Clark et al.
(2012)), it was expected that this sample contains a number of B[e]SGs.

Surprisingly, when analyzed in more detail (Clark et al. (2012); Humphreys et
al. (2014, 2017)), this bunch of newly identified LBV candidates in M31 and
M33 turned out to be a mixed bag containing not only B[e]SGs candidates, but
also so-called Fe ii emission stars (with neither [O i] nor [Fe ii] emission
and lacking warm dust), and warm hypergiants (with lots of dust, possible [O
i] and/or [Fe ii] but of spectral type A-F), with only a few objects left to
be considered as LBV candidates (with no [O i] emission and lacking hot dust).

As both, LBVs and B[e]SGs, are luminous blue supergiants, they share the same
optical colors. Hence, one step to distinguish these two groups of objects is
to inspect their location in infrared color-color diagrams (e.g. Zickgraf et
al. (1986); Gummersbach et al. (1995); Clark et al. (2012); Oksala et al.
(2013); Humphreys et al. (2017)). The hot ($\sim$ 1000 K) circumstellar dust
of B[e]SGs results in significantly increased near-IR emission. On the other
hand, LBVs can be associated to cold, dusty environments such as the
circumstellar shells recently discovered around many LBVs and LBV candidates
with the Spitzer Space Telescope at 24 $\mu$m (e.g. Gvaramadze et al. (2010);
Wachter et al. (2010)). The separation of B[e]SGs from quiescent LBVs, based
on their diverse IR properties, is demonstrated in Figure 5 for the known
samples of MC objects, limiting to the confirmed and generally accepted LBVs
in the LMC (Aadland et al. (2018)) and including one confirmed object from the
SMC (R40, Campagnolo et al. (2018)). Shown are two different color-color
diagrams ($J-H$ versus $H-K$ and $W1-W2$ versus $W2-W4$). The IR colors of the
objects are listed in Table LABEL:tab:JHK. They result from the $JHK$-band
magnitudes obtained from the 2MASS point source
catalog666http://cdsarc.u-strasbg.fr/viz-bin/cat/II/246 (Cutri et al. (2003)),
and from the mid-IR magnitudes ($W1,W2,W4$) collected with the Wide-field
Infrared Survey Explorer777http://cdsarc.u-strasbg.fr/viz-bin/cat/II/311
(WISE, Cutri, & et al. (2012)). From the many possibilities of near- and mid-
IR color-color diagrams, these two show the clearest separation between the
two groups of objects.

Figure 5: Demonstration of the separation of the B[e]SGs from the quiescent
LBVs within the near-IR ($J-H$ versus $H-K$ diagram, left panel) and the WISE
diagram ($W1-W2$ versus $W2-W4$, right panel). Shown are the positions of the
classical MC B[e]SG sample and of the MC LBV sample. IR colors of the objects
are provided in Table LABEL:tab:JHK. The solid line represents the positions
of regular supergiants with empirical colors taken from Worthey, & Lee (2011)
for solar metallicity stars.

Another clear distinctive feature between B[e]SGs and LBVs is the S Dor cycle
of the latter, while B[e]SGs are typically not undergoing this type of
variability. However, to identify such S Dor excursions of the stars within
the HR diagram is a time-consuming (though important) task, because it
requires regular monitoring of the whole sample.

In addition, dedicated spectroscopic observations are required to search for
the characteristic forbidden emission lines of [O i] and possible [Ca ii] in
the red portion of the optical spectra, and to search for possible molecular
emission of CO in their near-IR spectra, because LBVs typically do not show
these sets of forbidden lines and CO band emission888One exception to this
rule is the LBV star HR Car, which occasionally showed CO band emission
related to phases when the star was dimmer (Morris et al. (1997)). (McGregor
et al. (1988, 1988); Oksala et al. (2013)).

Table 3: IR colors of Magellanic Cloud B[e]SGs and LBVs. Object | J-H | H-K | W1-W2 | W2-W4 | Class
---|---|---|---|---|---
Large Magellanic Cloud
LHA 120-S 12 | 0.634$\pm$0.035 | 0.924$\pm$0.035 | 0.761$\pm$0.031 | 1.723$\pm$0.051 | B[e]SG
LHA 120-S 22 | 0.619$\pm$0.033 | 1.289$\pm$0.033 | 1.258$\pm$0.047 | 3.328$\pm$0.032 | B[e]SG
LHA 120-S 35 | 0.520$\pm$0.060 | 1.096$\pm$0.051 | 1.156$\pm$0.027 | 4.323$\pm$0.021 | B[e]SG
LHA 120-S 59 | 0.661$\pm$0.037 | 0.962$\pm$0.035 | 0.988$\pm$0.031 | 3.514$\pm$0.033 | B[e]SG
LHA 120-S 73 | 0.403$\pm$0.033 | 0.825$\pm$0.033 | 1.030$\pm$0.031 | 4.015$\pm$0.027 | B[e]SG
LHA 120-S 89 | 0.440$\pm$0.022 | 1.030$\pm$0.036 | 1.059$\pm$0.031 | 3.289$\pm$0.026 | B[e]SG
LHA 120-S 93 | 0.507$\pm$0.036 | 1.195$\pm$0.033 | 1.128$\pm$0.030 | 3.486$\pm$0.025 | B[e]SG
LHA 120-S 111a | 0.500$\pm$0.000 | 1.030$\pm$0.000 | … | … | B[e]SG
LHA 120-S 127 | 0.362$\pm$0.033 | 1.004$\pm$0.033 | 1.106$\pm$0.039 | 3.922$\pm$0.028 | B[e]SG
LHA 120-S 134b,c | 0.609$\pm$0.030 | 1.245$\pm$0.030 | 1.090$\pm$0.045 | 2.892$\pm$0.035 | B[e]SG
LHA 120-S 137 | 0.899$\pm$0.040 | 1.097$\pm$0.037 | 0.931$\pm$0.030 | 2.691$\pm$0.045 | B[e]SG
LHA 120-S 96 (= S Dor) | 0.173$\pm$0.054 | 0.172$\pm$0.057 | 0.374$\pm$0.028 | 5.586$\pm$0.022 | LBV
LHA 120-S 116 (= R110) | 0.116$\pm$0.034 | 0.161$\pm$0.036 | 0.076$\pm$0.032 | 3.391$\pm$0.034 | LBV
LHA 120-S 128 (= R127) | 0.115$\pm$0.037 | 0.286$\pm$0.036 | 0.255$\pm$0.029 | 6.573$\pm$0.021 | LBV
LHA 120-S 155 (= R71) | 0.027$\pm$0.035 | 0.097$\pm$0.038 | 0.069$\pm$0.033 | 7.095$\pm$0.024 | LBV
CPD-69 463 (= R143) | 0.244$\pm$0.033 | 0.201$\pm$0.035 | 0.259$\pm$0.030 | 7.851$\pm$0.049 | LBV
LHA 120-S 83 (= Sk $-$69 142a) | 0.115$\pm$0.041 | 0.287$\pm$0.041 | 0.239$\pm$0.031 | 1.609$\pm$0.055 | LBV
Small Magellanic Cloud
LHA 115-S 6b | 0.433$\pm$0.034 | 0.709$\pm$0.0319 | 0.769$\pm$0.031 | 4.037$\pm$0.035 | B[e]SG
LHA 115-S 18b,c | 0.418$\pm$0.050 | 0.822$\pm$0.0460 | 1.059$\pm$0.030 | 3.705$\pm$0.031 | B[e]SG
LHA 115-S 23 | 0.221$\pm$0.038 | 0.601$\pm$0.0382 | 0.946$\pm$0.031 | 2.484$\pm$0.096 | B[e]SG
LHA 115-S 65 | 0.292$\pm$0.035 | 0.800$\pm$0.0311 | 0.982$\pm$0.030 | 4.363$\pm$0.027 | B[e]SG
LHA 115-S 52 (= R40) | 0.182$\pm$0.036 | 0.065$\pm$0.033 | 0.066$\pm$0.031 | 2.851$\pm$0.043 | LBV
Note: IR photometry for all objects is taken from the 2MASS point source
catalog ($J,H,K$, Cutri et al. (2003)), except for the stars LHA 120-S 111
(McGregor et al. (1988)) and LHA 120-S 89 (Bonanos et al. (2009)), and from
the WISE All-Sky Data Release ($W1,W2,W4$, Cutri, & et al. (2012)).
aDespite of the lack of WISE colors, the presence of warm dust is proven by
its IR excess seen in the Spitzer data, and its IR spectrum that looks like a
twin of the one of LHA 120-S 73 (see Kastner et al. (2010)).

##  4 A census of B[e]SGs

With clearly defined classification characteristics and the proper
observational tools at hand, the massive star population within the local
Universe can be scanned for suitable B[e]SG candidates. At the moment of
writing this review, this is still an ongoing project that requires patience
and sufficient telescope time at both, optical and infrared facilities.
Nevertheless, many new, particularly extragalactic B[e]SG star discoveries
were reported in the literature within the past 20 years. The aim of this
section is, therefore, to take a closer and critical look at the suggested
B[e]SG candidates in order to sort out possible misclassified objects, to
check what type of observations are still missing for unambiguous
classification of the candidates, and to compile updated lists of confirmed
B[e]SGs for the galaxies with a reported B[e]SG population. Starting point for
this investigation are the Magellanic Clouds (Section 4.1), moving further out
into the Local Group (Section 4.2) and beyond (Section 4.3), before we finally
return to the Milky Way (Section 4.4).

The samples in each galaxy are presented in tables, which follow the same
structure. The objects are listed under a (if possible) homogeneous SIMBAD
identifier in column 1, reference(s) for the B[e]SG classification of the
stars follow in column 2. Where available, $E(B-V)$ values and their
references are provide in columns 3 and 4, and the four colors ($J-H$, $H-K$,
$W1-W2$, and $W2-W4$) are given along with their errors in the last four
columns. The tables are furthermore organized such that in the top part appear
the confirmed B[e]SGs. These are stars that fulfill all classification
criteria. In the middle part of each table stars with uncertain or
controversial classification are listed with their names in parentheses. These
are objects that lack one or more of the classification criteria due to
incomplete observational data sets. Also included are here objects for which
different research teams find controversial results so that clarification is
needed. In the bottom of each table erroneously classified objects are
gathered with their names in italic and within parentheses. These are stars
for which observational evidence (e.g. a specific color) excludes them from
belonging to the class of B[e]SGs.

###  4.1 Magellanic Clouds

As mentioned in Section 2, the "classical" sample of B[e]SGs resides within
the Magellanic Clouds. The first such object, for which the hybrid character
was reported, was the LMC star RMC 126 (LHA 120-S 127, Zickgraf et al.
(1985)), and soon after followed the identification of ten more B[e]SGs in the
LMC and four in the SMC (Stahl et al. (1989); Zickgraf et al. (1986, 1989,
1992, 1996); Gummersbach et al. (1995), see Table LABEL:tab:logTL) based on a
dedicated search for similar objects. Since then, a few more stars have been
suggested as B[e]SGs candidates. These are presented and discussed in the
following. The classical sample of B[e]SGs in the MCs, which fulfill all
classification criteria, has already been listed in Table LABEL:tab:logTL,
with their IR colors provided in Table LABEL:tab:JHK.

####  4.1.1 Large Magellanic Cloud

New B[e]SG candidates have been found either by dedicated searches, e.g. from
cross-matching catalogs of emission-line stars with near-IR catalogs (e.g
Miroshnichenko et al. (2011)), or more serendipitously as a by-product of deep
spectroscopic surveys of specific regions, such as the VLT-FLAMES Tarantula
Survey (VFTS, Evans et al. (2011, 2015); Kalari et al. (2014)) that was
devoted to the 30 Doradus starburst region. Also surveys for other purposes,
such as the search for post-asymptotic giant branch stars (Kamath et al.
(2015)), resulted in new B[e]SG candidates999I would like to point out that
from the proposed 12 newly discovered B[e] stars only one was found to be a
supergiant (see Table 4). The others have been carefully inspected in
collaboration with Devika Kamath, and they did not fulfill the requirements.
These results are yet unpublished.. Seven new B[e]SG objects have been
proposed in total, of which only two fulfill the criteria for B[e]SGs, two are
considered candidates, and three appear to be misclassified. All seven stars
are listed in Table 4. Their locations in the two color-color diagrams are
shown in Figure 6 in comparison with the confirmed B[e]SGs and LBVs from Table
LABEL:tab:JHK.

A sample of confirmed MC late-type stars and supergiants (González-Fernández
et al. (2015)) is included in these color-color diagrams. These serve as
reference for objects that might have a late-type companion. The outliers of
the late-type stars, especially in the WISE diagram, are objects with high
mass-loss.

Also shown is a sample of Galactic Herbig AeBe (HAeBe) stars (Hernández et al.
(2004)). Only stars with known extinction values and solid magnitudes in all
four bands were selected (rejecting objects with reported contamination). As
some of these pre-main sequence objects suffer from very high extinction
($A_{\rm V}>5$ mag), their colors were corrected using an $R_{\rm
V}$-dependent extinction law (Cardelli et al. (1989)) with $R_{\rm V}=5.0$
which has been found to be reasonable for HAeBe stars (Hernández et al.
(2004)). These pre-main sequence stars were included to check for possible
misclassification of objects that have a proposed luminosity ranging around
the lower limit for B[e]SGs, because this luminosity range is shared by the
most massive HAeBes. In the near-IR diagram, the HAeBes populate a stripe that
appears to be parallel and seems to connect seamlessly to the region occupied
by the classical B[e]SGs. In the WISE diagram, the HAeBes also seem to
populate a stripe adjacent to the B[e]SG domain.

In the following, the reasons for classification of the new LMC objects as
either confirmed or candidate, or for rejecting them as B[e]SGs are briefly
presented.

##### LHA 120-S 165

This object was first listed as a candidate young stellar object (Whitney et
al. (2008)), but was later classified as possible B[e]SG, based on its optical
spectrum displaying all characteristic emission features (SSTISAGEMC
J052747.62-714852.8, Kamath et al. (2015)). Its optical and infrared
brightness together with its position in both color-color diagrams support
this classification.

##### ARDB 54

Recent analysis of ARDB 54 revealed that it belongs to the (so far only few)
A[e]SGs, being the first of its kind in the LMC (Condori et al. (2019)). Its
position in the near-IR diagram supports this classification, although the
object is displaced from the region of classical B[e]SGs in the WISE diagram,
where it is located closer to the LBV region. Its luminosity of $\log
L/L_{\odot}\simeq 4.4$ (Condori et al. (2019)) is too low to be considered as
LBV candidate, and a bit too high to be considered as HAeBe star. From the
latter ARDB 54 is also offset in both color-color diagrams, so that a
classification as A[e]SG seems to be the most reasonable, despite its exposed
location in the WISE diagram, whose cause should be examined more closely.

Table 4: Confirmed and candidate B[e]SGs in the LMC. Misclassified objects are listed in the bottom part of the table. Object | Ref. | E(B-V) | Ref. | J-H | H-K | W1-W2 | W2-W4
---|---|---|---|---|---|---|---
Confirmed B[e]SGs
LHA 120-S 165 | Kamath et al. (2015) | … | … | 0.735$\pm$0.041 | 0.98$\pm$0.040 | 0.886$\pm$0.029 | 3.385$\pm$0.037
ARDB 54 | Levato et al. (2014); Condori et al. (2019) | 0.11 | Condori et al. (2019) | 0.340$\pm$0.057 | 0.810$\pm$0.04 | 0.402$\pm$0.023 | 2.057$\pm$0.054
Uncertain or controversial classification
(VFTS 1003) | Evans et al. (2011) | … | … | 0.540$\pm$0.094 | 0.87$\pm$0.0539 | … | …
(VFTS 822) | Kalari et al. (2014); Evans et al. (2015) | 0.56 | Kalari et al. (2014) | 0.689$\pm$0.039 | 1.261$\pm$0.035 | 0.952$\pm$0.033 | 6.09$\pm$0.046
Erroneous classification
(VFTS 698) | Dunstall et al. (2012) | 0.6 | Dunstall et al. (2012) | 0.436$\pm$0.030 | 0.444$\pm$0.034 | 0.585$\pm$0.030 | 9.81$\pm$0.023
([L72] LH 85-10) | Massey et al. (2000) | … | … | 0.108$\pm$0.036 | 0.211$\pm$0.048 | … | …
(NOMAD1 |  |  |  |  |  |  |
0181-0125572) | Levato et al. (2014) | … | … | …a | …a | … | …
Note: IR photometry for all objects is taken from the 2MASS point source
catalog ($J,H,K$, Cutri et al. (2003)) and from the WISE All-Sky Data Release
($W1,W2,W4$, Cutri, & et al. (2012)).
a The JHK magnitudes listed in SIMBAD were mistakenly taken from the paper of
Levato et al. (2014), but these belong to the star LHA 120-S 165.

##### (VFTS 1003)

This star was found from the VLT-FLAMES Tarantula Survey (Evans et al.
(2011)). In high angular-resolution near-IR images it appears as a single,
isolated object (Campbell et al. (2010)). The star’s blue emission-line
spectrum resembles closely the one of the Galactic B[e]SG GG Car. VFTS 1003
was suggested to be either a Herbig B[e] star or a B[e] supergiant (Evans et
al. (2011)). From its position in the near-IR diagram, a supergiant
classification seems more likely. But the lack of WISE photometry allows to
assign VFTS 1003 only a candidate status. In addition, for a definite B[e]
classification, red optical spectra are needed, because the optical spectra
reported in the literature do not cover the region of the [O i] lines.

Figure 6: Location of the new LMC (light blue) and SMC (purple) samples with
respect to those MC B[e]SGs that meet all the required classification criteria
(dark blue) and LBVs (red triangles) in the near-IR (top panel) and the WISE
diagram (bottom panel). Filled circles are used for confirmed B[e]SGs, filled
stars for candidates, and empty circles for misclassified objects. Also
included are a sample of late-type stars and supergiants in the MCs (small
gray crosses, from González-Fernández et al. (2015)) and a sample of
(dereddened, see text) Galactic HAeBe stars (black plus signs, from Hernández
et al. (2004)). The arrow in each panel indicates the direction of the
reddening, and their length complies with a value of $A_{\rm V}=3$. As not for
all MC objects the color excess is known, no extinction correction has been
applied. But the MC stars have in general relatively small color excess values
(see Tables 4 and LABEL:tab:SMC) which would shift them only marginally in the
diagrams.

##### (VFTS 822)

This late B-type star (Kalari et al. (2014)) is another object that was
identified in the VLT-FLAMES Tarantula Survey (Evans et al. (2015)) based on
its blue emission-line spectrum. As with VFTS 1003, a red spectrum is needed
to check for the presence of [O i] emission. And alike that star, VFTS 822 has
been proposed as possible Herbig B[e] pre-main sequence candidate. It displays
a UV excess typical for pre-main sequence stars, but its luminosity of $\log
L/L_{\odot}\sim 4$ is not conclusive and leaves as well room for an
interpretation as evolved star. In the near-IR diagram VFTS 822 appears within
the B[e]SG domain with a clear separation from the region hosting HAeBe stars,
but from the WISE diagram the star is located at the border region with the
HAeBes.

##### (VFTS 698)

This is the third object identified within in the VLT-FLAMES Tarantula Survey
and suggested as B[e]SG (Dunstall et al. (2012)). It also lacks a red optical
spectrum to ascertain the presence of [O i] emission in order to be considered
a B[e] star. Its spectral variability and photometric light curve unveiled
that the object is most likely an interacting binary consisting possibly of an
early B-type star and a veiled more massive companion. Its IR colors place it
in both diagrams into the region occupied by HAeBe stars, but the luminosity
estimates for both stars with $\log L/L_{\odot}\sim 5$ and $5.3$ are too high
for a pre-main sequence classification. Correcting for the extinction towards
VFTS 698 would shift the object closer to the LBV domain. In the WISE diagram,
the object appears particularly far off from the location of the classical
B[e]SGs. Hence, this object needs further investigations for a proper
classification.

##### ([L72] LH 85-10)

Searching for LBV candidates in OB associations in the LMC the object number
10, within the association LH 85, was found to be a luminous emission-line
star. Due to the similarity of its blue spectrum with other B[e]SGs, it has
been suggested as a new member of that group (Massey et al. (2000)). To date,
no information about the presence of [O i] emission from that object is
available. Moreover, its spectral energy distribution does not display the
characteristic near-IR excess emission of B[e]SGs questioning the validity of
this classification (Bonanos et al. (2009)). The star’s position in the near-
IR diagram coincides with the region populated by LBVs, supporting the
erroneous classification. No WISE photometry is available for this object.

##### (NOMAD1 0181-0125572)

This star appears neither in the 2MASS nor in the WISE catalog, although it
was reported to have 2MASS photometry (Levato et al. (2014)). From the
coordinates of the source labeled NOMAD1 0181-0125572 by Miroshnichenko et al.
(2011) it is obvious, that it was confused with the object LHA 120-S 165. The
star with SIMBAD identifier NOMAD1 0181-0125572 is not a B[e]SG.

####  4.1.2 Small Magellanic Cloud

In the SMC, a total of six new B[e]SGs has been reported. Five of them were
discovered as by-products from the spectroscopic survey of the hot, luminous
stars in the SMC (2dFS, Evans et al. (2004)), from the Runaways and Isolated O
Star Spectroscopic Survey of the SMC (RIOTS4, Graus et al. (2012)), and from
the SMC photometric catalog (Bonanos et al. (2010)). One more object was
serendipitously detected (Wisniewski et al. (2007)). From a critical
inspection of the properties of these six stars, only one object appears to
fulfill all required criteria, one object appears as very promising candidate
that lacks only some complementary information for unambiguous classification,
whereas the remaining four stars are considered as misclassified. All objects
are presented in the following, their IR colors are provided in Table
LABEL:tab:SMC, and their positions within the two color-color diagrams were
included in Figure 6.

##### [MA93] 1116 = NGC 346:KWBBe 200

The star is one of the few objects residing in a cluster, the SMC cluster NGC
346. Due to its H$\alpha$ emission it was first classified as a compact H ii
region (Meyssonnier, & Azzopardi (1993)), then as a Be star based on its
optical photometry (Keller et al. (1999)), and in the following as B[e]SG
based on its optical appearance and near-IR excess emission (Wisniewski et al.
(2007)). In the mid-IR, [MA93] 1116 displays silicate emission and strong PAH
bands, features often seen in HAeBe stars (e.g., Keller et al. (2008); Peeters
et al. (2002)). Additional cold dust is surrounding the object as is implied
by its detection at 24 $\mu$m with Spitzer/MIPS. These characteristics led to
the suggestion that [MA93] 1116 might be an evolved young stellar object
(Ruffle et al. (2015)) or a HAeBe star (Whelan et al. (2013)), despite of the
lack of clear evidence for infall, e.g., in form of inverse P Cygni profiles.
If true, its luminosity of $\log L/L_{\odot}\sim 4.4$ (Wisniewski et al.
(2007); Condori et al. (2019)) would place it at the upper limit for Herbig
objects, while it would get in lane with the group of less-luminous B[e]SGs in
the evolved scenario. The position of [MA93] 1116 within the WISE diagram
leaves room for a possible classification as HAeBe star where it falls on the
edge of the region populated by HAeBe stars, whereas it appears clearly off
the HAeBe region in the near-IR color-color diagram. It is hence considered as
B[e]SG, and the fifth object within a cluster (the first one in the SMC).

Table 5: Confirmed and candidate B[e]SGs in the SMC. Misclassified objects are listed in the bottom part of the table. Object | Ref. | E(B-V) | Ref. | J-H | H-K | W1-W2 | W2-W4
---|---|---|---|---|---|---|---
Confirmed B[e]SGs
$[$MA93] 1116 | Wisniewski et al. (2007); Condori et al. (2019) | 0.42 | Condori et al. (2019) | 0.665$\pm$0.077 | 1.084$\pm$0.0689 | 1.112$\pm$0.03 | 6.883$\pm$0.034
Uncertain or controversial classification
(LHA 115-S 38) | Bonanos et al. (2010) | 0.13 | Kamath et al. (2014) | 0.468$\pm$0.038 | 0.879$\pm$0.038 | 1.034$\pm$0.031 | 3.978$\pm$0.035
Erroneous classification
(LHA 115-N82) | Heydari-Malayeri (1990); Evans et al. (2004) | 0.12 | Heydari-Malayeri (1990) | 0.488$\pm$0.040 | 1.141$\pm$0.040 | 1.133$\pm$0.031 | 2.723$\pm$0.038
(LHA 115-S 29) | Graus et al. (2012) | 0.05 | Graus et al. (2012) | 0.089$\pm$0.030 | 0.180$\pm$0.032 | 0.346$\pm$0.030 | 2.479$\pm$0.020
(LHA 115-S 46) | Graus et al. (2012) | 0.05 | Graus et al. (2012) | 0.096$\pm$0.037 | 0.204$\pm$0.040 | 0.308$\pm$0.030 | 2.108$\pm$0.020
(LHA 115-S 62) | Graus et al. (2012) | 0.11 | Graus et al. (2012) | 0.084$\pm$0.039 | 0.175$\pm$0.048 | 0.315$\pm$0.034 | 3.076$\pm$0.024
Note: IR photometry for all objects is taken from the 2MASS point source
catalog ($J,H,K$, Cutri et al. (2003)) and from the WISE All-Sky Data Release
($W1,W2,W4$, Cutri, & et al. (2012)).

##### (LHA 115-S 38 = 2dFS 1804)

This object was found from the spectroscopic survey of the hot, luminous stars
in the SMC (Evans et al. (2004)). The blue spectrum displays numerous emission
lines, in particular of [Fe ii], but no information about [O i] emission is
available. The spectral energy distribution indicates a near-IR excess
(Bonanos et al. (2010)). With a luminosity estimate of $\log L/L_{\odot}\simeq
4.1$ the star was also considered as a post-AGB object (Kamath et al. (2014)),
but its positions within the two color-color diagrams places the star clearly
within the B[e]SG domain. As such, it is a strong candidate for another low-
luminosity B[e]SG.

##### (LHA 115-N82 = 2dFS 2837 = LIN 495)

This star displays all spectroscopic B[e]SG characteristics (Heydari-Malayeri
(1990); Evans et al. (2004)) and an IR excess emission (Bonanos et al.
(2010)). The optical spectrum was reported to be composite (Evans et al.
(2004)), and the detected photospheric lines display radial velocity
variations, whereas the emission from the circumstellar matter appears to be
stable (Condori et al. (2019)). The IR colors of LHA 115-N82 locate it clearly
within the B[e]SG domains. Curiously, the $V$ and $I$ band light curves
display a long-term brightening, which resembles LBV outbursts and is not
common in B[e]SGs. On the other hand, it displays both [O i] and [Ca ii]
emission which are typically not seen in LBVs. Since the star has too low
luminosity ($\log L/L_{\odot}\simeq 3.8$) to be an LBV (or even a B[e]SG), it
was recently assigned a classification as "LBV imposter" (Condori et al.
(2019)). This object clearly requires further investigations to pin down its
status.

##### (LHA 115-S 29 = RMC 15), (LHA 115-S 46 = RMC 38), (LHA 115-S 62 = RMC
48)

The blue optical spectra of these three objects show emission-line features
similar to B[e]SGs, and the luminosities derived for these objects, ranging
from $\log L/L_{\odot}\simeq 4.4$ to 4.8, assigns them a supergiant status
(Graus et al. (2012)). However, it is currently not known whether these
objects display [O i] line emission, one of the defining characters of the
B[e] phenomenon. Due to the lack of a pronounced near-IR excess emission,
these three stars have been proposed to be dust-poor B[e]SGs (Graus et al.
(2012)). However, the presence of warm dust is another main classification
criteria for a star to be considered as B[e]SG. It is not surprising that all
three objects fall clearly outside the B[e]SG domains in the color-color
diagrams. Instead, their positions coincide with the regions populated by
LBVs, which implies that these three stars have dense winds and circumstellar
ionized gas, which is exemplified by their emission-line spectra. These
objects require further investigations to unveil their true nature.

###  4.2 Local Group galaxies beyond the Magellanic Clouds

Moving further away, beyond the Magellanic Clouds, we may expect to find
B[e]SG stars and candidates in those galaxies, in which star-formation is
ongoing. For these galaxies, surveys such as the LGGS project mentioned in
Section 3, but also earlier surveys (e.g., Corral (1996); Fabrika, &
Sholukhova (1999)) provide indispensable information on the population of
luminous, evolved massive stars, and provide the base for systematic
investigations for unambiguous classification of these objects.

To date, systematic spectroscopic studies have been performed in the two large
spiral galaxies of the Local Group, M31 and M33, in which a number of B[e]SGs
were found amongst the putative LBV candidates (Clark et al. (2012); Kraus et
al. (2014); Sholukhova et al. (2015); Humphreys et al. (2017); Kourniotis et
al. (2018)). The suggested B[e]SG populations in each of these galaxies is
presented in the following subsections. The samples are provided in the Tables
LABEL:tab:M31 and LABEL:tab:M33, in which all objects are listed under their
Local Group Galaxy Survey (LGGS) identifiers (column 1). The properties, based
on which the decision to categorize an individual star as either a candidate
or a misclassified object has been made, are briefly depicted. The locations
of the confirmed, candidate, and possibly misclassified B[e]SGs in the two
color-color diagrams are shown in Figure 7.

####  4.2.1 M31

A total of 13 B[e]SGs has been proposed in M31 to date. Twelve of them
resulted from optical spectroscopic observations of LBV candidates that
display the typical B[e]SG characteristics (Humphreys et al. (2017);
Sholukhova et al. (2017)). Infrared spectroscopic observations of a small LBV
candidate sample revealed so far that three display CO band emission (Kraus et
al. (2014); Sholukhova et al. (2015)), making their classification as B[e]SG
very likely, especially since two of the CO band emission objects were also
found from optical spectroscopy to be a possible B[e]SG. However, a closer
look at this total sample of 13 objects, including the available information
from near- and mid-IR photometric observations, reveals that only four stars
can be considered as confirmed B[e]SGs. Seven objects require further
clarifications, and two appear to be misclassified (see Table LABEL:tab:M31).

##### J004320.97+414039.6, J004415.00+420156.2, J004417.10+411928.0

These three objects fulfill all classification criteria of the confirmed
B[e]SGs. Their positions in the near-IR and WISE diagrams coincides with the
domain populated by the B[e]SGs, except for J004320.97+414039.6, which appears
a bit off due to the uncertainties in its $J$ and $H$ magnitudes, which are
only upper limits.

##### J004522.58+415034.8

The star was assigned a warm hypergiant status due to detected photospheric
features pointing towards an A2Ia spectral-type (Humphreys et al. (2013)). As
it otherwise fulfills all criteria of a B[e]SG (even displaying CO band
emission, Kraus et al. (2014)), it is included into the list of confirmed
B[e]SGs, but keeping in mind that with its A spectral type it is actually a
representative of the (though few) A[e]SGs. Recent modeling of the star’s
spectral energy distribution resulted in an effective temperature estimate of
11 000 K (Humphreys et al. (2017)), which would point towards a late-B
spectral type.

##### (J004220.31+405123.2), (J004221.78+410013.4), (J004229.87+410551.8),
(J004442.28+415823.1)

These stars display the optical characteristics of B[e]SGs (Humphreys et al.
(2017)), and their WISE colors place them within the B[e]SG domain. But all
four objects lack $JHK$ band photometry, so that they cannot be located in the
$J-H$ versus $H-K$ diagram. The presence of He i emission in
J004221.78+410013.4 (Humphreys et al. (2017)) contradicts the proposed
effective temperature of 7200 K (Humphreys et al. (2017)) and requires
clarification.

##### (J004411.36+413257.2)

The star is reported to display He i, Fe ii, and [Fe ii] emission and was
suggested as B[e]SG star (Sholukhova et al. (2017)). Whether it displays [O i]
emission was not discussed by these authors and is hence yet unclear, although
the spectrum covers that spectral region. Its $J-H$ color is smaller than for
typical B[e]SGs (but the $H$ magnitude is uncertain), placing the star at the
lower boundary for the B[e]SGs in the color-color diagram. No WISE photometry
exists. The closest IR source is more than 6 arcsec away. The object is not
reported in other published spectroscopic investigations to display all B[e]SG
characteristics. Instead, it was listed as Fe ii emission line star (Humphreys
et al. (2014)) and more recently as LBV candidate (Humphreys et al. (2017)).

Table 6: Confirmed and candidate B[e]SGs in M31. Misclassified objects are listed at the bottom of the table. LGGS | Ref. | J-H | H-K | W1-W2 | W2-W4
---|---|---|---|---|---
Confirmed B[e]SGs
J004320.97+414039.6 | Humphreys et al. (2017); Sholukhova et al. (2017) | 0.358 | 1.522$\pm$0.143 | 0.983$\pm$0.051 | 5.284$\pm$0.084
J004415.00+420156.2a | Humphreys et al. (2017); Sholukhova et al. (2017) | 0.741$\pm$0.253 | 1.510$\pm$0.210 | 0.961$\pm$0.033 | 3.645$\pm$0.230
J004417.10+411928.0b,c | Kraus et al. (2014); Humphreys et al. (2017) | 0.387$\pm$0.148 | 0.853$\pm$0.149 | 0.882$\pm$0.068 | 4.455$\pm$0.247
J004522.58+415034.8c,d | Kraus et al. (2014) | 0.684$\pm$0.223 | 0.877$\pm$0.273 | 1.091$\pm$0.039 | 4.303$\pm$0.187
Uncertain or controversial classifications
(J004220.31+405123.2) | Humphreys et al. (2017) | … | … | 0.880$\pm$0.062 | 4.495$\pm$0.298
(J004221.78+410013.4) | Humphreys et al. (2017) | … | … | 0.927$\pm$0.072 | 5.253$\pm$0.107
(J004229.87+410551.8)e | Humphreys et al. (2017); Sholukhova et al. (2017) | … | … | 0.870$\pm$0.045 | 4.484$\pm$0.082
(J004411.36+413257.2) | Sholukhova et al. (2017) | 0.187$\pm$0.123 | 0.914$\pm$0.165 | … | …
(J004442.28+415823.1) | Humphreys et al. (2017); Sholukhova et al. (2017) | … | … | 0.781$\pm$0.076 | 4.994$\pm$0.406
(J004444.52+412804.0)c,d | Sholukhova et al. (2015) | 0.576$\pm$0.119 | 0.858$\pm$0.114 | 0.883$\pm$0.039 | 5.247$\pm$0.046
(J004621.08+421308.2)d | Sholukhova et al. (2017) | 0.374$\pm$0.171 | 1.283$\pm$0.128 | 1.091$\pm$0.039 | 4.303$\pm$0.188
Erroneous classifications
(J004043.10+410846.0) | Humphreys et al. (2017); Sholukhova et al. (2017) | 1.616 | 0.523$\pm$0.162 | 0.974$\pm$0.045 | 4.030$\pm$0.250
(J004057.03+405238.6) | Humphreys et al. (2017) | … | … | … | …
Note: IR photometry is taken from the 2MASS point source catalog ($J,H,K$,
Cutri et al. (2003)) and from the WISE All-Sky Data Release ($W1,W2,W4$,
Cutri, & et al. (2012)). Colors resulting from uncertain photometric values
are written in italic.
a Possible contamination in the photometric bands J, W1 and W2 due to
crowding.
b Possible contamination in the photometric bands J and H due to crowding.
c Has CO band emission (Kraus et al. (2014); Sholukhova et al. (2015)
d Also classified as warm hypergiant (Humphreys et al. (2013)).
e Possible contamination in the photometric bands W1 and W2 due to crowding.

##### (J004444.52+412804.0)

This object was first proposed to be a P Cyg-type LBV candidate (Massey et al.
(2007b)). With the detection of [Ca ii] emission and circumstellar dust
(Humphreys et al. (2014)), and with the determination of an effective
temperature of 6600 K (Humphreys et al. (2017)), it was classified as a warm
hypergiant. On the other hand, a B[e]SG classification with a central star of
15 000–20 000 K has been suggested, based on the presence of He i lines
displaying P Cyg profiles and the detection of intense CO band emission
(Sholukhova et al. (2015)). This controversial classification, based on
spectra taken at similar epochs, requires clarification. Infrared photometry
places J004444.52+412804.0 within the region occupied by B[e]SGs in the two
color-color diagrams.

Figure 7: As Figure 6 but for the location of the M31 (light blue) and M33
(purple) samples in the near-IR (top panel) and the WISE diagram (bottom
panel) based on their observed colors. Typical values for the objects’
reddening are $A_{\rm V}\leq 1.5$ mag (Humphreys et al. (2017)).

##### (J004621.08+421308.2)

A status of a late A-type warm hypergiant has been assigned to this object
based on the lack of detected He i emission (Gordon et al. (2016)). In
contrast to this, He i emission lines were reported by Sholukhova et al.
(2017), and an effective temperature of 10 000 K has been estimated from the
modeling of the stars’ spectral energy distribution (Humphreys et al. (2017)).
The position of the object in the near-IR and WISE diagrams coincides with the
B[e]SG domain.

##### (J004043.10+410846.0)

Its optical spectrum displays all typical characteristics of a B[e]SG
(Humphreys et al. (2017)), and its WISE colors confirm the presence of warm
dust, but its (though uncertain) unusually high $J-H$ value of 1.6 at a low
value of $H-K$ fits neither to B[e]SGs nor to LBVs.

##### (J004057.03+405238.6)

No IR photometry is available, so that currently no statement can be made
about the presence of (hot) dust. The absence of He i lines in the optical
spectra along with an effective temperature of 7700 K (Humphreys et al.
(2017)) speak against a B[e]SG classification.

####  4.2.2 M33

Also in M33, the sample of putative LBV candidates (Massey et al. (2007b,
2016)) served as a starting point for a more detailed investigation of the
individual objects. Optical (Fabrika et al. (2005); Clark et al. (2012);
Humphreys et al. (2017)), and infrared (Kourniotis et al. (2018))
spectroscopic studies revealed so far a total of ten possible B[e]SGs. They
are listed in Table LABEL:tab:M33. Combined with the available information
about their infrared colors, only two objects can currently be considered as
confirmed B[e]SGs, six need further clarification, and two objects appear to
be misclassified.

##### J013333.22+303343.4, J013350.12+304126.6

Both objects display all classification characteristics of B[e]SGs. Their
locations in the WISE diagram coincide with the B[e]SG domain. In the near-IR
diagram they appear slightly off the B[e]SG region, shifted towards the HAeBe
domain, but their high-luminosities clearly classify them as supergiants. The
CO band absorption detected from J013333.22+303343.4 cannot be explained with
a cool companion, but might originate from a pole-on seen jet (Kourniotis et
al. (2018)). Whether also CO band emission is present in this object, but
veiled by the intense absorption component, is currently unknown.

##### (J013324.62+302328.4), (J013342.78+303256.3), (J013349.28+305250.2),
(J013426.11+303424.7), (J013459.47+303701.9), (J013500.30+304150.9)

All six objects lack near-IR photometry, which renders it difficult to
unambiguously classify them as B[e]SGs, although their mid-IR colors and their
optical spectroscopic appearances support such a classification. The star
J013342.78+303256.3 has even no mid-IR photometry, so that it is unclear
whether it is surrounded by (warm) dust at all. There is an IR source at a
distance of $\sim$ 4.5 arcsec which is considered as too far off to be
identified with J013342.78+303256.3. The sources J013324.62+302328.4 and
J013500.30+304150.9 were reported as intrinsically faint objects with little
extinction, and were suggested as counterparts of the low-luminosity B[e]SG in
the MCs (Clark et al. (2012)). But according to the luminosity estimates for
these stars, for which values of $\log L/L_{\odot}\simeq 4.8-5.0$ were
obtained (Humphreys et al. (2017)), these stars fall rather onto the lower
boundary of the high-luminosity B[e]SGs sample within the MCs (see Figure 1).

##### (J013242.26+302114.1)

The optical spectra of this object display all typical B[e]SG characteristics,
but the star’s IR colors displace it clearly from the sample of confirmed
B[e]SGs. It has excess emission at 8 $\mu$m due to PAHs (Humphreys et al.
(2017)). The $JHK$ magnitudes suggest that J013242.26+302114.1 falls into the
region of late-type stars, respectively stars with a late-type companion
(Stahl et al. (1985); Gummersbach et al. (1995)). This classification is
supported by the WISE colors, although regular late-type stars typically have
slightly negative $W1-W2$ colors. Considering only the WISE photometry,
J013242.26+302114.1 would fit to the region populated by LBVs. This objects
clearly needs to be investigated in more detail for a proper classification.

##### (J013406.63+304147.8)

This star is also known as [HS80] B416. Despite the detected CO band emission
from this object (Kourniotis et al. (2018)), a classification as B[e]SG seems
not appropriate (as previously mentioned by Humphreys et al. (2014, 2017)),
because its near-IR colors show clearly the lack of warm dust and would place
the object to the region populated by LBVs. Also no [Oi] emission is seen from
J013406.63+304147.8 (Humphreys et al. (2014)). But so far no photometric
variability, typical for LBVs, is recorded (Clark et al. (2012)), and the star
can only be considered as a possible LBV candidate, as was previously
suggested (Humphreys et al. (2017)). The star is found to be surrounded by an
expanding ring-like nebula (Fabrika et al. (2005)) and shows excess emission
at 8 $\mu$m due to PAHs (Kourniotis et al. (2018)). The spectral energy
distribution leaves space for a companion (Humphreys et al. (2014); Kourniotis
et al. (2018)), and its spectral lines display radial velocity variations with
a period of 16.13 d, leading to the suggestion that the system might be an
interacting binary system causing mass loss in the equatorial plane
(Sholukhova et al. (2004)).

Table 7: Confirmed and candidate B[e]SGs in M33. Misclassified objects are listed at the bottom of the table. LGGS | Ref. | J-H | H-K | W1-W2 | W2-W4
---|---|---|---|---|---
Confirmed B[e]SGs
J013333.22+303343.4a | Clark et al. (2012); Humphreys et al. (2017); Kourniotis et al. (2018) | 1.144$\pm$0.234 | 1.128$\pm$0.154 | 0.900$\pm$0.063 | 5.269$\pm$0.098
J013350.12+304126.6 | Clark et al. (2012); Humphreys et al. (2017) | 0.653$\pm$0.089 | 0.683$\pm$0.091 | 0.764$\pm$0.041 | 4.132$\pm$0.186
Uncertain or controversial classifications
(J013324.62+302328.4) | Clark et al. (2012); Humphreys et al. (2017) | … | … | 1.050$\pm$0.047 | 4.235$\pm$0.414
(J013342.78+303256.3) | Humphreys et al. (2017) | … | … | … | …
(J013349.28+305250.2) | Humphreys et al. (2017) | … | … | 0.746$\pm$0.054 | 5.394$\pm$0.040
(J013426.11+303424.7) | Clark et al. (2012); Humphreys et al. (2017) | … | … | 1.208$\pm$0.066 | 4.845$\pm$0.278
(J013459.47+303701.9) | Clark et al. (2012); Humphreys et al. (2017) | … | … | 0.898$\pm$0.053 | 4.996$\pm$0.038
(J013500.30+304150.9) | Clark et al. (2012); Humphreys et al. (2017) | … | … | 0.939$\pm$0.040 | 4.142$\pm$0.029
Erroneous classifications
(J013242.26+302114.1) | Clark et al. (2012); Humphreys et al. (2017) | 0.861$\pm$0.059 | 0.376$\pm$0.064 | 0.050$\pm$0.040 | 4.543$\pm$0.031
(J013406.63+304147.8)b | Fabrika et al. (2005); Clark et al. (2012); Kourniotis et al. (2018) | 0.217$\pm$0.115 | 0.249$\pm$0.149 | … | …
Note: IR photometry is taken from the 2MASS point source catalog ($J,H,K$,
Cutri et al. (2003)) and from the WISE All-Sky Data Release ($W1,W2,W4$,Cutri,
& et al. (2012)). Colors resulting from uncertain photometric values are
written in italic.
a Has CO band absorption (Kourniotis et al. (2018)).
b Has CO band emission (Kourniotis et al. (2018)).

###  4.3 Byond the Local Group

Not much is known yet about the B[e]SG population beyond the Local Group. Most
promising for dedicated searches are the nearby large spiral galaxies M81,
M101, and NGC 2403, for which pioneering ground-based surveys have been
conducted already in the eighties and nineties of the last century (Sandage
(1983, 1984a, 1984b); Zickgraf, & Humphreys (1991)) and which are nowadays
extended to fainter objects thanks to the capabilities of the Hubble Space
Telescope (e.g., Grammer, & Humphreys (2013)). Spectroscopic follow-up
investigations revealed a number of variable luminous objects (Humphreys
(1980); Humphreys, & Aaronson (1987); Sholukhova et al. (1998a, b); Grammer et
al. (2015)), but only in one of these galaxies, M81, three B[e]SG candidates
have been found so far (Humphreys et al. (2019)). They all display the typical
B[e] features in their optical spectra, but without complementary information
about the presence of (warm) circumstellar dust, their classification remains
preliminary. These objects will certainly not remain the only of their kind,
because the search for more candidates has just begun.

The B[e]SG candidates in M81 are listed in Table LABEL:tab:M81. For lack of
proper SIMBAD identifiers for these objects, the table contains the star ID
(Humphreys et al. (2019)) along with the coordinates and, where available, the
$V$ band magnitudes.

Table 8: Candidate B[e]SGs in M81 Star ID | RA(J2000) | Dec(J2000) | V | Ref.
---|---|---|---|---
(10584-8-4) | 9:54:50.03 | +69:06:55.47 | … | Humphreys et al. (2019)
(10584-4-1) | 9:54:54.05 | +69:10:23.00 | 19.68 | Humphreys et al. (2019)
(10584-9-1) | 9:55:18.97 | +69:08:27.54 | 19.10 | Humphreys et al. (2019)

In the past few years, additional surveys have been performed, using large
ground-based facilities (e.g., Pustilnik et al. (2017)) and space telescopes
(e.g., Spetsieri et al. (2018, 2019)). These surveys were aimed at revealing
the luminous and variable massive star populations of other galaxies even
further away. They provide an excellent basis for follow-up spectroscopic
studies to classify their massive star content, so that many more LBV
candidates and B[e]SGs may be found in the (near) future. Also the Local Group
galaxies still need to be explored in more detail. In particular those
galaxies, that were already found to possess (even though in very small
numbers) LBV candidates (Massey et al. (2007b)) that are awaiting their proper
classifications. The upcoming era of the Extreme Large Telescope (ELT)
promises to become particularly fruitful. The next generation of high-
sensitivity instruments combined with the large collecting area of the
telescope will facilitate ground-based spectroscopic observations of faint
objects with very high spatial resolution.

###  4.4 Galactic objects

Finally, we return to our own Galaxy. Searching for B[e]SG stars in the Milky
Way is a difficult task. Though many B[e] stars are known, the assignment of a
supergiant status is significantly hampered due to highly uncertain distances
hence luminosities. The situation will hopefully change with the final data
release from the GAIA mission, from which one hopes for accurate parallax
measurements. But for now, the luminosities of the objects are subject to
large uncertainties, so that only objects with a reported luminosity of at
least $\log L/L_{\odot}\geq 5$ are considered as serious B[e]SG candidates.

The lower luminosity boundary of $\log L/L_{\odot}\sim 4.0$ for an evolved
star to be assigned a supergiant status is a further hindrance in the
classification of objects as B[e]SGs, because this luminosity domain is shared
with the massive pre-main sequence (HAeBe) stars. The latter have emission-
line spectra with numerous forbidden emission lines from [Fe ii] and [O i]
alike the B[e]SGs, and the stars are surrounded by significant amounts of
circumstellar dust within their massive accretion disks causing considerable
IR excess emission, just as the B[e]SGs. Hence, it is not surprising that
confusion exists about the proper classification for a number of objects
within this luminosity domain of $4.0<\log L/L_{\odot}<4.5$, and that Galactic
B[e]SG candidates also appear as candidates in catalogs of HAeBe stars (see,
e.g., Ababakr et al. (2017)). In the absence of clear indications for infall
of material, which is a typical characteristic of pre-main sequence stars,
alternative discriminators for the classification of such objects are needed.

A reasonable approach to this is to search for 13CO emission from the
circumstellar environments of the uncertain candidates. Many HAeBe stars have
been reported to display CO band emission from their massive accretion disks
(e.g., Bik & Thi (2004); Blum et al. (2004); Ilee et al. (2013, 2014, 2018)).
As these disks form from material provided by the interstellar medium in which
the 12C/13C isotope abundance ratio has typically a value of about 90 (Ekström
et al. (2012)), these pre-main sequence disks can clearly be distinguished
from the disks around evolved massive stars, which should be enriched in 13C
and hence give rise to clearly measurable emission in 13CO. $K$-band spectra
of these objects, covering the first-overtone bands of both 12CO and 13CO (see
Section 2.3 and Figure 3), are thus key for a proper discrimination between a
young (pre-main sequence) and an evolved status.

From the currently proposed 15 Galactic B[e]SGs listed in Table LABEL:tab:CO-
O-Ca, nine have been reported to display CO band emission. But so far, only
four of them have been observed in the region around the 13CO bands. The
spectra of all four stars have been found to display clear signatures of 13CO
emission, and model results revealed that the environments of all four objects
are clearly enriched in 13C. Two of these objects were already known to be
supergiants based on their confirmed high luminosities. These are the stars GG
Car (Kraus et al. (2013); Oksala et al. (2013)) and Hen 2-398 (Oksala et al.
(2013)). For the other two, which so far have also been considered as HAeBe
candidates (see Table LABEL:tab:Galactic), the detection of chemically
processed material can hence be regarded as the ultimate proof of their
evolved, supergiant nature. These are the objects MWC 137 (Muratore et al.
(2015)) and MWC 349 (Kraus et al. (2019)). These results are very promising
and encouraging, and they demonstrate that the 13CO molecular emission
provides a solid tool to unambiguously classify a star as either a pre-main
sequence or an evolved object. Clearly more observational effort needs to be
undertaken to search also for the signatures of 13CO in the spectra of the
remaining objects.

When collecting the IR magnitudes of the Galactic sample, it turned out that
only the near-IR measurements are reliable, whereas the WISE measurements for
all objects have been flagged as being contaminated by neighboring objects.
The latter are hence useless for classification purposes, and one can
currently only rely on the $JHK$-band magnitudes. The list of objects, their
observed colors, and literature values of their color excess are listed in
Table LABEL:tab:Galactic. The relatively high values of the color excess
requires correction for extinction before placing the objects to the near-IR
diagram. Corrections have been performed with the galactic extinction curve
using an $R_{\rm V}$ value of 3.1 has been utilized (Cardelli et al. (1989)).
The extinction corrected colors are included in Table LABEL:tab:Galactic, and
the positions of the objects are shown in Fig. 8 separately for the confirmed
(left panel) and candidate objects (right panel).

Table 9: Confirmed and candidate B[e]SGs in the Milky Way. Object | Ref. | E(B-V) | Ref. | (J-H) | (H-K) | (J-H)0 | (H-K)0
---|---|---|---|---|---|---|---
Confirmed B[e]SGs
MWC 137a | Esteban, & Fernandez (1998); Muratore et al. (2015) | 1.22 | Esteban, & Fernandez (1998) | 0.922$\pm$0.040 | 1.217$\pm$0.035 | 0.572$\pm$0.040 | 0.930$\pm$0.035
MWC 349a | Gvaramadze, & Menten (2012); Kraus et al. (2019) |  $\sim$ 3.2 | Cohen et al. (1985) | 1.472$\pm$0.038 | 1.603$\pm$0.369 | 0.554$\pm$0.038 | 0.851$\pm$0.369
GG Carb | Marchiano et al. (2012); Kraus et al. (2013); Oksala et al. (2013) | 0.51 | Marchiano et al. (2012) | 0.818$\pm$0.049 | 0.964$\pm$0.049 | 0.672$\pm$0.049 | 0.844$\pm$0.049
Hen 3-298 | Miroshnichenko et al. (2005); Oksala et al. (2013) | 1.7 | Miroshnichenko et al. (2005) | 1.009$\pm$0.065 | 1.139$\pm$0.060 | 0.522$\pm$0.065 | 0.739$\pm$0.060
CPD-52 9243 | Cidale et al. (2012) | 1.7 | McGregor et al. (1988) | 0.919$\pm$0.029 | 0.948$\pm$0.026 | 0.432$\pm$0.029 | 0.548$\pm$0.026
HD 327083b | Miroshnichenko et al. (2003) | 1.8 | Miroshnichenko et al. (2003) | 1.000$\pm$0.042 | 1.309$\pm$0.188 | 0.484$\pm$0.042 | 0.886$\pm$0.188
MWC 300a,b | Wolf, & Stahl (1985); Miroshnichenko et al. (2004) | 1.2 | Miroshnichenko et al. (2004) | 1.150$\pm$0.056 | 1.951$\pm$0.055 | 0.806$\pm$0.056 | 1.669$\pm$0.055
AS 381b | Miroshnichenko et al. (2002) | 2.3 | Miroshnichenko et al. (2002) | 1.285$\pm$0.029 | 1.117$\pm$0.028 | 0.625$\pm$0.029 | 0.576$\pm$0.028
CPD-57 2874 | Domiciano de Souza et al. (2007) | 1.75 | Domiciano de Souza et al. (2007) | 0.803$\pm$0.062 | 0.678$\pm$0.304 | 0.301$\pm$0.062 | 0.267$\pm$0.304
Uncertain or controversial classifications
(Hen 3-938) | Condori et al. (2019) | 1.64 | Condori et al. (2019) | 1.593$\pm$0.035 | 1.487$\pm$0.034 | 1.124$\pm$0.035 | 1.101$\pm$0.034
(MWC 342)a,b | Miroshnichenko, & Corporon (1999) | 1.5 | Miroshnichenko, & Corporon (1999) | 1.179$\pm$0.027 | 1.121$\pm$0.025 | 0.749$\pm$0.027 | 0.768$\pm$0.025
(Hen 3-303) | Miroshnichenko et al. (2005) | 1.7 | Miroshnichenko et al. (2005) | 1.366$\pm$0.063 | 1.443$\pm$0.063 | 0.879$\pm$0.063 | 1.042$\pm$0.063
(CD-42 11721)a | Borges Fernandes et al. (2007) | 1.4-1.6 | Cidale et al. (2012); Borges Fernandes et al. (2007) | 1.317$\pm$0.053 | 1.398$\pm$0.050 | 0.901$\pm$0.053 | 1.057$\pm$0.050
(HD 87643)a,b | McGregor et al. (1988); Millour et al. (2009) | 1.0 | McGregor et al. (1988) | 1.461$\pm$0.271 | 1.300$\pm$0.338 | 1.174$\pm$0.271 | 1.065$\pm$0.338
(HD 62623)b | Chentsov et al. (2010) | 0.17 | Plets et al. (1995) | 0.395$\pm$0.380 | 0.693$\pm$0.368 | 0.346$\pm$0.380 | 0.653$\pm$0.368
Note: IR photometry is taken from the 2MASS point source catalog ($J,H,K$,
Cutri et al. (2003)).
a Star appears also in HAeBe catalogs (see, e.g., Ababakr et al. (2017)).
b Confirmed or suspected binary.

The separation of confirmed from candidate B[e]SGs is based on two
characteristics: (i) Stars with detected enrichment in 13CO of their
circumstellar environments are considered as confirmed, as well as (ii) stars
with reported (by more than one research team) luminosity values of $\log
L/L_{\odot}\geq 5.0$. Objects with lower luminosities $4.0<\log
L/L_{\odot}<5.0$ are assigned a candidate status. Based on these criteria, the
Galactic sample splits into nine confirmed B[e]SGs and six candidates (see
Table LABEL:tab:Galactic).

##### MWC 137, MWC 349, GG Car, Hen 3-298

These four objects are considered as confirmed B[e]SGs based on the detected
enrichment of their circumstellar disk material with 13CO (see Table
LABEL:tab:CO-O-Ca). They all fall into the region of the confirmed B[e]SGs in
the near-IR diagram, regardless of the large errorbar for MWC 349.

##### CPD-52 9243, HD 327083, MWC 300

All three stars fulfill the high luminosity criterion. The near-IR colors of
CPD-52 9243 and HD 327083 place these two objects within the B[e]SG domain.
MWC 300 appears regularly in studies of HAeBe stars. Its near-IR colors locate
this star close to the B[e]SGs but far away from the HAeBe region, making a
pre-main sequence nature of this object rather unlikely. Its relatively high
color values might be influenced by a possible companion (Miroshnichenko et
al. (2004); Wang et al. (2012))

Figure 8: Near-IR diagrams as in Figure 6, showing the locations of the
Galactic confirmed B[e]SGs (top panel) and B[e]SG candidates (bottom panel).
The colors of the Galactic objects have been corrected for interstellar
extinction (Table LABEL:tab:Galactic).

##### AS 381, CPD-57 2874

These two objects are also known to have high luminosities, but they reside
slightly outside the classical B[e]SG domain in the near-IR diagram. AS 381 is
a reported binary (Miroshnichenko et al. (2002); Liermann et al. (2014))
consisting of a luminous B[e]SG and a K-type companion, which seems to
(significantly) contribute to the total near-IR flux, hence altering the
colors of the B[e]SG. The near-IR colors of CPD-57 2874 place it closer to the
LBVs rather than to the B[e]SGs, although its $H-K$ color is subject to large
uncertainty. Its pronounced emission in CO, and in the [O i] and [Ca ii]
forbidden lines (Maravelias et al. (2018)) speak against an LBV
classification.

##### (Hen 3-938), (MWC 342), (Hen 3-303), (CD-42 11721)

The rather low luminosities of these four objects and the closeness or even
coincidence of their location with the HAeBe domain requires clearly further
studies for an unambiguous classification. While Hen 3-938 and MWC 342 have to
our knowledge not yet been spectroscopically observed in the $K$-band, the
other two stars showed no evidence for CO band emission (Muratore et al.
(2012)). This renders their classification more difficult.

##### (HD 87643)

The near-IR colors of HD 87643 place it to the high end of the HAeBe regime.
However, these colors have large errors and require refinement. Studies based
on long-baseline interferometry revealed that this object consists of two
B-type stars, and each component might be surrounded by a dusty disk (Millour
et al. (2009)). Whether this object is a physical binary, is currently not
known.

##### (HD 62623)

It appeared in the literature as the first A[e]SG (Meilland et al. (2010);
Millour et al. (2011)) and is known to be surrounded by a detached gas and
dust disk, from which also CO band emission has been detected (Muratore et al.
(2012); Maravelias et al. (2018)). It has been speculated that the gap between
the star and the inner rim of the gas disk might have been cleared by a
companion. But no clear evidence of such a companion has been found yet. Its
low luminosity and large errors in the near-IR photometric measurements make
it challenging to unambiguously assign the star a B[e]SG status, although its
position within the near-IR seems to coincide more with the B[e]SG domain
rather than with any other classification. This object is an ideal candidate
to clarify its nature based on dedicated $K$-band observations to analysis the
13CO content within its circumstellar disk.

##  5 Discussion and Conclusions

B[e]SGs form a special class of evolved massive stars and are thought to
represent a short-lived transition phase either in their red-ward, post-main
sequence evolution, or in their blue-ward post-RSG evolution. The total number
of currently known B[e]SGs is low, supporting the idea of a short transition
phase. But in which direction the stars evolve, and whether all B[e]SGs evolve
into the same direction, is still an open issue. For the objects in M31 and
M33 it has been argued that the B[e]SGs are more isolated than LBVs and hardly
found in stellar associations, so that a post-RSG (or post-yellow supergiant)
evolution was proposed to be more likely (Humphreys et al. (2017)). This
assessment was based on the available numbers of putative B[e]SGs in these two
galaxies at that time. But after revision of the two samples (Tables
LABEL:tab:M31 and LABEL:tab:M33), two of the four confirmed B[e]SGs in M31 and
both confirmed B[e]SGs in M33 are associated with stellar groups, questioning
the conclusion that B[e]SGs are isolated and thus post-RSGs.

The best age indicator for B[e]SGs we have to date is their surface abundance
enrichment in 13C, as discussed in Section 2.3. If B[e]SGs would be post-RSGs,
their progenitors would all have started with a (very) low rotation speed.
While such a scenario cannot be excluded, it may not be very likely
considering that stars are born on average with a rotation rate of about 40%
of their critical velocity (Ekström et al. (2012)).

Interaction within a (close) binary system, maybe even up to a binary merger,
seems to be an alternative and popular scenario (e.g., Wang et al. (2012); de
Wit et al. (2014)), but the number of currently confirmed binaries amongst
B[e]SGs is still rather low to give preference to the binary channel as the
sole possible way for the formation of B[e]SGs. Likewise, some B[e]SGs have
been suggested as suitable supernova candidates. For example, the Galactic
object MWC 137 appears similar to Sher 25 (Hendry et al. (2008)) and SBW1
(Smith et al. (2013)), which both look like the progenitor of SN1987A. Also
the SMC object LHA 115-S 18 has been proposed to be a viable SN1987A
progenitor (Clark et al. (2013b)). In this respect, it is vital to resolve
B[e]SG populations and to study their properties.

In this review, a census of the currently known B[e]SG population in the Milky
Way and in nearby star-forming galaxies within and beyond the Local Group is
presented. The proposed candidates have been undertaken a critical
examination, sorted into confirmed B[e]SGs and B[e]SG candidates, and
unsuitable objects have been flagged as "misclassified"101010We would like to
caution that with insufficient knowledge of stellar properties, individual
objects may easily be misclassified, as it happened in recently published
catalogs (Smith, & Tombleson (2015); Aadland et al. (2018); Massey et al.
(2016); Richardson, & Mehner (2018)), in which erroneously a number of (even
confirmed) B[e]SGs are listed as LBV candidates..

During these investigations, a fundamental difference has been recognized
between the identification issues for objects in our Galaxy compared to those
in other galaxies. Extragalactic B[e]SGs bare the risk of being confused with
LBVs in quiescence, which share very similar optical spectroscopic
characteristics. To separate these two classes of objects, one can make use of
clearly defined classification criteria based on certain sets of emission
features identified in their optical and near-IR spectra as was outlined in
Section 3. In addition to these, inspection of the location of possible B[e]SG
candidates in the IR color-color diagrams is highly advisable, because B[e]SGs
and LBVs populate clearly separate domains. This fact might also be used as
starting point for future investigations of extragalactic samples. However,
for such future studies, infrared photometry with higher spatial resolution
than what is currently provided by 2MASS and WISE is desirable to prevent from
contamination with neighboring sources in densely populated regions. Moreover,
precise distances, as soon provided by GAIA, will help to separate foreground
stars in the directions to other galaxies that might have been misclassified
as luminous extragalactic stars.

In the Milky Way, an additional complication occurs due to the often uncertain
luminosity estimates, and the overlap of low-luminosity B[e]SGs with the most
luminous massive pre-main sequence objects (HAeBe). Here, special care needs
to be taken, especially since both classes of objects occupy adjacent regions
in the near-IR color-color diagram with a probable overlap. Without additional
distinctive features for such low-luminosity, borderline B[e]SG candidates,
their real nature remains elusive. One complementary classification criteria
that was discussed, is provided by the enrichment of the circumstellar
material of evolved objects with processed material that has been released
from the stellar surface, as opposed to the non-processed material with
interstellar abundance patterns found around HAeBes. The most ideal element to
search for is 13C, which is bound in 13CO molecules in the circumstellar
disks. Measuring the 13CO amount with respect to 12CO provides immediate
insight into the nature of the object. The current HAeBe samples might hide
such low-luminosity B[e]SG candidates, which can only be identified as such by
careful and honest analysis.

The result from this census, after strict application of the classification
criteria, is that we count nine confirmed and six candidate B[e]SGs in the
Galaxy. Moving out to the MCs, the numbers amount to 13 (+2) in the LMC and 5
(+1) in the SMC. The situation in other members of the Local Group is not much
better. There the numbers drop to 4 (+7) in M31 and 2 (+6) in M33. Even
further away, only 3 candidates have been reported from M81. The total number
of B[e]SGs found in the various galaxies are too small for statistical
analyses with respect to a metallicity dependence of their number, but the
comparable quantities within the Milky Way and the LMC suggest only a mild
dependence of the amount of B[e]SGs on (i.e. a possible drop with decreasing)
metallicity.

Further, we report that the Galactic sample of 15 B[e]SGs and candidates
contains currently seven confirmed or suspected binaries (see Table
LABEL:tab:Galactic), which is less than half of the population. In the MCs
this number is even lower, because so far only three stars have been reported
to be possible binaries, of which one, the SMC star LHA 115-S 6 (= RMC 4) has
been proposed to be the remnant of a binary merger within an initially triple
system (Langer, & Heger (1998); Pasquali et al. (2000)). The other two
objects, the LMC star LHA 120-S 134 and the SMC object LHA 115-S 18, have been
identified as optical counterparts of X-ray sources (Clark et al. (2013b);
Bartlett, & Clark (2015)). Nothing is known about possible binarity in the
B[e]SG samples from the other galaxies, although photometric variability was
seen in at least two M31 objects: the confirmed B[e]SG star
J004417.10+411928.0 (Sholukhova et al. (2015); Martin, & Humphreys (2017)) and
the candidate J004444.52+412804.0 (Stanek et al. (1999); Vilardell et al.
(2006)). Whether this variability is a sign of binarity or just of semi-
regular variability which might be interpreted with pulsation activity such as
reported from the $\alpha$ Cygni variables, needs to be studied in more
detail.

For the sake of completeness, a special class of objects, which have not been
discussed yet, should be briefly mentioned as well. These are the high-mass
X-ray binaries (HMXB) with possible B[e] supergiant (candidate) companion.
Some objects with an assigned B[e]SG status have been found to be too luminous
in X-rays for being considered single stars. These objects have been proposed
to be binary systems, in which the high energy emission is caused by either
accretion onto a compact object, or by shocks in a colliding wind binary with
a second massive star. Members of this group are the Galactic objects Cl*
Westerlund 1 W 9 (= Wd1-9), which is considered a colliding wind system (Clark
et al. (2013)), CI Cam (= MWC 84), which might be interpreted as supernova
imposter (Bartlett et al. (2019)), and the high-mass X-ray binary (HMXB) IGR
J16318-4848 in which the compact object was proposed to be a neutron star
(Filliatre, & Chaty (2004)). Two ultra-luminous X-ray sources (ULXs), Holmberg
II X-1 and NGC 300 ULX1, the latter being also named as supernova imposter
SN2010da, have also been proposed to be HMXBs including a B[e]SG (Lau et al.
(2016, 2017); Villar et al. (2016)). Whether these objects host indeed a
B[e]SG clearly needs to be investigated in more detail. But since the behavior
of these sources is considerably different from the confirmed B[e]SGs, I
hesitate to include them into the census.

The presented populations represent the current knowledge of B[e]SGs and
B[e]SG candidates in the closest star-forming galaxies. With the ever growing
sensitivities of instruments and telescope sizes, the future in B[e]SG star
research is bright, because many more candidates will be identified in even
more distant galaxies and with metallicities spreading over a large range.
With statistically meaningful samples it will finally be possible to unveil
the nature and fate of these fascinating objects.

This project has received funding from the Grant Agency of the Czech Republic
(GAČR, grant number 17-02337S). The Astronomical Institute of the Czech
Academy of Sciences, Ondřejov, is supported by the project RVO:67985815.

###### Acknowledgements.

I wish to thank the editor, Roberta Humphreys, for the invitation to write
this review. In addition, I am grateful to Michalis Kourniotis for inspiring
discussions on searching for extragalactic B[e]SGs, and to Dieter Nickeler and
Lydia Cidale for passionate discussions about B[e]SGs as well as for their
proof-reading of and suggestions on the draft versions. Moreover, I thank the
anonymous referees for their careful reading and suggestions on the draft
version. This research made use of the NASA Astrophysics Data System (ADS) and
of the SIMBAD database, operated at CDS, Strasbourg, France. This publication
makes use of data products from the 2MASS, which is a joint project of the
University of Massachusetts and the Infrared Processing and Analysis
Center/California Institute of Technology, funded by the National Aeronautics
and Space Administration and the National Science Foundation. This publication
makes use of data products from the Wide-field Infrared Survey Explorer, which
is a joint project of the University of California, Los Angeles, and the Jet
Propulsion Laboratory/California Institute of Technology, funded by the
National Aeronautics and Space Administration. The author declares no conflict
of interest. References

## References

  * Groh et al. (2013) Groh, J.H.; Georgy, C.; Ekström, S. Progenitors of supernova Ibc: a single Wolf-Rayet star as the possible progenitor of the SN Ib iPTF13bvn. Astron. Astrophys. 2013, 558, L1. https://doi.org/10.1051/0004-6361/201322369
  * Groh et al. (2013) Groh, J.H.; Meynet, G.; Georgy, C.; Ekström, S. Fundamental properties of core-collapse supernova and GRB progenitors: predicting the look of massive stars before death. Astron. Astrophys. 2013, 558, A131. https://doi.org/10.1051/0004-6361/201321906
  * Groh et al. (2013) Groh, J.H.; Meynet, G.; Ekström, S. Massive star evolution: luminous blue variables as unexpected supernova progenitors. Astron. Astrophys. 2013, 550, L7. https://doi.org/10.1051/0004-6361/201220741
  * Pauldrach et al. (2012) Pauldrach, A.W.A.; Vanbeveren, D.; Hoffmann, T.L. Radiation-driven winds of hot luminous stars XVI. Expanding atmospheres of massive and very massive stars and the evolution of dense stellar clusters. Astron. Astrophys. 2012, 538, A75. https://doi.org/10.1051/0004-6361/201117621
  * Hopkins et al. (2014) Hopkins, P.F.; Kereš, D.; Oñorbe, J.; Faucher-Giguère, C.-A.; Quataert, E.; Murray, N.; Bullock, J.S. Galaxies on FIRE (Feedback In Realistic Environments): stellar feedback explains cosmologically inefficient star formation. Mon. Not. R. Astrono. Soc. 2014, 445, 581-603. https://doi.org/10.1093/mnras/stu1738
  * Massey et al. (2016) Massey, P.; Neugent, K.F.; Smart, B.M. A Spectroscopic Survey of Massive Stars in M31 and M33. Astron. J. 2016, 152, 62. https://doi.org/10.3847/0004-6256/152/3/62
  * Zickgraf (2006) Zickgraf, F.-J. B[e] Supergiants in the Magellanic Clouds. In Stars with the B[e] Phenomenon. 2006, ASP Conference Series, 355, San Francisco: Astronomical Society of the Pacific, 135.
  * Georgy et al. (2013) Georgy, C.; Ekström, S.; Eggenberger, P.; Meynet, G.; Haemmerlé, L.; Maeder, A.; Granada, A.; Groh, J.H.; Hirschi, R.; Mowlavi, N.; Yusof, N.; Charbonnel, C.; Decressin, T.; Barblan, F. Grids of stellar models with rotation. III. Models from 0.8 to 120 M⊙ at a metallicity Z = 0.002. Astron. Astrophys. 2013, 558, A103. https://doi.org/10.1051/0004-6361/201322178
  * Cannon (1924) Cannon, A.J. Peculiar Spectra in the Large Magellanic Cloud. Harvard College Observatory Bulletin 1924, 801, 1-3.
  * Henize (1956) Henize, K.G. Catalogues of H$\alpha$-emission Stars and Nebulae in the Magellanic Clouds. Astrophys. J. Suppl. 1956, 2, 315. https://doi.org/10.1086/190025
  * Smith (1957) Smith, H.J. Spectra of Bright-Line Stars in the Large Magellanic Cloud. Publ. ASP 1957, 69, 137. https://doi.org/10.1086/127032
  * Feast et al. (1960) Feast, M.W.; Thackeray, A.D.; Wesselink, A.J. The brightest stars in the Magellanic Clouds. Mon. Not. R. Astrono. Soc. 1960, 121, 337. https://doi.org/10.1093/mnras/121.4.337
  * Allen, & Glass (1976) Allen, D.A.; Glass, I.S. Emission-line stars in the Large Magellanic Cloud: spectroscopy and infrared photometry. Astrophys. J. 1976, 210, 666-669. https://doi.org/10.1086/154872
  * Glass (1977) Glass, I.S. A dusty star in the Small Magellanic Cloud. Mon. Not. R. Astrono. Soc. 1977, 178, 9P. https://doi.org/10.1093/mnras/178.1.9P
  * Stahl et al. (1983) Stahl, O.; Wolf, B.; Zickgraf, F.J.; Bastian, U.; de Groot, M.H.J.; Leitherer, C. R66 (Aeq), an LMC B Supergiant with a Massive, Cool and Dusty Wind. Astron. Astrophys. 1983, 120, 287-296.
  * Stahl et al. (1984) Stahl, O.; Leitherer, C.; Wolf, B.; Zickgraf, F.-J. Three new hot stars with dust shells in the Magellanic clouds. Astron. Astrophys. 1984, 131, L5-L6.
  * Zickgraf et al. (1985) Zickgraf, F.-J.; Wolf, B.; Stahl, O.; Leitherer, C.; Klare, G. The hybrid spectrum of the LMC hypergiant R 126. Astron. Astrophys. 1985, 143, 421-430.
  * Zickgraf et al. (1986) Zickgraf, F.J.; Wolf, B.; Stahl, O.; Leitherer, C.; Appenzeller, I. B(e)-supergiants of the Magellanic Clouds. Astron. Astrophys. 1986, 163, 119-134.
  * Gummersbach et al. (1995) Gummersbach, C.A.; Zickgraf, F.-J.; Wolf, B. B[e] phenomenon extending to lower luminosities in the Magellanic Clouds. Astron. Astrophys. 1995, 302, 409.
  * McGregor et al. (1988) McGregor, P.J.; Hillier, D.J.; Hyland, A.R. CO Overtone Emission from Magellanic Cloud Supergiants. Astrophys. J. 1988, 334, 639\. https://doi.org/10.1086/166867
  * Zickgraf et al. (1996) Zickgraf, F.-J.; Kovacs, J.; Wolf, B.; Stahl, O.; Kaufer, A.; Appenzeller, I. R4 in the Small Magellanic Cloud: a spectroscopic binary with a B[e]/LBV-type component. Astron. Astrophys. 1996, 309, 505-514.
  * Zickgraf et al. (1989) Zickgraf, F.-J.; Wolf, B.; Stahl, O.; Humphreys, R.M. S 18 : a new B(e) supergiant in the Small Magellanic Cloud with evidence for an excretion disk. Astron. Astrophys. 1989, 220, 206-214.
  * Zickgraf et al. (1992) Zickgraf, F.-J.; Stahl, O.; Wolf, B. IR survey of OB emission-line stars in the SMC : detection of a new B E supergiant, AV 172. Astron. Astrophys. 1992, 260, 205-212.
  * Lamers et al. (1998) Lamers, H.J.G.L.M.; Zickgraf, F.-J.; de Winter, D.; Houziaux, L.; Zorec, J. An improved classification of B[e]-type stars. Astron. Astrophys. 1998, 340, 117–128.
  * Appenzeller et al. (1984) Appenzeller, I.; Klare, G.; Stahl, O.; Wolf, B.; Zickgraf, F.-J. CASPEC / Cassegrain Echelle Spectrograph / and IUE - a Perfect Match. The Messenger 1984, 38, 28.
  * Clark et al. (2013b) Clark, J.S.; Bartlett, E.S.; Coe, M.J.; Dorda, R.; Haberl, F.; Lamb, J.B.; Negueruela, I.; Udalski, A. The supergiant B[e] star LHA 115-S 18 - binary and/or luminous blue variable? Astron. Astrophys. 2013, 560, A10. https://doi.org/10.1051/0004-6361/201321216
  * Massey et al. (2014) Massey, P.; Neugent, K.F.; Morrell, N.; Hillier, D.J. A Modern Search for Wolf-Rayet Stars in the Magellanic Clouds: First Results. Astrophys. J. 2014, 788, 83. https://doi.org/10.1088/0004-637X/788/1/83
  * Bartlett, & Clark (2015) Bartlett, E.S.; Clark, J.S. Observational Constraints on the Nature of the X-ray bright supergiant B[e] stars LHA 115-S18 & LHA 120- S134. In Proceedings of the SALT Science Conference. 2015, Eds. G. E. Bromage, D. A. H. Buckley, K. N. Hainline, J. Mikołajewska, R. Srianand, E. Wilcots, T. J. Williams, Stellenbosch Institute of Advanced Study, South Africa. id.55
  * Gray (2005) Gray, D.F. The Observation and Analysis of Stellar Photospheres. 2005, UK: Cambridge University Press.
  * Magalhaes (1992) Magalhaes, A.M. Polarization and the Envelopes of B[e] Supergiants in the Magellanic Clouds. Astrophys. J. 1992, 398, 286. https://doi.org/10.1086/171856
  * Melgarejo et al. (2001) Melgarejo, R.; Magalhães, A.M.; Carciofi, A.C.; Rodrigues, C.V. S 111 and the polarization of the B[e] supergiants in the Magellanic Clouds. Astron. Astrophys. 2001, 377, 581-588. https://doi.org/10.1051/0004-6361:20011141
  * Magalhães et al. (2006) Magalhães, A.M.; Melgarejo, R.; Pereyra, A.; Carciofi, A.C. Polarimetry and the Envelopes of Magellanic B[e] Supergiants. In Stars with the B[e] Phenomenon. 2006, Eds. M. Kraus and A.S. Miroshnichenko. ASP Conference Series, 355. San Francisco: Astronomical Society of the Pacific, 147
  * Seriacopi et al. (2017) Seriacopi, D.B.; Carciofi, A.C.; Magalhães, A.M. The Envelopes of B[e] Supergiants in the Magellanic Clouds as Seen by Polarimetry. In The B[e] Phenomenon: Forty Years of Studies. 2017, Eds. A. Miroshnichenko, S. Zharikov, D. Korčáková and M. Wolf. ASP Conference Series, 508, San Francisco: Astronomical Society of the Pacific, 109.
  * Zickgraf, & Schulte-Ladbeck (1989) Zickgraf, F.-J.; Schulte-Ladbeck, R.E. Polarization characteristics of galactic Be stars. Astron. Astrophys. 1989, 214, 274-284.
  * McGregor et al. (1988) McGregor, P.J.; Hyland, A.R.; Hillier, D.J. Atomic and Molecular Line Emission from Early-Type High-Luminosity Stars. Astrophys. J. 1988, 324, 1071. https://doi.org/10.1086/165964
  * McGregor et al. (1989) McGregor, P.J.; Hyland, A.R.; McGinn, M.T. Emission-line stars in the Magellanic Clouds : infrared spectroscopy of Be and Ofpe/WN9 stars. Astron. Astrophys. 1989, 223, 237-240.
  * Morris et al. (1996) Morris, P.W.; Eenens, P.R.J.; Hanson, M.M.; Conti, P.S.; Blum, R.D. Infrared Spectra of Massive Stars in Transition: WNL, Of, Of/WN, Be, B[e], and Luminous Blue Variable Stars. Astrophys. J. 1996, 470, 597\. https://doi.org/10.1086/177892
  * Wheelwright et al. (2012) Wheelwright, H.E.; de Wit, W.J.; Weigelt, G.; Oudmaijer, R.D.; Ilee, J.D. AMBER and CRIRES observations of the binary sgB[e] star HD 327083: evidence of a gaseous disc traced by CO bandhead emission. Astron. Astrophys. 2012, 543, A77. https://doi.org/10.1051/0004-6361/201219325
  * Muratore et al. (2012) Muratore, M.F.; Kraus, M.; de Wit, W.J. Near-infrared spectroscopic survey of galactic B[e] stars. Boletín de la Asociación Argentina de Astronomía 2012, 55, 123-127.
  * Oksala et al. (2012) Oksala, M E.; Kraus, M.; Arias, M.L.; Borges Fernandes, M.; Cidale, L.; Muratore, M.F.; Curé, M. The sudden appearance of CO emission in LHA 115-S 65. Mon. Not. R. Astrono. Soc. 2012, 426, L56-L60. https://doi.org/10.1111/j.1745-3933.2012.01323.x
  * Oksala et al. (2013) Oksala, M.E.; Kraus, M.; Cidale, L.S.; Muratore, M.F.; Borges Fernandes, M. Probing the ejecta of evolved massive stars in transition. A VLT/SINFONI K-band survey. Astron. Astrophys. 2013, 558, A17. https://doi.org/10.1051/0004-6361/201321568
  * Kraus et al. (2013) Kraus, M.; Oksala, M.E.; Nickeler, D.H.; Muratore, M.F.; Borges Fernandes, M.; Aret, A.; Cidale, L.S.; de Wit, W.J. Molecular emission from GG Carinae’s circumbinary disk. Astron. Astrophys. 2013, 549, A28. https://doi.org/10.1051/0004-6361/201220442
  * Kraus et al. (2016) Kraus, M.; Cidale, L.S.; Arias, M.L.; Maravelias, G.; Nickeler, D.H.; Torres, A.F.; Borges Fernandes, M.; Aret, A.; Curé, M.; Vallverdú, R.; Barbá, R.H. Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73. Astron. Astrophys. 2016, 593, A112. https://doi.org/10.1051/0004-6361/201628493
  * Torres et al. (2018) Torres, A.F.; Cidale, L.S.; Kraus, M.; Arias, M.L.; Barbá, R.H.; Maravelias, G.; Borges Fernandes, M. Resolving the clumpy circumstellar environment of the B[e] supergiant LHA 120-S 35. Astron. Astrophys. 2018, 612, A113. https://doi.org/10.1051/0004-6361/201731723
  * Kraus et al. (2015) Kraus, M.; Oksala, M.E.; Cidale, L.S.; Arias, M.L.; Torres, A.F.; Borges Fernandes, M. Discovery of SiO Band Emission from Galactic B[e] Supergiants. Astrophys. J. Lett. 2015, 800, L20. https://doi.org/10.1088/2041-8205/800/2/L20
  * Torres et al. (2012) Torres, A.F.; Kraus, M.; Cidale, L.S.; Barbá, R.; Borges Fernandes, M.; Brandi, E. Discovery of Raman-scattered lines in the massive luminous emission-line star LHA 115-S 18. Mon. Not. R. Astrono. Soc. 2012, 427, L80-L84. https://doi.org/10.1111/j.1745-3933.2012.01349.x
  * Domiciano de Souza et al. (2007) Domiciano de Souza, A.; Driebe, T.; Chesneau, O.; Hofmann, K.-H.; Kraus, S.; Miroshnichenko, A.S.; Ohnaka, K.; Petrov, R.G.; Preisbisch, T.; Stee, P.; Weigelt, G.; Lisi, F.; Malbet, F.; Richichi, A. AMBER/VLTI and MIDI/VLTI spectro-interferometric observations of the B[e] supergiant CPD-57deg2874\. Size and geometry of the circumstellar envelope in the near- and mid-IR. Astron. Astrophys. 2007, 464, 81-86. https://doi.org/10.1051/0004-6361:20054134
  * Domiciano de Souza et al. (2011) Domiciano de Souza, A.; Bendjoya, P.; Niccolini, G.; Chesneau, O.; Borges Fernandes, M.; Carciofi, A. C.; Spang, A.; Stee, P.; Driebe, T. Fast ray-tracing algorithm for circumstellar structures (FRACS). II. Disc parameters of the B[e] supergiant CPD-57deg2874 from VLTI/MIDI data. Astron. Astrophys. 2011, 525, A22. https://doi.org/10.1051/0004-6361/201015194
  * Millour et al. (2011) Millour, F.; Meilland, A.; Chesneau, O.; Stee, Ph.; Kanaan, S.; Petrov, R.; Mourard, D.; Kraus, S. Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD 62623. Astron. Astrophys. 2011, 526, A107. https://doi.org/10.1051/0004-6361/201016193
  * Cidale et al. (2012) Cidale, L.S.; Borges Fernandes, M.; Andruchow, I.; Arias, M.L.; Kraus, M.; Chesneau, O.; Kanaan, S.; Curé, M.; de Wit, W.J.; Muratore, M.F. Observational constraints for the circumstellar disk of the B[e] star CPD-52 9243. Astron. Astrophys. 2012, 548, A72. https://doi.org/10.1051/0004-6361/201220120
  * Wang et al. (2012) Wang, Y.; Weigelt, G.; Kreplin, A.; Hofmann, K.-H.; Kraus, S.; Miroshnichenko, A.S.; Schertl, D.; Chelli, A.; Domiciano de Souza, A.; Massi, F.; Robbe-Dubois, S. AMBER/VLTI observations of the B[e] star MWC 300. Astron. Astrophys. 2012, 545, L10. https://doi.org/10.1051/0004-6361/201219800
  * Wheelwright et al. (2012) Wheelwright, H.E.; de Wit, W.J.; Oudmaijer, R.D.; Vink, J.S. VLTI/AMBER observations of the binary B[e] supergiant HD 327083. Astron. Astrophys. 2012, 538, A6. https://doi.org/10.1051/0004-6361/201117766
  * Aret et al. (2012) Aret, A.; Kraus, M.; Muratore, M.F.; Borges Fernandes, M. A new observational tracer for high-density disc-like structures around B[e] supergiants. Mon. Not. R. Astrono. Soc. 2012, 423, 284-293. https://doi.org/10.1111/j.1365-2966.2012.20871.x
  * Condori et al. (2019) Condori, C.A.H.; Borges Fernandes, M.; Kraus, M.; Panoglou, D.; Guerrero, C.A. The study of unclassified B[e] stars and candidates in the Galaxy and Magellanic Clouds. Mon. Not. R. Astrono. Soc. 2019, 488, 1090-1110. https://doi.org/10.1093/mnras/stz1540
  * Kraus et al. (2017) Kraus, M.; Liimets, T.; Cappa, C.E.; Cidale, L.S.; Nickeler, D.H.; Duronea, N.U.; Arias, M.L.; Gunawan, D.S.; Oksala, M.E.; Borges Fernandes, M.; Maravelias, G.; Curé, M.; Santander-García, M. Resolving the Circumstellar Environment of the Galactic B[e] Supergiant Star MWC 137 from Large to Small Scales. Astron. J. 2017, 154, 186. https://doi.org/10.3847/1538-3881/aa8df6
  * Kraus et al. (2007) Kraus, M.; Borges Fernandes, M.; de Araújo, F.X. On the hydrogen neutral outflowing disks of B[e] supergiants. Astron. Astrophys. 2007, 463, 627-634. https://doi.org/10.1051/0004-6361:20066325
  * Kraus et al. (2010) Kraus, M.; Borges Fernandes, M.; de Araújo, F.X. Neutral material around the B[e] supergiant star LHA 115-S 65. An outflowing disk or a detached Keplerian rotating disk? Astron. Astrophys. 2010, 517, A30. https://doi.org/10.1051/0004-6361/200913964
  * Maravelias et al. (2018) Maravelias, G.; Kraus, M.; Cidale, L.S.; Borges Fernandes, M.; Arias, M.L.; Curé, M.; Vasilopoulos, G. Resolving the kinematics of the discs around Galactic B[e] supergiants. Mon. Not. R. Astrono. Soc. 2018, 480, 320-344. https://doi.org/10.1093/mnras/sty1747
  * Muratore et al. (2015) Muratore, M.F.; Kraus, M.; Oksala, M.E.; Arias, M.L.; Cidale, L.; Borges Fernandes, M.; Liermann, A. Evidence of the Evolved Nature of the B[e] Star MWC 137. Astron. J. 2015, 149, 13. https://doi.org/10.1088/0004-6256/149/1/13
  * Aret et al. (2016) Aret, A.; Kraus, M.; Šlechta, M. Spectroscopic survey of emission-line stars - I. B[e] stars. Mon. Not. R. Astrono. Soc. 2016, 456, 1424-1437. https://doi.org/10.1093/mnras/stv2758
  * Kraus et al. (2000) Kraus, M.; Krügel, E.; Thum, C.; Geballe, T.R. CO band emission from MWC 349. I. First overtone bands from a disk or from a wind? Astron. Astrophys. 2000, 362, 158-168.
  * Kraus et al. (2019) Kraus, M.; Arias, M.L.; Cidale, L.S.; Torres, A.F. Evidence of an evolved nature of MWC 349A. In preparation.
  * Miroshnichenko et al. (2005) Miroshnichenko, A.S.; Bjorkman, K.S.; Grosso, M.; Hinkle, K.; Levato, H.; Marang, F. Properties of galactic B[e] supergiants. IV. Hen 3-298 and Hen 3-303. Astron. Astrophys. 2005, 436, 653-659. https://doi.org/10.1051/0004-6361:20052726
  * Liermann et al. (2014) Liermann, A.; Schnurr, O.; Kraus, M.; Kreplin, A.; Arias, M.L.; Cidale, L.S. A K-band spectral mini-survey of Galactic B[e] stars. Mon. Not. R. Astrono. Soc. 2014, 443, 947-956. https://doi.org/10.1093/mnras/stu1174
  * Miroshnichenko et al. (2002) Miroshnichenko, A.S.; Bjorkman, K.S.; Chentsov, E.L.; Klochkova, V.G.; Ezhkova, O.V.; Gray, R.O.; García-Lario, P.; Perea Calderón, J.V.; Rudy, R.J.; Lynch, D.K.; Mazuk, S.; Venturini, C.C.; Puetter, R. The luminous B[e] binary AS 381\. Astron. Astrophys. 2002, 383, 171-181. https://doi.org/10.1051/0004-6361:20011711
  * Andrillat, & Jaschek (1999) Andrillat, Y.; Jaschek, C. B[e] stars. VIII. MWC 342. Astron. Astrophys. Suppl. 1999, 136, 59-63. https://doi.org/10.1051/aas:1999461
  * Borges Fernandes et al. (2007) Borges Fernandes, M.; Kraus, M.; Lorenz Martins, S.; de Araújo, F.X. On the evolutionary stage of the unclassified B[e] star CD-42∘11721\. Mon. Not. R. Astrono. Soc. 2007, 377, 1343-1362. https://doi.org/10.1111/j.1365-2966.2007.11698.x
  * Kraus et al. (2008) Kraus, M.; Borges Fernandes, M.; Kubát, J.; de Araújo, F.X. From B[e] to A[e]. On the peculiar variations of the SMC supergiant LHA 115-S 23 (AzV 172). Astron. Astrophys. 2008, 487, 697-707. https://doi.org/10.1051/0004-6361:200809779
  * Chentsov et al. (2010) Chentsov, E.L.; Klochkova, V.G.; Miroshnichenko, A.S. Spectral variability of the peculiar A-type supergiant 3Pup. Astrophysical Bulletin 2010, 65, 150-163. https://doi.org/10.1134/S1990341310020057
  * Kraus, & Lamers (2003) Kraus, M.; Lamers, H.J.G.L.M. Ionization structure in the winds of B[e] supergiants. I. Ionization equilibrium calculations in a H plus He wind. Astron. Astrophys. 2003, 405, 165-174. https://doi.org/10.1051/0004-6361:20030518
  * Kraus (2006) Kraus, M. Ionization structure in the winds of B[e] supergiants. II. Influence of rotation on the formation of equatorial hydrogen neutral zones. Astron. Astrophys. 2006, 456, 151-159. https://doi.org/10.1051/0004-6361:20065204
  * Zsargó et al. (2008) Zsargó, J.; Hillier, D.J.; Georgiev, L.N. Axi-symmetric models of B[e] supergiants. I. The effective temperature and mass-loss dependence of the hydrogen and helium ionization structure. Astron. Astrophys. 2008, 478, 543-551. https://doi.org/10.1051/0004-6361:20078293
  * Glatzel, & Kiriakidis (1993) Glatzel, W.; Kiriakidis, M. Stability of Massive Stars and the Humphreys-Davidson Limit. Mon. Not. R. Astrono. Soc. 1993, 263, 375-384. https://doi.org/10.1093/mnras/263.2.375
  * Kiriakidis et al. (1993) Kiriakidis, M.; Fricke, K. J.; Glatzel, W. The stability of massive stars and its dependence on metallicity and opacity. Mon. Not. R. Astrono. Soc. 1993, 264, 50-62. https://doi.org/10.1093/mnras/264.1.50
  * Glatzel, & Mehren (1996) Glatzel, W.; Mehren, S. Non-radial pulsations and stability of massive stars. Mon. Not. R. Astrono. Soc. 1996, 282, 1470-1482. https://doi.org/10.1093/mnras/282.4.1470
  * Kurfürst et al. (2014) Kurfürst, P.; Feldmeier, A.; Krtička, J. Time-dependent modeling of extended thin decretion disks of critically rotating stars. Astron. Astrophys. 2014, 569, A23. https://doi.org/10.1051/0004-6361/201424272
  * Kurfürst et al. (2018) Kurfürst, P.; Feldmeier, A.; Krtička, J. Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks. Astron. Astrophys. 2018, 613, A75. https://doi.org/10.1051/0004-6361/201731300
  * Pelupessy et al. (2000) Pelupessy, I.; Lamers, H.J.G.L.M.; Vink, J.S. The radiation driven winds of rotating B[e] supergiants. Astron. Astrophys. 2000, 359, 695-706.
  * Curé (2004) Curé, M. The Influence of Rotation in Radiation-driven Wind from Hot Stars: New Solutions and Disk Formation in Be Stars. Astrophys. J. 2004, 614, 929-941. https://doi.org/10.1086/423776
  * Curé et al. (2005) Curé, M.; Rial, D. F.; Cidale, L. Outflowing disk formation in B[e] supergiants due to rotation and bi-stability in radiation driven winds. Astron. Astrophys. 2005, 437, 929-933. https://doi.org/10.1051/0004-6361:20052686
  * Kraus (2017) Kraus, M. Spectroscopic Diagnostics for Circumstellar Disks of B[e] Supergiants. In The B[e] Phenomenon: Forty Years of Studies. 2017, Eds. Miroshnichenko, A., Zharikov, S., Korčáková, D., Wolf, M. ASP Conference Series, 508, San Francisco: Astronomical Society of the Pacific, 219-228.