# Decade-long time-monitoring of candidate Luminous Blue Variable Stars in the
two very metal-deficient compact dwarf galaxies DDO 68 and PHL 293B

N. G. Guseva1, T. X. Thuan2,3 and Y. I. Izotov1
1Bogolyubov Institute for Theoretical Physics, Ukrainian National Academy of
Sciences, Metrologichna 14b, Kyiv 03143, Ukraine,
<EMAIL_ADDRESS><EMAIL_ADDRESS>
2Astronomy Department, University of Virginia, P.O. Box 400325,
Charlottesville, VA 22904-4325,
<EMAIL_ADDRESS>
3Institut d’Astrophysique de Paris (UMR 7095 CNRS & Sorbonne Université) 98
bis Bd Arago, F-75014 Paris France

###### Abstract

We have studied the spectral time variations of candidate luminous blue
variable stars (cLBV) in two low-metallicity blue compact dwarf galaxies, DDO
68 and PHL 293B. The LBV in DDO 68, located in H ii region #3, shows an
outburst, with an increase of more than 1000 times in H$\alpha$ luminosity
during the period 2008–2010. The broad emission of the H i and He i lines
display a P Cygni profile, with a relatively constant terminal velocity of
$\sim$800 km/s, reaching a maximum luminosity $L$(H$\alpha$) of $\sim$
2$\times$1038 erg/s, with a FWHM of $\sim$1000–1200 km/s. On the other hand,
since the discovery of a cLBV in 2001 in PHL 293B, the fluxes of the broad
components and the broad-to-narrow flux ratios of the H i and He i emission
lines in this galaxy have remained nearly constant over 16 years, with small
variations. The luminosity of the broad H$\alpha$ component varies between
$\sim$2$\times$1038 erg/s and $\sim$1039 erg/s, with the FWHM varying in the
range $\sim$500–1500 km/s. Unusually persistent P Cygni features are clearly
visible until the end of 2020 despite a decrease of the broad-to-narrow flux
ratio in the most recent years. A terminal velocity of $\sim$800 km/s is
measured from the P Cygni profile, similar to the one in DDO 68, although the
latter is 3.7 more metal-deficient than PHL 293B. The relative constancy of
the broad H$\alpha$ luminosity in PHL 293B suggests that it is due to a long-
lived stellar transient of type LBV/SN IIn.

###### keywords:

galaxies: dwarf – galaxies: starburst – galaxies: ISM – galaxies: abundances.

††pagerange: Decade-long time-monitoring of candidate Luminous Blue Variable
Stars in the two very metal-deficient compact dwarf galaxies DDO 68 and PHL
293B–References††pubyear: 2022

##  1 Introduction

During their evolution, massive stars are known to go through a very short and
high luminosity important transitional phase, called the luminous blue
variable (LBV) phase. During this phase, LBVs undergo significant variations
in photometric magnitudes and spectral features, characterized in particular
by the appearance of broad components in the hydrogen and helium emission
lines and in some heavy element ion lines in the UV and optical ranges. The
broad emission in the LBVs may be due to sharp eruptions of the massive stars,
reaching a total mass loss of up to $\sim$30 – 100 M⊙. It can also be caused
by stellar winds and expanding dense circumstellar envelopes. In these cases,
H$\alpha$ luminosities are in the range of 1036 – 1039 ergs s-1 (Izotov et
al., 2007). The mass loss rate of hydrogen-rich layers through stellar winds
is $\sim$10-6 – $\sim$10-3M⊙ yr-1 (Humphreys & Davidson, 1994; Smith et al.,
1994; Drissen et al., 1997, 2001). However, very strong broad emission (FWHM
$>$ 1000 km s-1) can also be present in the spectra of objects other than
massive stars, such as Type IIn Supernovae (SNe) and Active Galactic Nuclei
(AGNs). In these objects, the luminosities of the broad H$\alpha$ component
are larger and can reach values up to 1040 – 1042 ergs s-1 (Izotov et al.,
2007; Sobral et al., 2020; Kokubo, 2021; Burke et al., 2021).

LBVs frequently show recurring eruptive events through various evolution
phases, during their transition from young massive main-sequence stars to WR
stars, SN explosions or massive black holes (BHs). It is believed that stars
with masses greater than 20 – 30 M⊙ and luminosities $L$ $\sim$
103–106$L_{\odot}$ go through the LBV phase (Crowther, 2007; Solovyeva et al.,
2020). Among all types of variable stars, only LBV stars show significant
variability both in photometric brightness and spectroscopic features: rapidly
amplified broad emission and blueward absorption lines, strongly enhanced
continuum that becomes bluer in the UV and optical spectra.

To date, about a few hundreds LBVs and candidate LBVs (cLBVs) are known to
show irregular cyclic quasi-periodic brightness variations of $\sim$0.5 - 2
mag on timescales from several years to decades. They are called S Dor LBVs
(see e.g. Massey et al., 2000; Humphreys et al., 2013, 2017; Humphreys, 2019;
Grassitelli et al., 2020; Weis & Bomans, 2020). On the other hand, there
exists a tiny number of LBV stars which show giant eruptions, with amplitudes
more than 2.5 - 3 mag, on timescales of up to thousands years (Davidson &
Humphreys, 1997; Smith et al., 2011; Vink, 2012; Weis & Bomans, 2020). Well-
known prototypes of this category are $\eta$ Carinae and P Cygni with
luminosities $\sim$1040 ergs s-1 (Lamers et al., 1983; Davidson, 1999). In
some cases, the peak luminosity during the outbursts can reach $\sim$1042 ergs
s-1 (Kokubo, 2021). Nearly all these known luminous LBVs are either in our
Galaxy or in nearby galaxies.

High intensity broad and very broad components of emission lines, with P Cygni
profiles, have also been observed in the integral spectra of star-forming
galaxies (SFGs) underlying their strong narrow emission lines produced in H ii
regions (see e.g. Schaerer et al., 1999; Guseva et al., 2000; Leitherer et
al., 2001; Izotov & Thuan, 2008; Guseva et al., 2012). The most prominent
spectral features in SFGs with LBVs are broad components of hydrogen and often
helium lines with blueward absorption, and Fe ii emission.

To understand the physical mechanism responsible for the broad emission, time-
monitoring of the broad spectral features is necessary. The reason is that, as
said previously, broad emission occurs not only in LBV spectra but also in
those of SNe and AGNs. It is difficult to distinguish between these different
possibilities without a long-term time monitoring of the broad features. It
has now been established that a significant number of the objects detected in
supernova surveys are not true supernovae, but belong to a category of objects
called “supernova impostors”. Ordinary LBVs of the S Dor type or LBVs with
giant eruptions, like $\eta$ Car at maximum luminosity, appear among these
”impostors”. Despite the fact that many stars with LBV features have been
discovered (Weis & Bomans, 2020), only a few dozen of them are confirmed as
genuine Galactic and extragalactic LBVs (Wofford et al., 2020). The remaining
are cLBV which require time-monitoring to confirm their true nature. A genuine
LBV would show a significant enhancement of the spectral and photometric
features on a time scale of tens of years, followed by the disappearance of
these features. This disappearance is necessary to rule out SNe, AGNs or other
physical mechanisms (Kokubo, 2021).

In addition, it is of particular interest to understand how LBV evolution in
SFGs depends on the properties of the host galaxy, such as gas metallicity,
interstellar medium density, star formation rate (SFR) and specific SFR
(sSFR). Only very few LBVs are known up to date in metal-poor SFGs with strong
star-forming activity (see e.g. Weis & Bomans, 2020, and references therein).
We discuss in this paper the time monitoring over two decades of the
photometric and spectroscopic properties of cLBV stars in two extremely metal-
deficient dwarf star-forming galaxies (SFG), DDO 68 (located in H ii region
#3) with 12 + log (O/H) = 7.15 and PHL 293B with 12 + log (O/H) = 7.72. These
two SFGs are the lowest-metallicity galaxies where LBV stars have been
detected, allowing the study of the LBV phenomenon in the extremely low
metallicity regime, and shedding light of the evolution of the first
generation of massive stars possibly born from primordial gas.

The paper is structured as follows: the LBT/MODS and 3.5m APO observations and
data reduction are described in Sect. 2. In Sect. 3 we present the study of
multi-epoch optical spectra of DDO 68 #3 and PHL 293B. Finally, in Sect. 4 we
summarize our main results.

Table 1: Journal of observations  Date |  | exposure | slit width | airmass
---|---|---|---|---
|  | (seconds) | (arcsec) |
DDO 68 #3, 3.5m APO
2008-10-27 |  | 2700 | 1.5 | 1.24
2008-11-06 |  | 2400 | 1.5 | 1.17
2009-02-22 |  | 1800 | 2.0 | 1.53
2009-11-19 |  | 1800 | 2.0 | 1.18
2010-02-06 |  | 2700 | 1.5 | 1.36
2010-03-20 |  | 1800 | 1.5 | 1.16
2010-10.31 |  | 1500 | 1.5 | 1.13
2012-02-15 |  | 2700 | 1.5 | 1.20
2012-05-16 |  | 1800 | 1.5 | 1.27
2013-06-01 |  | 986 | 1.5 | 1.42
2016-04-09 |  | 1800 | 0.9 | 1.51
2018-04-07 |  | 1860 | 1.5 | 1.06
PHL 293B, 3.5m APO
2010-10-06 |  | 1800 | 1.5 | 1.56
2014-11.17 |  | 2700 | 0.9 | 1.19
2015-11-06 |  | 1800 | 0.9 | 1.21
2017-12-15 |  | 1800 | 0.9 | 1.27
PHL 293B, 2$\times$8.4m LBT/MODS
2020-11-18 |  | 2700 | 1.2 | 1.19
Table 2: Parameters of H$\alpha$ narrow and broad components in DDO 68 #3
spectra, observed with the 3.5m APO telescope and presented in Fig. 1

Date |  $F_{nar}$a | FWHMnarb | FWHMc |  $F_{br}$a | FWHMbrb |  $v$termd |  $L_{br}$e
---|---|---|---|---|---|---|---
|  |  | ([O iii]4959) |  |  |  |
2008-10-27 | 64.26 | 115(2.51) | 2.42 | 84.01 | 1072 | 706 | 16.2
2008-11-06 | 59.32 | 79(1.74) | 2.00 | 66.86 | 1029 | 722 | 12.9
2009-02-22 | 47.13 | 88(1.92) | 2.33 | 74.41 | 1006 | 764 | 14.4
2009-11-19 | 56.61 | 87(1.91) | 2.16 | 93.66 | 1110 | 808 | 18.1
2010-02-06 | 27.58 | 78(1.72) | 1.94 | 30.94 | 1251 | 749 | 6.0
2010-03-20 | 34.85 | 78(1.72) | 2.15 | 49.48 | 1107 | 819 | 9.6
2010-10-31 | 56.30 | 78(1.71) | 2.55 | 73.84 | 1130 | 871 | 14.3
2012-02-15 | 48.22 | 69(1.52) | 1.86 | 29.10 | 742 | … | 5.6
2012-05-16 | 73.16 | 78(1.72) | 1.80 | 31.87 | 734 | … | 6.2
2013-06-01 | 45.34 | 74(1.61) | 1.61 | 35.82 | 560 | … | 6.9
2016-04-09 | 13.43 | 63(1.39) | 1.54 | 2.44 | 326 | … | 0.5
2018-04-07 | 8.48 | 86(1.87) | 1.63 | … | … | … | 0.0

aFluxes of H$\alpha$ narrow ($F_{nar}$) and broad ($F_{br}$) components in
units of 10-16 erg s-1 cm-2.
---
bFull widths at half maximum (FWHM) of H$\alpha$ narrow and broad components
in units of km s-1, and in
angstroms (in brackets).
cFWHM of [O iii]$\lambda$4959Å emission line in units of angstroms.
dTerminal velocity in units of km s-1.
eLuminosity of H$\alpha$ broad component in units of 1037 erg s-1, calculated
with a distance $D$=12.71 Mpc
which is a mean from determinations by Cannon et al. (2014); Sacchi et al.
(2016); Makarov et al. (2017).

Table 3: Parameters of H$\alpha$ narrow and broad components in PHL 293B
spectra, presented in Fig. 3

Date | tel.a | $F^{\rm b}$ | FWHMc | FWHMd | $F^{\rm b}$ | FWHMc | $F^{\rm b}$ | FWHMc | $F^{\rm e}$ |  $v$termf | $L^{\rm g}$
---|---|---|---|---|---|---|---|---|---|---|---
|  | nar | nar | 4959 | br | br | v.br | v.br | sum br | br | sum br
2001-08-22 | SDSS | 238.02 | 235(5.1) | 3.8 | 63.74 | 1379 | 7.86 | 2602 | 71.60 | 938 | 44.2
2010-10-06 | APO | 261.50 | 71(1.6) | 2.2 | 27.13 | 181 | 42.99 | 999 | 70.12 | 702 | 43.2
2014-11-17 | APO | 71.04 | 66(1.4) | 1.6 | 11.77 | 162 | 12.62 | 627 | 24.39 | … | 15.0
2015-11-06 | APO | 471.20 | 74(1.6) | 1.6 | 45.96 | 180 | 95.08 | 705 | 141.04 | … | 87.0
2017-12-15 | APO | 91.16 | 69(1.5) | 1.6 | 9.64 | 165 | 15.02 | 903 | 24.66 | … | 15.2
2020-11-18 | LBT | 401.30 | 280(6.1) | 4.0 | 23.08 | 504 | 11.64 | 1320 | 34.72 | … | 21.4

atelescopes: 2.5m APO (labelled as SDSS), 3.5m APO (labelled as APO),
2$\times$8.4m LBT (labelled as LBT).
---
bFlux of narrow ($F_{nar}$), broad ($F_{br}$) and very broad ($F_{v.br}$)
components of H$\alpha$ in units of 10-16 erg s-1 cm-2.
cFull width at half maximum (FWHM) of narrow, broad and very broad components
of H$\alpha$ in km s-1 and in angstroms
(in brackets).
dFWHM of the [O iii]$\lambda$4959Å emission line in angstroms.
eFlux of the whole broad bump (i.e. sum of the broad and very broad
components) in units of 10-16 erg s-1 cm-2.
fTerminal velocity in units of km s-1.
gLuminosity of the whole broad bump (i.e. the sum of broad and very broad
components) in 1037 erg s-1, calculated with the
distance $D$=22.7 Mpc of PHL 293B, the same as that adopted by Izotov & Thuan
(2009).

Figure 1: The rest-frame H$\alpha$ emission line profile in the DDO 68 #3
spectrum at different epochs, observed with the 3.5m APO telescope.
Wavelengths are in Å and fluxes are in units of 10-16 erg s-1 cm-2 Å-1.

Figure 2: The most time-separated spectra of PHL 293B. Panels a) and b) show
blue and red parts of the SDSS spectrum obtained with the 2.5m APO telescope
on August 22, 2001. Panels c) and d) show the spectrum obtained with LBT/MODS
on November 18, 2018. Absorption features of P Cygni profiles for hydrogen and
helium emission lines are indicated in the rest-frame spectra by red and blue
arrows, respectively. Wavelengths are in Å and fluxes are in units of 10-16
erg s-1 cm-2 Å-1.

Figure 3: The rest-frame H$\alpha$ emission line profiles in the PHL 293B
spectra at the different epochs listed in Table 3. Wavelengths are in Å and
fluxes are in units of 10-16 erg s-1 cm-2 Å-1.

Figure 4: Decomposition by Gaussians of the H$\alpha$ narrow (blue dotted
lines) and broad emission (blue solid lines) profiles in the spectra of DDO 68
#3, listed in Table 2 and shown in Fig. 1. The observed profile is shown by
the black solid line whereas the summed flux of narrow+broad components is
represented by the red solid line. Wavelengths are in Å and fluxes are in
units of 10-16 erg s-1 cm-2 Å-1.

Figure 5: The observed profile of H$\alpha$ in DDO 68 #3 with an excess flux
at high velocities in redward wings. This excess is seen only for the
observations on 15 February and 16 May 2012 (see Fig. 4h and Fig. 4i). Three
Gaussians were used to fit the narrow, broad and very broad components (blue
dots, turquoise solid and blue solid lines, respectively). The total fitted
profile is represented by a red solid line. Wavelengths are in Å and fluxes
are in units of 10-16 erg s-1 cm-2 Å-1.

Figure 6: The H$\alpha$ profiles in the PHL 293B spectra obtained with
different telescopes during different epochs. The black lines represent the
observed spectra. The fits of H$\alpha$ by three or four Gaussians are shown
by blue dotted lines (narrow component), by turquoise solid lines (broad
component), by blue solid lines (very broad component) and by magenta dashed
lines (for absorption features of P Cygni profiles whenever possible). Note
that, in contrast to Fig. 4 and Fig. 5, the y-axes have different scales, to
better see the shape of the broad components. Fluxes are in units of 10-16 erg
s-1 cm-2 Å-1 and wavelengths are in Å.

Figure 7: a) Temporal variations of the flux ratio of broad-to-narrow
H$\alpha$ components in DDO 68 #3. The black solid line shows the APO
observations discussed here while the blue parts of the curve indicate Special
Astrophysical Observatory (SAO) and APO observations by Pustilnik et al.
(2005, 2008) (dotted lines) and by Izotov & Thuan (2009) (solid line). The
temporal variations of the same flux ratio in PHL 293B is shown by the red
dashed line. Note that the total flux of the whole broad bump (i.e. the sum of
the broad and very broad emission components, and of the absorption component
when appropriate) was taken to be the broad component when calculating this
broad-to-narrow ratio. b) Temporal variations of H$\alpha$ fluxes in different
components for DDO 68 #3. The thick black line shows the APO observations
discussed here while the thin black line represents data by Pustilnik et al.
(2005, 2008) and Izotov & Thuan (2009). The colored lines show the data for
PHL 293B.

##  2 OBSERVATIONS AND DATA REDUCTION

Over the course of more than a decade, starting in October 2008, we have
obtained a series of spectroscopic observations for the two SFGs DDO 68 (its H
ii region #3) and PHL 293B, using two different telescopes and instrumental
setups.

###  2.1 APO Observations

The first instrumental setup consisted of the Dual Imaging Spectrograph (DIS)
mounted on the ARC 3.5m Apache Point Observatory (APO) telescope. The blue and
red channels of the DIS permitted to simultaneously observe the objects over a
wide range of wavelengths. A long slit giving medium resolution ($R$ = 5000)
was used during all APO observations. The B1200 grating giving medium
resolution ($R$ = 5000) with a central wavelength of $\sim$4800 Å and a linear
dispersion of 0.62 Å pixel-1 was used in the blue range. In the red range, we
employ the R1200 grating with a central wavelength of $\sim$6600 Å and a
linear dispersion of 0.56 Å pixel-1. In this way, APO medium resolution
spectra of the two SFGs DDO 68 #3 and PHL 293B were obtained, that span two
wavelength ranges, from $\sim$4150 to 5400 Å in the blue range, and from
$\sim$6000 to 7200Å in the red range.

The journal of the APO observations, giving the observation dates, the
exposure times, the slit widths and the airmasses, are shown in Table 1.

###  2.2 LBT Observations

For PHL 293B, one high signal-to-noise ratio spectroscopic observation was
also carried out with the 2$\times$8.4m Large Binocular Telescope (LBT). We
employ the LBT in the binocular mode utilizing both the MODS1 and MODS2
spectrographs, equipped with 8022$\times$3088 pixel CCDs. Two gratings, one in
the blue range, G400L, with a dispersion of 0.5Å pixel-1, and another in the
red range, G670L, with a dispersion of 0.8Å pixel-1, were used. The PHL 293B
spectrum was obtained in the wavelength range $\sim$3150 – 9500 Å with a
60$\times$1.2 arcsec slit, giving a resolving power $R$ $\sim$ 2000\. The
seeing during the observation was in the range 0.5 – 0.6 arcsec. Three
subexposures were derived, resulting in an effective exposure time of
2$\times$2700 s when adding the fluxes obtained with both spectrographs, MODS1
and MODS2. The spectrum of the spectrophotometric standard star GD 71,
obtained during the LBT observation with a 5 arcsec wide slit, was used for
flux calibration. It was also used to correct the red part of the PHL 293B
spectrum for telluric absorption lines. The journal of the LBT observation is
also shown in Table 1.

###  2.3 Data reduction

The two-dimensional APO spectra were bias- and flat-field corrected, fixed for
distortion and tilt of frame and background subtracted using iraf routines.
For the LBT observation, the MODS Basic CCD Reduction package modsCCDRed
(Pogge, 2019) was used for flat field correction and bias subtraction.
Wavelength calibrations of both LBT and APO observations were performed using
spectra of comparison lamps obtained every night before and after the
observations. Each two-dimensional spectrum was aligned along the brightest
part of the galaxy. After wavelength and flux calibration and removal of
cosmic particle trails, all subexposures were summed. One-dimensional spectra
were then extracted along the spatial axis so that the entire bright part of
the H ii region falls into the selected aperture.

In summary, we have obtained twelve new APO observations of DDO 68 #3 and four
new APO observations of PHL 293B, together with one new LBT spectrum of PHL
293B. To increase the time baseline for PHL 293B, we have also included in our
analysis the Sloan Digital Sky Survey (SDSS) spectrum obtained with the 2.5m
APO telescope on 22 August 2001 and available in the SDSS archive. This brings
the total number of PHL 293B spectra to be analyzed to six. More details are
given in Tables 2 and 3. The spectra are shown in Fig. 1 and Fig. 3,
respectively. All spectra are displayed in the wavelength range around
H$\alpha$ to better emphasize the temporal changes of this line. In Fig. 2 we
have also shown two spectra of PHL 293B over the whole wavelength range of
observations, and that are most separated in time.

##  3 RESULTS

###  3.1 Profile decomposition

The most remarkable features in the DDO 68 #3 and PHL 293B spectra are the
strong broad components with P Cygni profiles underlying the narrow nebular
emission of the hydrogen and helium lines. For derive quantitative
measurements of these features, we have decomposed the profiles of the
hydrogen emission lines into the sum of several Gaussian profiles: a high-
intensity narrow component, and low-intensity broad and very broad (the latter
when needed) components, using the iraf/splot deblending routine.

The fluxes and full widths at half maximum (FWHM) of the H$\alpha$ narrow and
broad components, along with the terminal velocities and the total
luminosities of the broad components in the two SFGs are given in Table 2 and
Table 3 for all spectra shown in Figs.1 and 3. The data for the very broad
components in PHL 293B are also given in Table 3. Details of the profile
decomposition of the H$\alpha$ line can be seen in Figs. 4, 5, 6. For
luminosity determination, the observed fluxes of narrow and broad emission
lines were corrected for the extinction and underlying stellar absorption
taken from Izotov & Thuan (2009), derived in accordance with the prescription
of Izotov et al. (1994). Note that the extinction coefficient $C$(H$\beta$)
and equivalent width of the underlying absorption (EWabs) derived from the
high signal-to-noise ratio MMT spectrum of DDO 68 #3 and the VLT spectrum of
PHL 293B (see table 3 in Izotov & Thuan, 2009) are very small in both
galaxies. $C$(H$\beta$) and EWabs have zero values in DDO 68 #3 and are
respectively equal to 0.08 and 0.05 in PHL 293B.

###  3.2 DDO 68 #3

DDO 68 (UGC 05340) is one of the most metal-deficient SFG known (12 + logO/H =
7.15$\pm$0.04, 7.14$\pm$0.07, Izotov & Thuan, 2009; Annibali et al., 2019a).
It is likely in the process of forming by hierarchical merging (Pustilnik et
al., 2017; Annibali et al., 2019b). Broad spectral features with blueshifted
absorption in the lines of the hydrogen series as well as in some He i lines
were first noticed in DDO 68 by Pustilnik et al. (2008). Those authors
attributed the broad features to the outburst of a LBV located in one of the H
ii region of DDO 68 named Knot 3, and which we will designate by #3 in the
remainder of this paper.

Based on photometric and spectroscopic observations, Izotov & Thuan (2009)
dated the start of the strong LBV outburst in DDO 68 #3 to be between 2007
February and 2008 January. They confirmed the presence of P Cygni profiles in
both the H i and He i emission lines and found that the Fe ii emission lines
are not present, in contrast to “typical” LBVs (Pustilnik et al., 2008; Izotov
& Thuan, 2009). The absence of Fe ii could be due to extremely low metallicity
and thus low optical depth precluding considerable radiative pumping.
Pustilnik et al. (2017) described the state of the LBV in 2015-2016 as being
in a fading phase. Observations carried out by Annibali et al. (2019a) with
LBT/MODS1 and LBT/MODS2 in February 2017 do not show the characteristic signs
of a LBV star. This led them to conclude that, by 2017, the LBV was back in a
quiescent phase.

We provide here a more complete picture of the time evolution of the LBV in
DDO 68 #3 by monitoring its spectrum. Our observations start from 2008, and
occur as often and as regularly as possible afterwards, ending in 2018. We
show in Fig. 1 the time evolution of the H$\alpha$ line. We emphasize that all
spectra were obtained with the same telescope and instrumental setup (3.5m
APO/DIS) and reduced in an uniform manner, so they are directly comparable. We
remark that, as seen in Fig. 1 and more clearly in Fig. 4, the [N ii] emission
lines near H$\alpha$ are nearly not detected, due to the low metallicity of
DDO 68 #3.

Our new APO observations in the monitoring series of DDO 68 #3 reveal that the
fluxes of the H$\alpha$ broad components are nearly an order of magnitude
higher at the end of October 2008 as compared to January 2008, when the LBV
was discovered by Pustilnik et al. (2008) (Table 2 and Fig. 7b, thick solid
black line). The MMT observation of DDO 68 #3 on March 2008 of Izotov & Thuan
(2009) is consistent with that trend. It showed a broad H$\beta$ flux $\sim$2
times lower than the one in the first observation in our APO monitoring series
starting 6 months later, in October 2008.

Our more than a decade APO monitoring showed that the features characteristic
of eruption in the LBV in DDO 68 #3 persist until the period somewhere between
the end of 2010 and the beginning of 2012, with small variations in the
component fluxes. The width of the broad component remains unchanged until
October 2010 with a FWHM $\sim$1000–1200 km s-1 (Table 2). After 2011 the flux
and the velocity of the broad component markedly decreased. Furthermore, the
absorption feature in the P Cygni profile disappeared toward the beginning of
2012. That disappearance together with the sharp decrease of the fluxes of the
broad features are possibly due to the dense stellar envelope being destroyed
and scattered.

In addition to the presence of broad components with absorption components,
observed during the period from 2008 up to 2012 (Fig. 4 a – g), a low-
intensity high-velocity redward tail made its appearance in 2012. It can be
seen as a flux excess above the Gaussian fit to the broad component (Fig. 4h
and 4i). It should be noted, however, that the best fit to the 2012 profiles
could only be obtained by including two Gaussians for the broad component
(Fig. 5, a broad one and a very broad one). Thus, a supplementary broad
component with a small FWHM = 300 – 500 km s-1 is present, in addition to the
very broad component with a larger FWHM $\sim$ 1000 km s-1 (see Fig. 5, solid
turquoise curves). Note the blueward shift in the broad components in Fig. 5
(solid turquoise curves). Likely, the appearance of the high-velocity tail and
the subsequent disappearance of the P Cygni profile can be interpreted as the
break-up of a dense circumstellar envelope by a rapid outflow. In 2013, the
spectrum, despite being somewhat noisy, still clearly shows the broad blueward
component (Fig. 4j). By 2018, the broad components have completely gone from
the spectra, suggesting that the LBV shell has disappeared. The outburst event
has thus lasted from 2008 to 2013, for a duration of about 5 years.

Assuming that the broad components of the emission lines belong exclusively to
the LBV, and that the narrow emission components are predominantly nebular, we
can trace the decay of the LBV outburst by using the flux ratio of the broad-
to-narrow components in each of the hydrogen and helium lines. This can be
done most reliably for the brightest H$\alpha$ line. The temporal evolution of
the H$\alpha$ broad-to-narrow flux ratio is shown in Fig. 7a. A maximum for
this flux ratio is seen between 2009 and 2010. It is more pronounced than the
maximum for the flux of the broad component (Fig. 7b). However, in general,
the variation of the broad-to-narrow flux ratio follows tightly the flux
temporal evolution of the broad component.

To derive line flux luminosities for DDO 68, we need to know the distance of
the dwarf galaxy. Based on the same imaging HST observations of the dwarf
galaxy (GO 11578, PI: Aloisi) Cannon et al. (2014); Sacchi et al. (2016);
Makarov et al. (2017) have applied the TRGB (Tip of the Red Giant Branch)
method to derive distances $D$ = 12.74, 12.65 and 12.75 Mpc for DDO 68,
respectively. New HST observations of a stream-like system associated with DDO
68 by Annibali et al. (2019b) gives $D$ = 12.8 $\pm$ 0.7 Mpc, using the same
TRGB method. The mean of these values is $D$ = 12.7 Mpc, which we adopt. This
distance is more than twice as large than the previous indirect distance
determination of $D$$\sim$5 – 7 Mpc (NED) (see e.g. Pustilnik et al., 2005;
Izotov & Thuan, 2009).

With this distance, the luminosity of the H$\alpha$ broad component at the
maximum is $\sim$ 2$\times$1038 ergs s-1. This maximum luminosity and the FWHM
($\sim$1000–1200 km s-1) of the broad component of the lines during the
transient event are in the ranges of those observed for LBV stars. The data
thus suggest that a LBV star in the H ii region DDO 68 #3 has undergone an
outburst during the period 2008–2013.

The terminal velocity $v$term of the LBV stellar wind is an important
parameter for confronting theory with observation. It is obtained from the
wavelength difference between the maximum of the broad emission line profile
and the minimum of the blue absorption line profile. The terminal velocity
$v$term in DDO 68 #3 is of $\sim$800 km s-1 and does not change significantly
over time (Table 2). It remains also unchanged during the “maximum” of the
eruption, as well as during the decay period starting in 2013. Nearly the same
value of the expanding wind terminal velocity was reported by Izotov & Thuan
(2009) and Pustilnik et al. (2017).

To summarize, the new data suggest that the broad component fluxes of the
hydrogen lines of the LBV reached a maximum during the time interval $\sim$
2008–2012. The narrow component also reached a maximum during the same period,
but less pronounced. The broad-to-narrow component flux ratio reached a
maximum at about the same period.This also supports the hypothesis that the
LBV star underwent an eruptive event. It can be seen in Fig. 7 that the
H$\alpha$ luminosity of the LBV star is now at a minimum or close to it.

###  3.3 PHL 293B

The situation with understanding the physical processes taking place in PHL
293B is more complex than in DDO 68 #3. PHL 293B (J2230–0006) is a well known
SFG with a moderately low metallicity 12+logO/H $\sim$ 7.6–7.7 (e.g. Papaderos
et al., 2008; Izotov et al., 2011; Fernández et al., 2018). Izotov & Thuan
(2009) first detected a broad component in the strong hydrogen emission lines
with P Cygni profiles of PHL 293B in a VLT UVES spectrum obtained on 8
November 2002, and in a SDSS spectrum taken on 22 August 2001\. Those authors
suggested that these broad features are due to a transient LBV phenomenon.
Earlier observations of PHL 293B by Kinman (1965) and French (1980) did not
mention any broad emission. The broad component was again detected by Izotov
et al. (2011) in a VLT X-Shooter observation on 16 Aug 2009. Later
observations have been carried out, and various other hypotheses have been
proposed to interpret them. Thus, Terlevich et al. (2014) have suggested that
the observations of PHL 293B can be explained by a young dense expanding
supershell driven by a stellar cluster wind and/or two supernova remnants or
by a stationary wind. Hydrodynamic models have been built by Tenorio-Tagle et
al. (2015).

On the basis of 10.4m GTC (Gran Telescopio Canarias)/MEGARA observations
performed in July 2017, Kehrig et al. (2020) found a flux ratio $I$br/$I$nar
$\sim$0.10 in the H$\alpha$ emission line of all integrated regions. They
interpret this low value as due to a diminution of the broad H$\alpha$
emission.

Allan et al. (2020), from spectroscopic observations including the 2019 VLT
X-Shooter data and radiation transfer modeling, report the absence of the
broad emission component since 2011 and conclude that the LBV was in an
eruptive state during 2001 - 2011 that has ended. Burke et al. (2021) also
report the decrease of the $I$br/$I$nar ratio from 0.41 (SDSS, 2001) to
$\sim$0.10, using Gemini observations taken in December 2019. Despite the fact
that a AGN-like damped random walk model works well to fit the observed light
curve, those authors concluded that a long-lived stellar transient of type SN
IIn can better explain all the data for PHL 293B.

On the other hand, Prestwich et al. (2013) have emphasized the lack of X-ray
emission in the galaxy, establishing an upper limit of $L_{X}$ $\sim$ 3
$\times$ 1038 erg s-1 (their table 6). This casts some doubt on the supernova
model, thought to be the most probable one for explaining the PHL 293
phenomenon. Kehrig et al. (2020) have detected P Cygni profiles in the
H$\alpha$ and H$\beta$ emission lines in July 2017, while those features were
not found by Burke et al. (2020) in H$\alpha$ in December 2019. These
variations suggest that long-time monitoring, such as the observations
described here, is important for distinguishing between various models.

Contrary to the conclusions of Izotov & Thuan (2009), based on the archival
VLT/UVES observations on 2002-11-08, that P Cygni profiles are seen only in
the hydrogen lines, and those of Terlevich et al. (2014), our new PHL 293B
observations, as well as a reconsideration of the SDSS archival data
(2001-08-22) and the VLT/X-Shooter observation by Izotov et al. (2011) on
2009-08-16, show that blue-shifted absorption lines are detected not only in
hydrogen emission lines, but also in He i lines (Fig. 2). In other words, the
situation is the same as in the case of DDO 68 #3 which has a much lower
metallicity. However, the permitted Fe ii emission lines, which generally
originate in dense circumstellar envelopes and are usually seen in the spectra
of LBV stars experiencing a giant eruption and creating an envelope, have not
been detected in our new observations. Note that Terlevich et al. (2014) found
blue-shifted Fe ii absorption with a terminal velocity of $\sim$ 800 km s-1.
We did not detect permitted Fe ii lines in the LBT spectrum obtained in 2020\.
Only forbidden [Fe iii]$\lambda$$\lambda$4658, 4702, 4755:, 4986, 5270Å
emission lines were found in this spectrum, with no absorption features.

In the decomposition of the strong emission lines, we have always attempted to
fit the broad component in the simplest way possible, i.e., with the smallest
number (preferably one) of Gaussian profiles. We also tried to perform fitting
with Lorentzian profiles. But the results were always worse than in the case
of Gaussian profiles. In the case of PHL293B, not one but two broad components
are required to match the observed profiles well. The decomposition of
H$\alpha$ for all spectra from Table 3 into narrow, broad, very broad (and
absorption lines, whenever warranted), are presented in Fig. 6.

The P Cygni profile has persisted over some two decades, during the period
2001–2020, as is clearly seen in Fig. 2. The blueward absorption feature with
wavelength $\sim$6545Å is close to the nitrogen emission line [N
ii]$\lambda$6548Å so that in many cases decomposition of the absorption
profile in H$\alpha$ emission line is difficult. The absorption is likely
present, but masked by strong broad and very broad components and the nitrogen
emission line (Fig. 3 and Fig. 6).

We obtain a high terminal velocity $v_{term}$$\sim$800 km s-1 for the
absorption component in the Balmer lines. This value is the same as the ones
derived by Izotov & Thuan (2009), Izotov et al. (2011) and Terlevich et al.
(2014) from X-Shooter observations made in Aug. 2009 and in Aug.-Sep. 2009,
respectively. The luminosity of the broad bump (i.e. the sum of broad and very
broad components) varies from a few 1038 erg s-1 up to $\sim$1039 erg s-1.

The very broad component derived by us from the SDSS spectrum taken in August
2001 (Fig. 6, Table 3) has nearly the same redshift as Terlevich et al.
(2014)’s faint ultra-broad component measured from their 2009 observations and
shown in their figure 2 but a flux that is $\sim$100 times lower. Note that
Terlevich et al. (2014) also fitted H$\alpha$ in their X-Shooter observation
by two broad components. At about the same redshift, Izotov et al. (2011) also
saw an excess in their fit of the H$\beta$ broad component (their figure 3).
In our subsequent series of observations of PHL 293B starting in 2010, the
very broad component does not show any redward shift. On the contrary, the
broad and very broad components are centered practically at the systemic
velocity of the galaxy. This means that, at least since 2010, there has been
no large velocity outflows. We remark also that the broad components in the
SDSS spectrum dated 2001 (Izotov & Thuan, 2009), and in the VLT X-Shooter
spectra dated 2009 (Izotov et al., 2011; Terlevich et al., 2014), have quite
large FWHMs, $\sim$ 1500, 1000 and 1000 km s-1, respectively. After 2010,
these components become much narrower, with FWHM $\sim$ 160 – 180 km s-1. This
signifies the fact that the velocity dispersions of the moving and radiating
matter have decreased, at least in the broad components.

If the ionization parameter is close to the limiting value of log$U$ $\la$–2,
the radiation pressure can prevail over the ionized gas pressure (Dopita et
al., 2002), and some contribution of a radiation-driven (i.e. by LyC and/or
Ly$\alpha$) superwind from a young stellar cluster, producing a high-velocity
very broad component, will be possible (Komarova et al., 2021). To check this
possibility, we use accurate oxygen abundance and an observational indicator
of the ionization parameter, O32 = [O iii] 4959,5007/[O ii]3727, for PHL 293B
(12 + logO/H = 7.72 and O32 = 15.52 from Izotov & Thuan (2009) and 12 + logO/H
= 7.71 and O32 = 14.21 from Izotov et al. (2011)) to estimate log$U$. We get
(following Kobulnicky & Kewley, 2004) log$U$ = –2.2 and log$U$ = –2.3,
respectively, which are close to the maximum value, but still below it. Note
that it is higher than log$U$ = –2.9 derived from the MMT observation by
Izotov & Thuan (2009) for DDO 68 #3.

In general, the temporal variations of both the broad-to-narrow component flux
ratios and of the H$\alpha$ fluxes of the narrow and broad components for PHL
293B (Fig. 7, colored lines) are very different from those of DDO 68 #3 (Fig.
7, black lines). A strong variability of the H$\alpha$ fluxes, simultaneously
in both the narrow and broad and very broad components, from 2011 to 2018, is
seen in PHL 293B (Fig. 7b) . Flux jumps by a factor of $\sim$6 are observed in
both the narrow and the sum of the broad and very broad components (Table 3).
We have checked the logs of the observations and have determined that all
observations were carried out in photometric conditions, so that the flux
variations cannot be attributed to sky variability. The flux variations, both
in the narrow and broad and even very broad components (a sharp flux decrease
in 2014 followed by a rise at the end of 2015, followed by another decrease in
2017) can be explained by inaccurate pointing when observing with a narrow
slit of 0.9 arcsec. However the variations are likely to be real because the
light curves derived by Burke et al. (2021) from SDSS and DES (Dark Energy
Survey) imaging between 1998 and 2018 (their figure 2) also show small-
amplitude variability in the $g$ and $r$ bands during the same years. The
synchronicity in the variability of fluxes in the narrow and broad components
does not hold anymore starting 2018 (see also the fading of the broad
component in Allan et al., 2020; Burke et al., 2021).

In contrast to the outburst nature of the variability over time of the broad-
to-narrow flux ratio in DDO 68 #3, which manifests itself in the form of a
peak (Fig. 7a, black line), PHL 293B does not undergo such a type of
variation. Its broad-to-narrow flux ratios depend weakly on time, at least
over the past two decades. However, a decrease in the broad-to-narrow flux
ratio can be seen starting from the very end of 2017 and continuing to 2020
(Fig. 7a). This decrease can be partly explained by an increase of the narrow
component relative to the broad one (Fig. 7b, blue and red dashed and green
solid lines).

In the case of PHL 293B, the behavior of the broad component may be explained
by several effects. Enduring broad hydrogen Balmer P Cygni profiles with
absorption feature blue-shifted by $\sim$800 km s-1, can indicate the presence
of fast moving ejecta. The persistent luminosity of the broad Balmer emission
and the possibility that the Balmer emission could be due to a long-lived
stellar transient, like LBV/SN, motivate additional follow-up spectroscopy to
distinguish between these effects.

###  3.4 LBVs in other SFGs

We now compare the behavior of the candidate LBVs in DDO 68 #3 and PHL 293B
with those in other SFGs.

The LBV in the relatively low-metallicity galaxy Mrk 177 = UGCA 239 at the
post-merger stage, with 12 + logO/H = 8.58 and $D_{L}$ = 28.9 Mpc, has
undergone multiple outbursts during the last 20 years (Kokubo, 2021). The
luminosity of the broad H$\alpha$ component of Mrk 177 varies from a maximum
of 1040 erg s-1 during the strongest explosion to 1039 erg s-1 during the next
two strong explosions. This is an order of magnitude higher than the
corresponding values of $L$(H$\alpha$) (broad) = (2 – 9)$\times$1038 erg s-1
in PHL 293B. As for the H$\alpha$ broad-to-narrow flux ratio for the LBV in
Mrk 177, it varies in the interval 3.4 – 4.5 during last two outbursts. This
is in the same range as the LBV outburst maximum in DDO 68 #3, but one order
of magnitude higher than in PHL 293B. The strongest outburst in Mrk 177 during
2001 is characterised by a H$\alpha$ broad-to-narrow ratio that is
approximately 10 times higher than those during subsequent explosions, i.e.
$\sim$ two orders of magnitude higher than in PHL 293B.

On the other hand, the luminosity $L$(H$\alpha$) $\sim$1038 erg s-1 (Drissen
et al., 2001) of the LBV-V1 in the low-metallicity (12 + logO/H = 7.89, Izotov
et al., 1997) galaxy Mrk 71 (NGC 2363A) is lower than that of the cLBV in PHL
293B, with a similar metallicity.

##  4 CONCLUSIONS

Over nearly two decades, we have monitored the time variation of the broad
component fluxes and of the broad-to-narrow flux ratios of the strong hydrogen
and helium emission lines in two low metallicity star-forming galaxies (SFG),
DDO 68 (12 + logO/H = 7.15) and PHL 293B (12 + logO/H = 7.72). These two SFGs
have the particularity of harboring candidate luminous blue variable (cLBV)
stars. We have carried out this monitoring by obtaining over time spectra of
these two objects with the 3.5m Apache Point Observatory (APO) telescope and
the 2$\times$8.4m Large Binocular Telescope (LBT). Our main results are the
following:

(1) The broad emission with a P Cygni profile of the H$\alpha$ line emitted by
the DDO 68 H ii region #3 shows a marked increase of its luminosity during the
period 2005 to 2017, reaching a maximum $L$(H$\alpha$) of $\sim$ 1038 erg s-1
in 2008 – 2011, adopting the distance derived from brightness of the Tip of
the Red Giant branch (TRGB). The absorption feature of the P Cygni-like
profile and the broad component rapidly decayed and disappeared in 2018. These
properties are characteristic of an eruptive event in a LBV star. The derived
H$\alpha$ luminosity and the FWHM $\sim$1000–1200 km s-1 of the broad
component during the eruption event are in the range of those observed for LBV
stars. A terminal velocity $v$term$\sim$800 km s-1 is measured from the
absorption profile. It does not change significantly over time. These
observations are also consistent with the earlier findings of Pustilnik et al.
(2017) who described the LBV in DDO 68 #3 as going through a fading phase
during 2015–2016, and with those of Annibali et al. (2019a) who did not find
any characteristic sign of a LBV star at the beginning of 2017. Thus, our
spectroscopic monitoring indicates that the LBV has passed through the maximum
of its eruption activity and is now in a fairly quiet phase.

(2) The situation is quite different in the BCD PHL 293B. Since the discovery
of the cLBV in it on the basis of SDSS 2001 observations, the fluxes of the
broad component and the broad-to-narrow flux ratios of the hydrogen emission
lines in PHL 293B have remained nearly constant, with small variations, over
16 years, at least until 2015. A decrease of the broad-to-narrow flux ratio in
recent years (2017–2020) can partly be attributed to an increase of the narrow
component flux relative to that of the broad component. The luminosity of the
broad H$\alpha$ component varies from $\sim$2$\times$1038 erg s-1 to
$\sim$1039 erg s-1, and the FWHM varies in the range $\sim$500–1500 km s-1
over the whole period of monitoring of PHL 293B, from 2001 to 2020.

Unusually persistent P Cygni features with broad and very broad components of
hydrogen emission lines and blueward absorption in the H i and He i emission
lines are clearly visible, even at the very end of 2020, despite the several
decreases of the broad-to-narrow flux ratio in the most recent years
(2017-2020). A terminal velocity $v$term$\sim$800 km s-1 is measured in PHL
293B, similar to the one obtained for DDO 68 #3, although the latter is 3.7
times more metal deficient than the former. The terminal velocity does not
change significantly with time. The near-constancy of the H$\alpha$ flux
suggests that the cLBV in PHL 293B is a long-lived stellar transient of type
LBV/SN IIn. However, other mechanisms such as a stationary wind from a young
stellar cluster, cannot be ruled out. Further spectroscopic time monitoring of
the BCD is needed to narrow down the nature of the phenomenon in PHL 293B.

## Acknowledgements

N.G.G. and Y.I.I. acknowledge support from the National Academy of Sciences of
Ukraine (Project No. 0121U109612 “Dynamics of particles and collective
excitations in high energy physics, astrophysics and quantum microsystem”).
T.X.T. thanks the hospitality of the Institut d’Astrophysique in Paris where
part of this work was carried out. The APO 3.5 m telescope is owned and
operated by the Astrophysical Research Consortium (ARC). The LBT is an
international collaboration among institutions in the United States, Italy and
Germany. LBT Corporation partners are: The University of Arizona on behalf of
the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT
Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the
Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State
University, and The Research Corporation, on behalf of The University of Notre
Dame, University of Minnesota and University of Virginia. This paper used data
obtained with the MODS spectrographs built with funding from NSF grant
AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), with
additional funds from the Ohio Board of Regents and the Ohio State University
Office of Research. iraf is distributed by the National Optical Astronomy
Observatories, which are operated by the Association of Universities for
Research in Astronomy, Inc., under cooperative agreement with the National
Science Foundation. Funding for the Sloan Digital Sky Survey IV has been
provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy
Office of Science, and the Participating Institutions. SDSS-IV acknowledges
support and resources from the Center for High-Performance Computing at the
University of Utah. The SDSS web site is www.sdss.org. SDSS-IV is managed by
the Astrophysical Research Consortium for the Participating Institutions of
the SDSS Collaboration. This research has made use of the NASA/IPAC
Extragalactic Database (NED), which is operated by the Jet Propulsion
Laboratory, California Institute of Technology, under contract with the
National Aeronautics and Space Administration.

##  5 DATA AVAILABILITY

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

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