###### Abstract

Massive stars expel large amounts of mass during their late evolutionary
phases. We aim to unveil the physical conditions within the warm molecular
environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs),
which are known to be embedded in circumstellar shells and disks. We present
$K$-band spectra of two B[e]SGs from the Large Magellanic Cloud and four
Galactic YHGs. The CO band emission detected from the B[e]SGs LHA 120-S 12 and
LHA 120-S 134 suggests that these stars are surrounded by stable rotating
molecular rings. The spectra of the YHGs display a rather diverse appearance.
The objects 6 Cas and V509 Cas lack any molecular features. The star [FMR2006]
15 displays blue-shifted CO bands in emission, which might be explained by a
possible close to pole-on oriented bipolar outflow. In contrast, HD 179821
shows blue-shifted CO bands in absorption. While the star itself is too hot to
form molecules in its outer atmosphere, we propose that it might have
experienced a recent outburst. We speculate that we currently can only see the
approaching part of the expelled matter because the star itself might still
block the receding parts of a (possibly) expanding gas shell.

###### keywords:

stars: massive; stars: supergiants; stars: winds; outflows; circumstellar
matter

1 1 0 Academic Editor: Oleg Malkov 14 March 2023 5 June 2023 12 June 2023
https://doi.org/ Dense Molecular Environments of B[e] Supergiants and Yellow
Hypergiants Dense Molecular Environments of B[e] Supergiants and Yellow
Hypergiants Michaela Kraus 1,*, Michalis Kourniotis 1, María Laura Arias 2,3,
Andrea F. Torres 2,3 and Dieter H. Nickeler 1 Michaela Kraus, Michalis
Kourniotis, María Laura Arias, Andrea F. Torres and Dieter H. Nickeler Kraus,
M.; Kourniotis, M.; Arias, M.L.; Torres, A.F.; Nickeler, D.H. Correspondence:
<EMAIL_ADDRESS>
##  1 Introduction

The evolution of massive stars ($M_{\rm ini}\gtrsim 8$ M⊙) bears many
uncertainties, which render it difficult to trace such objects from the cradle
up to their spectacular explosion as supernova. One major hindrance is the
poorly constrained mass loss due to stellar winds that the stars experience
along the course of their evolution. Furthermore, the post-main sequence
evolution of massive stars encounters phases in which the stars lose a
significant amount of mass due to episodically enhanced mass loss or
occasional mass eruptions, both of poorly understood origin. The ejected mass
can accumulate around the star in rings, shells, or bipolar lobes, as seen in
some B- or B[e]-type supergiants (e.g., Sher 25 Brandner et al. (1997); Hendry
et al. (2008), MWC 137 Marston and McCollum (2008); Kraus et al. (2017), SBW1
Smith et al. (2013)), yellow hypergiants (IRC+10 420 Oudmaijer and de Wit
(2013) and Hen 3-1379 Lagadec et al. (2011)), many luminous blue variables
Weis (2011); Liimets et al. (2022), and Wolf-Rayet stars Chu (1981); Marston
et al. (1994a, b); Maryeva et al. (2020); Sévigny et al. (2021).

Two groups of evolved massive stars are particularly interesting. These are
the B[e] supergiants (B[e]SGs) and the yellow hypergiants (YHGs). Both types
of objects have dense and warm circumstellar environments, and representatives
of both classes of objects show (at least occasionally) emission from hot
molecular gas.

###  1.1 B[e] Supergiants

The group of B[e]SGs consists of luminous ($\log(L_{*}/L_{\odot})\geq 4.0$)
post-main sequence B-type emission line stars. Besides large-scale ejecta
(with sizes of several pc) detected in some B[e]SGs Marston and McCollum
(2008); Liimets et al. (2022), all objects have intense winds and are
surrounded by massive disks on small scales (up to $\sim$100 AU) Zickgraf et
al. (1986); Domiciano de Souza et al. (2007); Maravelias et al. (2018), giving
rise to the specific emission features characterizing stars with the B[e]
phenomenon Lamers et al. (1998). These disks give shelter to a diversity of
molecular and dust species, and the near-infrared (NIR) is an ideal wavelength
regime to detect molecular emission features that, when resolved with high
spectral resolution, provide insight into the physical properties of the disks
and reveal the disk dynamics.

The most commonly observed molecule is CO. The emission from its first-
overtone bands arises in the $K$-band around $2.3\,\upmu$m and has been
detected in about 50% of the B[e]SGs Kraus (2019). Besides the main isotope
12CO, emission from 13CO is seen in considerable amounts Liermann et al.
(2010); Oksala et al. (2013), confirming that the matter from which the disks
have formed contains processed material that must have been released from the
stellar surface Kraus (2009).

Emission from the first-overtone bands of SiO, arising in the $L$-band around
$4\,\upmu$m, has been reported for some Galactic B[e]SGs Kraus et al. (2015).
SiO has a lower binding energy than CO. It thus forms at distances farther
away from the star than CO. The individual ro-vibrational lines in both CO and
SiO are kinematically broadened with a double-peaked profile, and
(quasi-)Keplerian rotation of the molecular gas around the central object has
been suggested as the most likely explanation to interpret the spectral
appearance Kraus et al. (2015, 2016, 2013); Muratore et al. (2015); Maravelias
et al. (2018); Torres et al. (2018); Cidale et al. (2012).

Observations of B[e]SGs in the NIR are sparse. But persistent CO band emission
over years and decades has been detected in numerous objects McGregor et al.
(1988a, b); Oksala et al. (2013) and has been used as one of the criteria to
identify and classify stars as B[e]SGs in Local Group galaxies Kraus et al.
(2014); Sholukhova et al. (2015). However, in a few cases, considerable
variability in these emission features has been reported as well. The most
striking object is certainly the B[e]SG star LHA 115-S 65 in the Small
Magellanic Cloud (SMC), for which a sudden appearance of CO band emission has
been recorded Oksala et al. (2012). The disk around this object is seen edge-
on, and in addition to its rotation around the central object, it also drifts
outwards, very slowly and with a velocity decreasing with distance from the
star and reaching about zero Kraus et al. (2010). This slowdown might have
resulted in a build-up of density in regions favorable for molecule
condensation and for excitation of the CO bands.

Furthermore, LHA 115-S 18 in the SMC showed no CO band emission back in
1987/1989 McGregor et al. (1989), whereas follow-up observations in November
1995 taken with a more than three times higher resolution displayed intense CO
bands Morris et al. (1996), which were also seen in the observations acquired
in October 2009 Oksala et al. (2013).

In the spectrum of the Galactic B[e]SG MWC 349, intense CO band emission
appeared in the early 1980s Hamann and Simon (1986). It was still observable
in 2013, but by then the CO gas had clearly cooled and the emission intensity
had significantly decreased, which has been interpreted as due to expansion
and dilution of the circumstellar disk Kraus et al. (2020). Two more objects
in the Large Magellanic Cloud (LMC) displayed indications of CO band
variability, most likely related to inhomogeneities within the distribution of
the molecular gas around the central star. These are LHA 120-S 73 Kraus et al.
(2016) and LHA 120-S 35 Torres et al. (2018).

In the optical range, indications for emission from TiO molecular bands have
been found in six B[e]SGs Zickgraf et al. (1989); Torres et al. (2012); Kraus
et al. (2016); Torres et al. (2018). All six objects reside in the Magellanic
Clouds, and five of them also have CO band emissionThe sixth object is LHA
120-S 111. To our knowledge, it has been observed in the $K$-band only once,
in January 1987 McGregor et al. (1988a). At that time, no CO band emission was
detected.. No Galactic B[e]SG has been reported to date to display TiO band
emission Kraus (2019).

###  1.2 Yellow Hypergiants

With temperatures in the range $T_{\rm eff}\simeq 4000-8000$ K and
luminosities $\log(L/L_{\odot})$ spreading from $5.2$ to $5.8$, the YHGs
populate a rather narrow domain in the Hertzprung-Russel (HR) diagram. The
stars are in their post-red supergiant (post-RSG) evolutionary phase Oudmaijer
et al. (2009), and their luminosities place them on evolutionary tracks of
stars with initial masses in the range $M_{\rm ini}\simeq 20-40$ M⊙.
Evolutionary calculations of (rotating) stars in this mass range have shown
that these objects can indeed evolve back to the blue, hot side of the HR
diagram Ekström et al. (2012), whereas stars with lower initial mass just
reach the RSG stage before they explode as SNe of type II-P. Support for this
theoretical scenario is provided by the lack of high-mass ($M_{\rm ini}\geq
18$ M⊙, i.e., with luminosities $\log L/L_{\odot}>5.1$) RSG progenitors for
this type of supernovae Smartt et al. (2009); Smartt (2015).

As post-RSGs, the YHGs might be expected to be embedded in envelopes, remnants
of the previous mass-losing activities during the RSG stage. However,
surprisingly, so far, only about half of the YHGs have been reported to have a
dusty and/or cold molecular envelope. These are the Galactic objects IRC
+10420 Oudmaijer et al. (1996); Castro-Carrizo et al. (2001); Tiffany et al.
(2010), HR 5171A Deguchi et al. (1990), HD 179821 Jura and Werner (1999) and
Hen 3-1379 Lagadec et al. (2011); Koumpia et al. (2020), three YHGs in the LMC
(HD 269953, HD 269723, HD 268757, Kourniotis et al. (2022)), as well as Var A
Humphreys et al. (2006) and three more YHG candidates in the galaxy M33
Kourniotis et al. (2017).

A typical classification characteristic of YHGs is the occurrence of outbursts
that can be clearly discriminated from the more regular (cyclic) brightness
variability due to stellar pulsations. During such an outburst event, the star
inflates, its brightness drastically decreases, and the object seems to
undergo a red loop evolution in the HR diagram. Molecules such as TiO and CO
can form in the cool, outer atmospheric layers, leading to intense absorption
structures in the optical and NIR, respectively, and the object’s entire
spectral appearance resembles that of a much later spectral type. The
outbursts are most likely connected with enhanced mass loss or mass eruptions
from the star, which might be connected to non-linear instabilities such as
finite-time singularities or blow-ups typically occurring in fluid dynamics
Nickeler and Karlický (2008) or to strange-mode instabilities Glatzel (1994);
Glatzel et al. (1999) as recent computations propose Kraus et al. (2022). The
duration of the outbursts can range from months to decades before the star
appears back at its real location in the HR diagram.

The bona-fide YHG, $\rho$ Cas, experienced four documented outbursts during
the past 80 years with variable duration (from weeks up to three years) and
amplitude (0.29 to 1.69 mag), connected with significant changes in its
spectral appearance Lobel et al. (2003); Klochkova et al. (2018); Kraus et al.
(2019); Maravelias and Kraus (2022), whereas Var A in M33 presumably underwent
an eruption around 1950 that lasted $\sim$45 years Humphreys et al. (2006).
The object V509 Cas experienced mass-loss events in the seventies, during
which the star’s apparent temperature decreased significantly. Since then, the
star has displayed a steady increase in its effective temperature from
$\sim$5000 K in 1973 to $\sim$8000 K in 2001 Nieuwenhuijzen et al. (2012), and
since then it has stabilized at that temperature Aret et al. (2017).
Furthermore, IRC +10420 changed its spectral type from F8 to mid- to early A,
connected with an increase in temperature over a period of about 30 years with
an average rate of $\sim$120 K per year Oudmaijer (1998); Klochkova et al.
(2002). The light curves of other YHG candidates also display outburst
activity in connection with variable mass loss Kourniotis et al. (2017, 2022).
However, in many cases, the mass-loss episodes appear to be short, so that the
released material expands and dilutes without creating detectable large-scale
circumstellar envelopes Schuster et al. (2006).

Nevertheless, many YHGs are surrounded (or were for some period in the past)
by hot molecular gas traced by first-overtone CO band emission, suggesting
that the objects are embedded in a dense and hot environment. Whether the
molecular gas is arranged in a ring revolving around the object, as in the
case of the B[e]SGs, is currently not known. However, the CO band spectral
features in YHGs seem to be much more variable than in their hotter B[e]SG
counterparts, especially because they often appear superimposed on
photospheric CO band absorption that forms during the expansion and cooling
periods of the long-term pulsation cycles, especially of the cooler YHGs, or
during outburst events. One such candidate with cyclic CO band variability is
$\rho$ Cas. During its pulsation cycles, CO band emission appears when the
star is hottest (maximum brightness) and most compact, whereas CO bands are
seen in absorption when the star is coolest (minimum brightness) and most
inflated Gorlova et al. (2006). It has been speculated that the appearance of
CO bands in emission might be related to propagating pulsation-driven shock
waves in the outer atmosphere of the star Gorlova et al. (2006). An
alternative scenario would also be conceivable, in which the CO emission could
be permanent, arising from a circumstellar ring or shell and being detectable
only during phases in which no photospheric absorption compensates the
emission Kraus et al. (2019). Support for the latter scenario is provided by
the fact that the CO emission features remain at constant radial velocities
(along with other emission lines formed in the circumstellar environment such
as [Ca ii] and Fe i, Kraus et al. (2019)) whereas the absorption components
change from red- to blue-shifted, in phase with the pulsation cycle.

The object $\rho$ Cas is, to date, the best monitored YHG in the NIR. For many
other YHGs observations in the $K$-band have been taken only sporadically.
Hence, not much can be said or concluded about the variability rhythm of their
CO band features. Occasional CO band emission has been reported for the
galactic objects HD 179821 Hrivnak et al. (1994); Oudmaijer et al. (1995),
V509 Cas Lambert et al. (1981), [FMR2006] 15 Davies et al. (2008), and the two
LMC objects HD 269723 and HD 269953 McGregor et al. (1988a); Oksala et al.
(2013). The latter even displays emission from hot water vapor and is, so far,
the only evolved massive star with such emission features from its environment
Kraus et al. (2022).

###  1.3 Motivation and Aims

The appearance of CO band emission in the NIR spectra of B[e]SGs and YHGs
suggests that similar physical conditions may prevail in the circumstellar
environments of both groups of objects. In depth studies of these conditions
are rare though.

For the B[e]SGs in the Magellanic Clouds CO, column densities, temperatures,
and 12CO/13CO isotopic ratios were determined based on medium-resolution
$K$-band spectra Oksala et al. (2013). The resolution of these spectra was,
however, too low to derive the gas kinematics with high confidence. On the
other hand, high-resolution $K$-band spectra of the Galactic B[e]SG sample
allowed to obtain the CO dynamics Muratore et al. (2012); Maravelias et al.
(2018), whereas in most cases the spectral coverage was too short to infer
about the density and temperature of their hot molecular environment.

The situation for the YHGs is even worse. Besides for $\rho$ Cas Gorlova et
al. (2006) and for the LMC object HD 269953 Oksala et al. (2013); Kraus et al.
(2022) no attempts have been undertaken so far to study their warm molecular
environments in more detail and to derive the parameters of the CO band
emitting regions.

Therefore, we started to systematically observe both the B[e]SGs and the YHGs
in the Milky Way and Magellanic Clouds to fill this knowledge gap. A further
motivation for our research is provided by the location of the B[e]SGs with CO
band emission in the HR diagram. As has been mentioned previously for the
Magellanic Clouds sample Kraus (2019), these objects cluster around luminosity
values $\log L/L_{\odot}\simeq 5.0-{5.9}$, whereas B[e]SGs that are more
luminous than $\sim$5.9 or less luminous than $\sim$5.0 do not show CO band
emission. The same holds when inspecting the Galactic B[e]SGs. In the left
panel of Figure 1 we depict the LMC objects, whereas in the right panel we
display the Galactic sampleWe excluded two Galactic B[e]SGs from this plot.
With a literature luminosity value of $\log L/L_{\odot}=4.33\pm 0.09$ Millour
et al. (2011), the luminosity of HD 62623 is considerably lower than for the
other B[e]SGs with CO bands. However, its distance with a parallax value of
$0.59\pm 0.17$ is not well constrained. The object HD 327083 turned out to be
misclassified and has been removed from the B[e]SGs list (Cidale et al., in
preparation). from Maravelias et al. (2018); Kraus et al. (2020). The YHGs in
the LMC Kourniotis et al. (2022) and the Milky Way de Jager (1998) have been
added to the plotsWe omit the SMC objects, because we do not have new data for
any of the SMC B[e]SGs, and there are currently no confirmed YHGs in the SMC..
Their positions with maximum and minimum effective temperature values, as
reported in the literature, have been connected with dashed lines. Errors in
temperature and luminosity are indicated when provided by the corresponding
studies.

Figure 1: HR diagram with evolutionary tracks for LMC ($Z=0.006$, Eggenberger
et al. (2021), left panel) and solar ($Z=0.014$, Ekström et al. (2012), right
panel) metallicity for models of rotating stars ($v/v_{\rm crit}=0.4$) with
initial masses from $20-40$ M⊙. Shown are the positions of LMC (blue symbols)
and Galactic (red symbols) objects. Only B[e]SGs with hot circumstellar
molecular CO gas are shown. These populate similar evolutionary tracks as the
YHGs. The minimum and maximum temperature values (where known) of the YHGs are
connected by dashed lines. YHGs with reported (at least once) CO band emission
are shown with triangles. The stars of the current study are labeled.

Interestingly, the YHGs and the B[e]SGs with CO band emission share similar
evolutionary tracks, which is particularly evident for the Galactic objects.
This raises the question of whether evolved stars in this particular mass
range suffer from specific instabilities in the blue and yellow temperature
regimes independent of their evolutionary state. Such instabilities need to
have the potential to drive mass ejections or eruptions, and the released mass
would have to be dense and cool enough to create the required conditions for
the formation of significant amounts of molecules generating intense band
emission.

###  1.4 Selection of Targets

For our current study, we have selected two B[e]SGs from the LMC. These are
the objects LHA 120-S 12 and LHA 120-S 134. Both are known to display CO band
emission (see, e.g., Oksala et al. (2013)), but both of them lack high-
resolution $K$-band spectra to derive the CO kinematics. Furthermore, we have
selected four Galactic YHGs, [FMR2006] 15, HD 179821, V509 Cas, and 6 Cas. Of
these, only the former three objects were reported in the literature to
display (at some epochs) CO band emission. The basic stellar parameters of all
objects, as obtained from the literature, are listed in Table 1.

Table 1: Stellar parameters. Errors are given where available.

Object | $G$ | $K_{s}$ | $\log T_{\rm eff}$ | $\log L/L_{\odot}$ | Ref. | $d$ | Ref.
---|---|---|---|---|---|---|---
| [mag] | [mag] | [K] |  |  | [kpc] |
LHA 120-S 12 | $12.4$ | $10.2$ | $4.36$ | $5.34\pm 0.04$ | Zickgraf et al. (1986) | $49.6\pm 0.5$ | Pietrzyński et al. (2019)
LHA 120-S 134 | $11.4$ | $8.6$ | $4.41$ | $5.90\pm 0.06$ | Zickgraf et al. (1986) | $49.6\pm 0.5$ | Pietrzyński et al. (2019)
[FMR2006] 15 | $19.5$ | $6.7$ | $3.84$ | $5.36\pm 0.15$ | Davies et al. (2008) | $6.6\pm 0.9$ | Davies et al. (2008)
6 Cas | — | $3.4$ | $3.93$ | $5.13$ | Kudritzki et al. (1999) | $2.8\pm 0.3$ | Maíz Apellániz et al. (2021)
V509 Cas | $5.0$ | $1.7$ | $3.90$ | $5.60$ | Aret et al. (2017); de Jager and Nieuwenhuijzen (1997) | $1.4\pm 0.5$ |  Nieuwenhuijzen et al. (2012) a
HD 179821 | $7.5$ | $4.7$ | $3.83$ | $5.30$ | Kipper (2008); de Jager (1998) | $5.3\pm 0.3$ | Kipper (2008); Gaia Collaboration (2020)

Note: The $G$ and $K_{s}$-band magnitudes are from GAIA Early Data Release 3
Gaia Collaboration (2020) and from the 2MASS point source catalog Cutri et al.
(2003), respectively. The YHG effective temperatures refer to the hot state.
No GAIA $G$-band measurement is available for 6 Cas. a Listed distance is
based on the HIPPARCOS parallax of $0.73\pm 0.25$. The new GAIA Early Data
Release 3 parallax of $0.2507\pm 0.0633$ Gaia Collaboration (2020) places the
object at a distance of $\sim$4 kpc.

##  2 Observations and Data Reduction

High-resolution spectra ($R\sim$ 50,000) of the two B[e]SGs were acquired with
the visitor spectrograph Phoenix Hinkle et al. (2003) mounted at GEMINI-South.
The spectra were taken on 20 December 2004 and 30 November 2017 under program
IDs GS-2004B-Q-54 and GS-2017B-Q-32. The observations were carried out in the
$K$-band with two different filters, K4396 and K4308. The central wavelength
was chosen such that the wavelength ranges cover the first and second band
heads of the first-overtone CO band emission.

Medium-resolution $K$-band spectra of the YHGs have been acquired with the
Gemini Near-InfraRed Spectrograph (GNIRS, Elias et al. (2006a, b)) at GEMINI-
North under Program IDs GN-2019A-Q-204, GN-2019B-Q-418, and GN-2021A-Q-315.

The spectrum of [FMR2006] 15 was observed on 12 May 2019 centered on
$\lambda=2.35\,\upmu$m. The instrument configuration was a short camera (0.15”
per pixel) with the 0.3” slit and the 111 l mm-1 grating, resulting in a
resolving power of $R\sim$ 5900.

V509 Cas and 6 Cas were observed on 21 December 2019 with the instrumental
configuration: Long camera (0.05” per pixel) with the 0.10” slit and the 32 l
mm-1 grating which provides a resolving power of $R\sim 5100$. The
observations were centered on $\lambda=2.35\,\upmu$m.

HD 179821 was observed on 7 April 2021 with two different central wavelengths
$\lambda=2.14\,\upmu$m and $2.33\,\upmu$m and with the following instrument
configuration: a short camera (0.15” per pixel), the 0.3” slit, and the 111 l
mm-1 grating, resulting in a resolving power of $R\sim 5900$.

For all objects, a telluric standard star (usually a late B-type main sequence
star) was observed close in time and airmass. For optimal sky subtraction, the
star was positioned at two different locations along the slit (A and B), and
the observations were carried out in ABBA cycles. Data reduction and telluric
correction were performed using standard IRAFIRAF is distributed by the
National Optical Astronomy Observatory, which is operated by the Association
of Universities for Research in Astronomy (AURA) under cooperative agreement
with the National Science Foundation. tasks. The reduction steps consist of
subtraction of AB pairs, flat-fielding, wavelength calibration (using the
telluric lines), and telluric correction. The observing log is given in Table
LABEL:tab:obs, where we list the star name, object class, observing date (UT),
used instrument, covered wavelength range, spectral resolution $R$, and
resulting signal-to-noise ratio (SNR).

Table 2: Observation log. Object | Class | Obs Date | Instrument | $\lambda_{\rm min}-\lambda_{\rm max}$ | R | SNR
---|---|---|---|---|---|---
|  | [yyyy-mm-dd] |  |  [$\upmu$m] |  |
LHA 120-S 12 | B[e]SG | 2004-12-20 | Phoenix | 2.291–2.300 | 50,000 | 35
|  | 2017-11-30 | Phoenix | 2.319–2.330 | 50,000 | 20
LHA 120-S 134 | B[e]SG | 2017-11-30 | Phoenix | 2.290–2.299 | 50,000 | 40
|  | 2017-11-30 | Phoenix | 2.319–2.330 | 50,000 | 100
[FMR2006] 15 | YHG | 2019-05-12 | GNIRS | 2.257–2.440 | 5900 | 300
6 Cas | YHG | 2019-12-21 | GNIRS | 2.232–2.453 | 5100 | 100
V509 Cas | YHG | 2019-12-21 | GNIRS | 2.233–2.453 | 5100 | 140
HD 179821 | YHG | 2021-04-07 | GNIRS | 2.046–2.424 | 5900 | 250

##  3 Results

###  3.1 Description of the Spectra

CO band head emission is detected in both B[e]SGs (black lines in Figure 2)
despite the low quality of some of the spectral pieces and some telluric
remnants in the red parts of the short wavelength portions (left lower
panels). The spectrum of LHA 120-S 134 contains additional emission lines from
the hydrogen Pfund series. Intense emission in these recombination lines has
already been reported from that star based on medium-resolution spectra Oksala
et al. (2013). No contribution from the Pfund lines is seen in the spectra of
LHA 120-S 12.

Figure 2: Best fitting model (red) to the normalized Phoenix spectra (black)
of LHA 120-S 12 (top) and LHA 120-S 134 (bottom). For each star we display the
entire fit to the total spectrum (top panels) and the zoom to the band heads
(bottom panels). Emission from the Pfund series (blue dashed line), detected
in the spectrum of LHA 120-S 134, is included in the total fit.

The $K$-band spectra of the YHGs display a diverse appearance, as depicted in
Figure 3. Two stars possess just Pfund lines in absorption (V509 Cas and 6
Cas) and an otherwise featureless spectrum, in agreement with their high
effective temperature ($T_{\rm eff}\geq 8000$ K, see Table 1). The star
[FMR2006] 15 shows CO band emission overimposed on the atmospheric spectrum of
a presumably late-type star. In HD 179821 the CO bands and the Br $\gamma$
line are in absorption along with numerous other photospheric lines, whereas
the Na i $\lambda\lambda$2.206,2.209 doublet shows prominent emission.

Figure 3: Normalized medium-resolution $K$-band spectra of the YHGs taken with
GNIRS. For better visualization, the spectra have been offset along the flux
axis. Positions of the CO band heads and of the lines from the Pfund series
are marked by ticks. The lines of Br$\gamma$ and of the Na i doublet are
labeled as well.

###  3.2 Modeling of the CO Band and Pfund Line Emission

We model the CO emission using our molecular disk code Kraus et al. (2000)
that has been developed to compute the ro-vibrational bands from a rotating
ring (or disk) of circumstellar gas under local thermodynamic equilibrium
conditions. The calculations are carried out for the two main isotopes, 12CO
and 13CO Kraus (2009); Oksala et al. (2013).

The high-resolution spectra of the two B[e]SGs display kinematic broadening of
the individual ro-vibrational lines in the form of a double-peaked profile
(Figure 2). Such a profile can be interpreted either as rotation around the
central object or as an equatorial outflow. Since B[e]SGs are known to be
surrounded by (quasi-)Keplerian rotating disks, the assumption of rotation as
the most likely broadening mechanism seems to be justified. The situation for
the YHG star [FMR2006] 15 is less clear. The medium-resolution spectrum
provides no clear hint about a possible double-peaked shape of the individual
lines. Therefore, we can only derive an upper limit for a possible rotational
(or outflow) contribution to the total dynamics.

Due to the high sensitivity of the CO band intensity to the gas temperature
and column density, the observed emission traces the hottest and densest
molecular regions. Therefore, it is usually sufficient to consider a single
ring of gas with constant density and temperature, reducing the number of free
parameters to the column density $N_{\rm CO}$, temperature $T_{\rm CO}$, the
isotope ratio 12CO/13CO, and the gas kinematics split into contributions of
the rotation velocity projected to the line of sight $v_{\rm rot,los}$, and of
a combined thermal and turbulent velocity in the form of a Gaussian component
$v_{\rm Gauss}$.

The best-fitting CO parameters obtained for the three objects are listed in
Table 3, and the total emission spectra are included (in red) in Figure 2 for
the two B[e]SGs. It is noteworthy that the two CO band heads of LHA 120-S 12
can be reproduced fairly well with the same model parameters, despite the fact
that the spectral pieces have been observed 13 years apart. This implies that
the ring of CO gas around LHA 120-S 12 is stable on longer timescales.

Table 3: Best fitting CO parameters.

Object | $T_{\rm CO}$ | $N_{\rm CO}$ | $v_{\rm rot,los}$ | $v_{\rm Gauss}$ |  12CO/13CO
---|---|---|---|---|---
| [K] |  [$\times 10^{21}$ cm-2] | [km s-1] | [km s-1] |
LHA 120-S 12 | $2800\pm 200$ | $1.5\pm 0.2$ | $27\pm 2$ | $3\pm 1$ | $20\pm 2$
LHA 120-S 134 | $2300\pm 100$ | $2.0\pm 0.1$ | $30\pm 1$ | $1.5\pm 0.5$ | $15\pm 2$
[FMR2006] 15 | $3000\pm 200$ | $2.0\pm 0.2$ | $20\pm 5$ | $15\pm 5$ | $4\pm 2$
[FMR2006] 15 | $3000\pm 200$ | $2.0\pm 0.2$ | $0$ | $20\pm 5$ | $4\pm 2$

Note: The 12CO/13CO values for LHA 120-S 12 and LHA 120-S 134 have been
derived by Liermann et al. (2010); Oksala et al. (2013), respectively. Our
Phoenix spectra do not reach the wavelength region of the 13CO bands.

The spectrum of LHA 120-S 134 displays emission from the hydrogen Pfund line
series superimposed on the CO band spectrumPfund line emission is also
reported from LHA 120-S 12 Liermann et al. (2010), but in that star the
maximum detected Pfund transition is with Pf(31) arising at 2.34 $\mu$m
clearly outside our spectral coverage.. These Pfund lines appear to be broad
with no indication of a double-peaked profile shape. To include the
contribution of these lines to the total emission spectrum, we apply our code
developed for the computation of the hydrogen series according to Menzel case
B recombination, assuming that the lines are optically thin Kraus et al.
(2000). We fix the electron temperature at 10,000 K, which is a reasonable
value for ionized gas around an OB supergiant star and, using a Gaussian
profile, we obtain a velocity of $53\pm 3$ km s-1. Similar velocity values for
the lines from the Pfund series have been found for various B[e]SGs (see
Liermann et al. (2010); Oksala et al. (2013); Muratore et al. (2015)). These
rather low values compared with the wind velocities of classical B supergiants
might suggest that the Pfund lines form in a wind emanating from the surface
of the ionized part of the circumstellar disk. The electron density can be
derived from the maximum number of the Pfund series visible in the spectrum.
For LHA 120-S 134, this number corresponds to the line Pf(57), resulting in an
electron density of $(5.8\pm 0.5)\times 10^{12}$ cm-3 within the Pfund line
forming region. Having the parameters for the Pfund emission fixed, we compute
the contribution of the Pfund series to the total emission spectrum of LHA
120-S 134. This contribution is shown in blue in Figure 2.

The best-fitting CO model for the YHG [FMR2006] 15 is depicted in Figure 4
(top). It should be noted that the contribution of the rotation (respectively
outflow) component should be considered an upper limit. A double-peaked
profile corresponding to such a velocity might be hidden within the CO band
structure. Only high-resolution observations will tell whether such a profile
component is really included. We found that a model omitting this velocity
component and using instead just a single Gaussian profile (added to Table 3)
results in a similar but slightly less satisfactory fit because the intensity
of many individual ro-vibrational lines is overestimated in the short
wavelength domain (Figure 4, bottom).

Figure 4: Best fitting model (red) to the observed (black) $K$-band spectrum
of [FMR2006] 15 for the model including a rotational component (top) and a
pure Gaussian broadening (bottom).

The rotation velocity of the CO gas allows us to estimate the distances of the
CO-emitting rings from the central object. For this, we need to know the
current stellar masses. Considering the stellar effective temperatures,
luminosities (from Table 1), and the carbon isotope ratios (from Table 3), we
search for the best matching evolutionary models for each of our targets
utilizing the stellar evolution track interpolation tool
SYCLISThttps://www.unige.ch/sciences/astro/evolution/en/database/syclist/. To
reproduce the observed carbon isotope ratios, we vary the initial stellar
rotation rate between zero and the maximum offered value of 0.4 and select
those evolutionary tracks that fit both the stellar location in the HRD and
the carbon isotope ratio. The metallicities used are 0.006 for the LMC objects
and 0.014 for the Galactic star. Table 4 lists the stellar radii of our
targets along with the most likely initial masses, initial rotation rate, and
the resulting current masses. We note that according to these evolutionary
tracks, the B[e]SGs seem to have evolved just off the main sequence, whereas
the low carbon isotope ratio measured in [FMR2006] 15 clearly places this
object on the post-RSG path. For the reason that no proper values for the disk
inclination angles are known, we provide the distances of the CO-emitting
rings as a function of the inclination angle. These are lower limits to the
real distances. We refrain from adding a distance of the CO emitting region
for [FMR2006] 15 because of the speculative nature of its rotation component.

Table 4: Estimated current stellar masses and distances of the CO emitting rings from the star. Object | $R_{*}$ | $M_{\rm in}$ | $v/v_{\rm crit}$ | $M_{*}$ | $r_{\rm CO}/(\sin i)^{2}$ | $r_{\rm CO}/(\sin i)^{2}$
---|---|---|---|---|---|---
| [$R_{\odot}$] | [$M_{\odot}$] | [km s-1] | [$M_{\odot}$] | [cm] | [$R_{*}$]
LHA 120-S 12 | $30$ | $26$ | $0.25$ | $25$ | $4.6\times 10^{14}$ | $218.0$
LHA 120-S 134 | $44$ | $45$ | $0.36$ | $40$ | $5.9\times 10^{14}$ | $192.7$
[FMR2006] 15 | $333$ | $25$ | $0.40$ | $13$ | — | —

##  4 Discussion

We have detected CO band emission in three objects and CO band absorption in
one object in our sample. To assess a possible CO variability, we summarize in
Table 5 information about previous detections of CO band features in the
$K$-band from the literature, including our results.

The number of observations in the $K$-band over the last 3–4 decades is sparse
for all objects, but the two B[e]SGs continuously display emission over a time
interval of 32 years. The diversity in spectral resolution used for the
observations makes it difficult to compare the shape and intensity of the
emission bands and, hence, to judge variability. Only for LHA 120-S 12, we can
say that no significant variability seems to have taken place between our
observations acquired in 2004 and in 2017.

For the YHGs, the situation is different. Three of them clearly show
variability in their CO bands, ranging from emission over complete
disappearance to absorption (or a combination of emission and absorption). The
changes are on time scales ranging from months to years. The exception is 6
Cas, which has not been reported previously to have CO bands, and our own
observations taken 25 years later also lack any CO band features.

In the following, we briefly describe the known characteristics of each of our
targets with respect to stellar variability and the properties of the
circumstellar environments in order to incorporate our new observational
results.

Table 5: CO band detection in the $K$-band and CO variability.

Object |  Obs Date |  $R$ |  CO Bands |  Ref.
---|---|---|---|---
LHA 120-S 12 |  1985-12-28 |  450 |  em |  McGregor et al. (1988a)
|  2004-12-17 |  50,000 |  em |  TW
|  2009-10-14 |  4500 |  em |  Liermann et al. (2010); Oksala et al. (2013)
|  2017-11-30 |  50,000 |  em |  TW
LHA 120-S 134 |  1985-12-29 |  450 |  em |  McGregor et al. (1988a)
|  2009-11-10 |  4500 |  em |  Oksala et al. (2013)
|  2017-11-30 |  50,000 |  em |  TW
$[$FMR2006] 15 |  2005-09-15 |  1000 |  weak abs |  Figer et al. (2006)
|  2006-05-05 |  17,000 |  em |  Davies et al. (2008)
|  2006-08-12 |  17,000 |  none |  Davies et al. (2008)
|  2019-05-12 |  5900 |  em |  TW
6 Cas |  1996-08-31 |  1800 |  none |  Vandenbussche et al. (2002)
|  2019-12-21 |  5100 |  none |  TW
V509 Cas |  1979-1980 |  32,000 |  em + abs (variable) |  Lambert et al. (1981)
|  1988 |  N.A. |  none |  Gorlova et al. (2006)
|  2003-11-20 |  300 |  none |  Gorlova et al. (2006)
|  2004-10-30 |  300 |  none |  Gorlova et al. (2006)
|  2019-12-21 |  5100 |  none |  TW
HD 179821 |  1989-07-14 |  1600 |  em |  Oudmaijer et al. (1995)
|  1990-09-26 |  760 |  em |  Hrivnak et al. (1994)
|  1991-11-04 |  330 |  em |  Hrivnak et al. (1994)
|  1992 August |  N.A. |  none |  Oudmaijer et al. (1995)
|  1997-04-19 |  1800 |  none |  Vandenbussche et al. (2002)
|  2000-10-18 |  N.A. |  none |  Gorlova et al. (2006)
|  2021-04-07 |  5900 |  abs |  TW

Note: em = emission; abs = absorption; TW = this work; N.A. = no information
available.

#### LHA 120-S 12 (= SK $-$67 23)

The object LHA 120-S 12 was first mentioned in the catalog of
H$\alpha$-emission stars in the Magellanic Clouds Henize (1956). Follow-up
observations recorded an intense IR excess due to hot dust Stahl et al.
(1984). The two-component wind associated with the star led to its
classification as B[e]SG Zickgraf et al. (1986), and the high degree of
intrinsic polarization suggested a high, but not fully edge-on, viewing angle
towards the object and its dusty disk Magalhaes (1992). Shell-line profiles
were seen in the NIR, consistent with the high inclination angle of the system
Oksala et al. (2013). Observations with the Spitzer Space Telescope revealed
silicate dust within the circumstellar disk, based on weak features in the
star’s mid-IR spectrum, but no IR nebulosity has been detected in association
with the object Kastner et al. (2010).

The first detection of CO band emission from LHA 120-S 12 was in 1985 based on
a low-resolution $K$-band spectrum McGregor et al. (1988a). This spectrum
clearly depicted four first-overtone band heads, similar to the observations
collected in 2009. Modeling of the latter Liermann et al. (2010) resulted in
similar parameters for the CO temperature and column density to those we found
from the high-resolution spectra taken before (2004) and afterwards (2017).
Moreover, we see no changes in the rotation velocity, projected to the line of
sight, within the time span of 13 years between our two observations.
Therefore, we believe that the CO-emitting ring revolving around LHA 120-S 12
is rather stable, with no detectable outflow or inflow.

A similar (projected) rotation velocity to that for CO can be inferred from
the double-peaked profiles of the [Caii] lines resolved in high-resolution
optical spectra of LHA 120-S 12 Aret et al. (2012), although a Gaussian
component with a higher value might be necessary to smooth out the sharp
synthetic double-peaked rotation profile in order to reproduce the shape of
the observed forbidden lines.

#### LHA 120-S 134 (= HD 38489, SK $-$69 259, MWC 126)

The object LHA 120-S 134 was listed in the catalog of B and A-type stars with
bright hydrogen lines published in 1933 Merrill and Burwell (1933) under the
Mount Wilson Catalogue (MWC) number 126 with classification as Beq. Its hybrid
spectra with narrow (equivalent widths of 30–50 km s-1) emission lines of
neutral and low-ionized metals in the optical spectral range Zickgraf et al.
(1986) and very broad P Cygni profiles (implying a wind terminal velocity of
$\sim$2300 km s-1) of high-ionized metals in the ultraviolet Shore and
Sanduleak (1983), along with the detected intense IR excess emission
characteristic of hot circumstellar dust Stahl et al. (1984), resulted in the
classification of the star as B[e]SG. A small inclination angle has been
proposed for LHA 120-S 134 Zickgraf et al. (1986), and the relatively low
measured degree of polarization supports a close to pole-on orientation of the
star plus disk system Magalhaes (1992).

LHA 120-S 134 is one of only two B[e]SGs showing broad emission from He ii
$\lambda$4686 Zickgraf et al. (1986). The other object is LHA 115-S 18 in the
SMC, which displays this (time variable) He ii line in emission in concert
with Raman-scattered O vi emission and TiO molecular emission features Torres
et al. (2012). While for LHA 115-S 18, it was proposed that the peculiar
spectral characteristics might point towards an LBV-like status of the star
Torres et al. (2012), a possible Wolf-Rayet companion was postulated to
explain the spectral features of LHA 120-S 134 Massey et al. (2014). Although
a solid proof for a Wolf-Rayet companion is still missing, LHA 120-S 134 has
appeared since then in the catalog of LMC Wolf-Rayet stars (WR 146).

The mid-IR spectrum of LHA 120-S 134, obtained with the Spitzer Space
Telescope, shows intense $10\,\upmu$m and weak $20\,\upmu$m emission features
of amorphous silicate dust, and a faint and wispy nebulosity around the IR
bright star was found with the telescope’s imaging facilities Kastner et al.
(2010). Follow-up optical imaging revealed that LHA 120-S 134 is located on
the northeast rim of the superbubble of DEM L269 and on the western rim of the
H ii region SGS LMC-2 Hung et al. (2021). Therefore, it is unclear if and how
much of the optical and IR nebulosity might be related to LHA 120-S 134
itself.

In the NIR regime, the first mention of CO first-overtone emission dates back
to 1985, when the star was observed with low-resolution McGregor et al.
(1988a). The next $K$-band spectrum was taken only 24 years later (see Table
5). The new spectrum had higher spectral resolution, but the CO bands appeared
to be similar to the previous detection. Our new, high-resolution spectrum was
acquired after another time gap of 8 years. Our modeling of the band spectrum
revealed basically the same CO parameters (temperature and column density) as
in 2009, with one addition, the projected rotational velocity. As for LHA
120-S 12, this velocity is comparable to the one that might be inferred from
the [Ca ii] lines Aret et al. (2012), and we may conclude that also LHA 120-S
134 is surrounded by a stable, rotating ring of atomic and molecular gas.

#### [FMR2006] 15 (= 2MASS J18375778-0652320)

The star [FMR2006] 15 was recorded as object number 15 in a survey of the cool
supergiant population of the massive young star cluster RSGC1 Figer et al.
(2006). Based on the weak CO absorption detected in its $K$-band spectrum, a
spectral type of G6 I was allocated. In a follow-up investigation, it was
proposed that [FMR2006] 15 is most likely a YHG, based on the star’s
luminosity and spectral similarity to $\rho$ Cas Davies et al. (2008). The
star was assigned an effective temperature of $6850\pm 350$ K and a luminosity
of $\log L/L_{\odot}=5.36^{+0.14}_{-0.16}$. With this temperature, the
spectral type of [FMR2006] 15 is more likely G0 ($\pm 2$ subtypes), and the
luminosity implies that the star lies with about $M_{\rm ini}\sim 25$ M⊙ in
the lower mass range of stars developing into YHGs (see Figure 1). This
relatively low initial mass was considered to be the reason for the lack of
detectable intense IR excess emission and of maser emission in contrast to the
high-mass YHGs such as, e.g., IRC +10420 Davies et al. (2008).

The first low-resolution $K$-band spectrum of [FMR2006] 15 was taken in
September 2005. At that time, very weak CO band absorption was seen Figer et
al. (2006). Soon thereafter, the object was re-observed twice with
considerably higher spectral resolution. In May 2006, the spectrum displayed
CO band emission and in August 2006, the emission had disappeared Davies et
al. (2008). Our detection of intense CO band emission 13 years later is a
clear indication that [FMR2006] 15 is embedded in circumstellar matter, even
if the molecular gas is with 3000 K much too hot for the condensation of dust
grains. When modeling the CO bands from [FMR2006] 15, we noticed that the CO
emission displays a blue-shift of $-77$ km s-1 with respect to the atmospheric
absorption line spectrum. A similar behavior has been seen in IRC +10420. This
star displays blue-shifted emission in its IR hydrogen recombination lines
Oudmaijer et al. (1994) and in its CO rotational transitions Oudmaijer et al.
(1996), which has been interpreted as emission formed in a close to pole-on
seen bipolar outflow with the central star eclipsing the receding part of the
emission. The outflow velocity from IRC +10420 has been measured to be on the
order of $\sim$40 km s-1, which is about half the value we measure for
[FMR2006] 15. To date, nothing is known about a possible orientation of
[FMR2006] 15, but we might speculate that a similar scenario as for IRC +10420
might hold for that object as well. In such a case, we would expect to have a
Gaussian-like distribution of the velocity (see bottom model in Figure 4).

On the other hand, if we consider the contribution from the rotation as real,
the blue-shifted CO gas might revolve around a hidden companion, as in the
case of the YHG HD 269953 in the LMC Kourniotis et al. (2022). In this object,
the companion was proposed to be surrounded by a gaseous disk traced by
numerous emission lines that display a time-variable radial velocity offset
with respect to the photospheric lines of the YHG star. If [FMR2006] 15 is
indeed a binary system, time-resolved K-band observations would be essential
to derive the orbital parameters and to characterize the hidden companion.

The best-fitting CO model has been subtracted from the $K$-band spectrum of
[FMR2006] 15, and the residual is shown in Figure 5. Also included in this
plot is a synthetic spectrum of a cool supergiant star with $T_{\rm eff}=6800$
K and $\log g=1.5$, similar to the values derived for [FMR2006] 15 Davies et
al. (2008). This spectrum has been computed with the spectrum synthesis code
Turbospectrum using MARCS atmospheric models Plez (2012). We note that the
main photospheric features are decently well represented by this model. The
short wavelength coverage of our spectrum and a possible alteration of the
intrinsic stellar spectrum caused by the absorbing circumstellar gas impede a
more decent classification of the object.

Figure 5: Residual spectrum of [FMR2006] 15 after subtraction of the blue-
shifted CO band emission. For illustration purposes, a synthetic model
spectrum of a cool supergiant is shown as well (shifted down along the flux
axis for better visualization) with parameters similar to those determined for
[FMR2006] 15.

#### 6 Cas (= V566 Cas, HR 9018, HD 223385)

The object 6 Cas was reported to be a spectroscopic binary based on its
composite spectrum in the ultraviolet Talavera and Gomez de Castro (1987).
While the main component is traced by the resonance lines of low-ionized
metals with terminal wind velocities of about 330 km s-1, typical for A-type
supergiants, the high-ionized resonance lines with P Cygni profiles and
terminal wind velocities of about 2400 km s-1 can be assigned to a
significantly hotter O-type companion. Disentangling the spectra in the
optical, to which the O star only very weakly contributes, resulted in the
classification of the system as a A3 Ia (A) and a O9.5 II (B) component Maíz
Apellániz et al. (2021). The positions of the two stars are separated by about
$1\mbox{$.\\!\\!^{\prime\prime}$}5$ at a position angle of $\sim$195∘. Whether
these two stars form indeed a bound binary system or whether they are just
close in projection could not be solved yet. The small radial velocity
variations detected for the A supergiant seem to be due to pulsations rather
than orbital motion in a binary system Maíz Apellániz et al. (2021).

The H$\alpha$ profile of 6 Cas resembles those seen in other hypergiants, such
as HD 33579 and Schulte 12. It is superimposed on broad electron scattering
wings Klochkova and Chentsov (2018). Discrete absorption components (DACs)
traveling through the broad absorption components of the wind profiles of H
and Fe ii lines were detected in 6 Cas Chentsov (1995). Based on the atypical
characteristics of a regular A-type supergiant, the star was assigned the
status of a hypergiant.

We are aware of only one former $K$-band spectrum of 6 Cas, which has been
taken with the ISO-SWS instrument Vandenbussche et al. (2002). This spectrum
shows no indication of CO bands, just the lines of the Pfund series in
absorption, similar to our spectrum (Figure 3). Such an otherwise featureless
spectrum is consistent with a hot ($T_{\rm eff}>8000$ K) star, in agreement
with its previous classification (see Table 1).

#### V509 Cas (= HR 8752, HD 217476)

The star V509 Cas is one of the YHGs for which an outburst was recorded based
on combined photometric and spectroscopic monitoring of the star
Nieuwenhuijzen et al. (2012). This outburst must have taken place around 1973,
after a preceding 16-year-long period of reddening and cooling of the object
from about 5000 K down to about 4000 K Luck (1975). Thereafter, the effective
temperature of the star gradually increased until it reached a value of about
8000 K around 2001 Nieuwenhuijzen et al. (2012), where it has stabilized since
then Aret et al. (2017). During the “heating” period, several short-term drops
in temperature were recorded, which were associated with phases of enhanced
mass loss Nieuwenhuijzen et al. (2012).

Despite the outburst and the multiple mass-loss events, no extended nebulosity
was detected at optical wavelengths so far Schuster et al. (2006).
Nevertheless, the star must be embedded in an ionized circumstellar envelope
traced by thermal emission at radio wavelengths Smolinski et al. (1977); Higgs
et al. (1978). The ionization of this envelope is most likely performed by the
radiation field of a distant, hot main-sequence B1-type companion Stickland
and Harmer (1978). The envelope is also the place where other emission lines
are formed. Most prominent are the nebular lines of [N ii] $\lambda\lambda$
6548,6583 Sargent (1965), but also those of [O i] $\lambda\lambda$ 6300,6364
and [Ca ii] $\lambda\lambda$ 7291,7324 were identified Aret et al. (2017),
which were proposed to trace a possible Keplerian disk or ring based on the
double-peaked profiles, particularly of the [Ca ii] lines, for which a
rotation velocity, projected to the line of sight, of about 40 km s-1 was
derived Aret et al. (2017). Support for such an interpretation comes from an
optical spectroscopic monitoring of V509 Cas between 2015 and 2022. The
observations revealed that the [Ca ii] lines are stable in position and shape
over the observing period of $\sim$7 years, in contrast to the photospheric
absorption lines, whose shape and radial velocity are strongly influenced by
the pulsation activity of the star Kasikov et al. (2022).

In the NIR, observations dating back to 1979–1980 detected CO features
displaying both emission and absorption components with variable strength (see
Table 5, Lambert et al. (1981)). Comparison with the brightness curve revealed
that the CO emission was strongest when the star was around maximum brightness
Gorlova et al. (2006). Furthermore, the CO emission appeared at the stellar
systemic velocity, whereas the absorption was either blue- or red-shifted.
This behavior is very similar to what has been observed for the YHG star
$\rho$ Cas Gorlova et al. (2006). Since about 1988, the CO features have
disappeared from the NIR spectra of V509 Cas, and our spectrum also shows no
traces of molecular emission. The absorption lines of the hydrogen Pfund
series seen in our otherwise featureless $K$-band spectrum are in agreement
with a stellar temperature of about 8000 K.

#### HD 179821 (= RAFGL 2343, IRAS 19114+0002, V1427 Aql)

The star is embedded within a detached, almost spherical shell of cold
(120–140 K) dust Jura and Werner (1999) responsible for an intense far-IR
excess emission van der Veen et al. (1989). In the radio regime, CO
($J=1\longrightarrow 0$) emission has been detected, which traces a cold
molecular outflow with a velocity of $\sim$33–35 km s-1 Zuckerman and Dyck
(1986). The object is oxygen-rich, as is inferred from its OH maser emission
Likkel (1989).

The evolutionary state of the star is highly debated in the literature due to
its uncertain distance value. Distance estimates range from about 1.5 kpc to
about 6 kpc, which would classify the star as either a post-asymptotic giant
branch star or a YHG. Considering the newest parallax measurements of
$0.1893\pm 0.0206$ provided by the GAIA Early Data Release 3 Gaia
Collaboration (2020), a distance closer to the upper value seems to be more
likely. Such a high value is also in agreement with the star’s kinematic
distance derived from its large heliocentric systemic velocity of 84–88 km s-1
Zuckerman and Dyck (1986); Likkel (1989); Kipper (2008).

The spectral classification of HD 179821 has been rather controversial as well
in the past decades ranging from F3–5 Zacs et al. (1996); Reddy and Hrivnak
(1999); Kipper (2008), over G5 Hrivnak et al. (1989) to K4 Odenwald (1986).
The value for $\log g$, derived from high-resolution spectroscopy, ranges
around $0.5\pm 0.5$. Such a low $\log g$ value assigns the star a luminosity
class I.

Extinction values obtained for HD 179821 range from $A_{V}=2.0$ Hrivnak et al.
(1989) over $3.1$ Arkhipova et al. (2001) to $\sim$4 Reddy and Hrivnak (1999),
and it has been suggested that the total extinction might be variable due to
possible changes in the circumstellar contribution from the dust shell
Arkhipova et al. (2009).

In a recent work, data from long-term photometric and spectroscopic monitoring
have been presented Ikonnikova et al. (2018). The colors imply that the star
first became bluer between 1990 and 1995 and has displayed a systematic
reddening since 2002. The fastest change in color took place between 2013 and
2017, with a simultaneous brightening of the star. The spectra confirm this
trend and record a more or less stable temperature of $T_{\rm eff}=6800\pm
100$ K between 1994 and 2008, in agreement with previous temperature
determinations We note that a higher effective temperature of $\sim 7350$ K
was proposed in the same period Şahin et al. (2016). Zacs et al. (1996); Reddy
and Hrivnak (1999); Kipper (2008); Ikonnikova et al. (2018). Since then, it
decreased and reached a value of about 5900 K in 2017 Ikonnikova et al.
(2018). The reddening and change in temperature indicate the onset of a
possible new red loop evolution of HD 179821, i.e., an excursion to the cool
edge of the HR diagram, related to a significant increase in the stellar
radius and an increase in mass loss.

In the NIR, HD 179821 displayed CO-band emission, during observations taken
between 1989 and 1991 (see Table 5), whereas no indication for CO band
features (neither emission nor absorption) was seen between 1992 and 2000. The
presence and disappearance of CO band emission might indicate a prior phase of
higher mass loss or some mass ejection episode followed by the subsequent
expansion and dilution of the released circumstellar material. Such a scenario
might be supported by the redder color of the star around 1990 Ikonnikova et
al. (2018). The absence of CO bands in the spectra between 1992 and 2000
supports the classification of the star as early to mid F-type, because stars
in this temperature range are too hot for the formation of molecules in their
atmospheres.

In contrast to all previous NIR observations, our data from 2021 clearly
display CO band absorption. One possible explanation could be that the trend
of cooling has continued since 2017\. When comparing our observed spectra with
synthetic spectra, we found that the intensity of the first band head of CO
can be achieved for a stellar effective temperature of $\sim$5400 K (see
Figure 6), although the entire CO band structure signals a considerably cooler
temperature for the molecular gas due to the only weakly pronounced higher
band heads. Hotter stars display less intense and cooler stars more intense CO
bands. However, a star with a temperature of $5400$ K should show
significantly stronger absorption in all other atomic photospheric lines,
which is not the case. Instead, the intensity and specific line ratios in the
$K$-band spectrum are more in line with an effective temperature of about 6600
K. But such a hot stellar photosphere contains no CO molecular absorption
features (Figure 6). Based on this discrepancy, we believe that our $K$-band
spectrum is composite. It shows a hotter stellar photosphere along with CO
absorption formed in a presumably cooler gas shell or outflow. Support for
such a scenario is provided by the fact that the CO absorption bands display a
blue-shift of about $-43\pm 1$ km s-1 with respect to all other photospheric
atomic lines and the circumstellar emission lines, such as the Na i doublet.
If interpreted as outflow velocity, then this value is comparable to the
outflow seen in IRC +10420 Oudmaijer et al. (1994, 1996) but is slightly
higher than the isotropic expanding cold molecular gas and dust shell around
HD 179821 ($\sim$35 km s-1, Zuckerman and Dyck (1986); Castro-Carrizo et al.
(2007)). We exclude a cool binary component as an explanation for the
velocity-shifted CO absorption bands because, in this case, the cool companion
would imprint (besides CO) significantly stronger blue-shifted absorption
lines onto the $K$-band spectrum, which are not observed. If the proposed
scenario of a new outflowing shell or gas layer is correct, possibly initiated
during the reddening of the star and the increase in stellar radius recorded
in the years 2014–2017 Ikonnikova et al. (2018), or a possible outburst event
that might have followed this reddening as in the case of V509 Cas (as we
mentioned before, see Nieuwenhuijzen et al. (2012); Luck (1975)), then we may
speculate that with further expansion of this matter, future $K$-band
observations will display CO bands in emission before the material dilutes and
the CO features might disappear again.

Besides CO, our spectrum of HD 179821 also shows Br $\gamma$ in absorption and
intense emission of the Nai doublet. The line of Br $\gamma$ was in absorption
in previous observations taken in 1989 Oudmaijer et al. (1995) and 1990
Hrivnak et al. (1994). The latter spectrum also displays intense emission of
the (blended) Nai doublet, as well as the spectrum taken in 2000 Gorlova et
al. (2006). The remaining previous spectra either do not cover the spectral
region of Br $\gamma$ and/or the Nai doublet, or these lines have not been
mentioned by the corresponding authors.

Figure 6: Comparison of the $K$-band spectrum of HD 179821 (top) with
synthetic spectra for effective temperatures of 5400 K (middle) and 6600 K
(bottom). For illustration purposes, the synthetic model spectra are included
in this figure and shifted down along the flux axis for better visualization.

Na i emission is a clear indicator for circumstellar material. It has been
reported from the NIR spectra of numerous evolved massive stars: (i) the YHGs
$\rho$ Cas, V509 Cas, Hen 3-1379 (the Fried Egg Nebula), IRC +10420 Lambert et
al. (1981); Koumpia et al. (2022), and the IRC +10420 analog IRAS 18357-0604
Clark et al. (2014), (ii) most of the B[e]SGs McGregor et al. (1988a); Oksala
et al. (2013); Arias et al. (2021), and (iii) also many luminous blue
variables Oksala et al. (2013); Kourniotis et al. (2017). The equivalent
widths of the intense Na i lines from the YHG IRC +10420 Oudmaijer and de Wit
(2013) are about three times higher than for HD 179821, for which we measured
values of $-1.0670\pm 0.015$ Å and $-0.9231\pm 0.018$ Å. Recent spatially
resolved observations revealed that the Na i emission in IRC +10420 and Hen
3-1379 is confined within a compact spherical envelope around the star Koumpia
et al. (2022, 2020). The emission lines in our spectrum are symmetric, and
their wavelengths coincide with the systemic velocity of the star. While their
formation region can be a compact spherical shell as well, they cannot be
related to the possible new blue-shifted outflow traced by the CO band
absorption.

##  5 Conclusions

We present new medium- and high-resolution $K$-band spectra for two B[e]SGs
and four YHGs. The spectra of both B[e]SGs show rotationally broadened CO band
emission, from which we could derive, for the first time, the projected
rotation velocity of the CO gas for both stars. On the other hand, our model
parameters for the CO temperature and column density are very similar to those
reported in previous studies based on spectra with significantly lower
resolution Liermann et al. (2010); Oksala et al. (2013). The similarities of
the detected CO band features over more than 30 years suggest that the CO
emitting gas rings around these two B[e]SGs are stable structures, neatly
fitting to the findings of most of the B[e]SGs.

With respect to the YHGs, we detect CO band emission from only one star, the
highly reddened cluster member [FMR2006] 15, which previously showed time-
variable CO features (see Table 5), and which has an effective temperature
that is clearly too high to form molecules within its atmosphere.
Consequently, the CO emission must be of circumstellar origin. A second
object, HD 179821, shows CO bands in absorption, while it had CO emission
during 1989–1991 but lacked any CO features in its spectrum since 1992 (Table
5). The latter is consistent with the star’s high effective temperature, which
prevents the formation of molecules. For both YHGs, our detected CO features
are clearly blue-shifted with respect to the photospheric absorption lines,
suggesting that both stars most likely had recent mass ejection events and the
CO emission/absorption forms within the expelled matter. For [FMR2006] 15, we
propose that the blue-shifted emission arises in a possible pole-on seen
bipolar outflow as in the case of the YHG star IRC +10420 Oudmaijer and de Wit
(2013), but nothing can be said yet about the geometry of the outflow seen
from HD 179821 because the (highly inflated) star itself might still block
large portions of the possibly receding parts of the ejecta.

Conceptualization, M.K. (Michaela Kraus), M.K. (Michalis Kourniotis) and
D.H.N.; methodology, M.K. (Michaela Kraus), M.K. (Michalis Kourniotis), M.L.A.
and A.F.T.; formal analysis, M.K. (Michaela Kraus) and M.K. (Michalis
Kourniotis); investigation, M.K. (Michaela Kraus), M.K. (Michalis Kourniotis)
and D.H.N.; resources, M.K. (Michaela Kraus), M.K. (Michalis Kourniotis),
M.L.A. and A.F.T.; writing—original draft preparation, review and editing,
M.K. (Michaela Kraus), M.K. (Michalis Kourniotis), M.L.A., A.F.T. and D.H.N.;
visualization, M.K. (Michaela Kraus) and M.K. (Michalis Kourniotis); funding
acquisition, M.K. (Michaela Kraus), M.L.A. and A.F.T. All authors have read
and agreed to the published version of the manuscript.

This research was funded by the Czech Science foundation (GA ČR, grant number
20-00150S), by CONICET (PIP 1337), by the Universidad Nacional de La Plata
(Programa de Incentivos 11/G160), Argentina, and by the European Union’s
Framework Programme for Research and Innovation Horizon 2020 (2014-2020) under
the Marie Skłodowska-Curie Grant Agreement No. 823734. The Astronomical
Institute of the Czech Academy of Sciences is supported by the project
RVO:67985815.

Not applicable.

Not applicable.

The data underlying this article will be shared on reasonable request to the
corresponding author.

###### Acknowledgements.

We thank the anonymous referees for their valuable comments and suggestions.
This research made use of the NASA Astrophysics Data System (ADS) and of the
SIMBAD database, operated at CDS, Strasbourg, France. This paper is based on
observations obtained with the Phoenix infrared spectrograph, developed and
operated by the National Optical Astronomy Observatory and based on
observations obtained at the international Gemini Observatory, a program of
NSF’s NOIRLab, which is managed by the Association of Universities for
Research in Astronomy (AURA) under a cooperative agreement with the National
Science Foundation on behalf of the Gemini Observatory partnership: the
National Science Foundation (United States), National Research Council
(Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio
de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência,
Tecnologia Inovações e Comunicações (Brazil), and Korea Astronomy and Space
Science Institute (Republic of Korea) under program IDs GS-2004B-Q-54,
GS-2017B-Q-32, GN-2019A-Q-204, GN-2019B-Q-418, and GN-2021A-Q-315. The authors
declare no conflict of interest. Abbreviations The following abbreviations are
used in this manuscript:
B[e]SG | B[e] supergiant
---|---
NIR | Near infrared
LMC | Large Magellanic Cloud
SMC | Small Magellanic Cloud
YHG | Yellow hypergiant
[custom] References

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