Spectral variability of LBV star V 532 (Romano’s star)

Abstract

We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33.
The photometric variations are traced from 1960 to 2010, spectral variations—from 1992 to 2009. The star has revealed an absolute
maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness
difference of ΔB ≈ 2.3
m
. The temperature estimates in the absolute maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its
photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the
brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to
the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type
profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness
declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The
wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type
of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution
of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL
star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate
sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain
to the bona fide WNL stars.

Key wordsStars–massive—stars–individual–V 532

Full text

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arXiv:1105.0123v1 [astro-ph.SR] 30 Apr 2011
Spectral Variability of LBV star V532 (Romano’s star)
O. N. Sholukhova,1S. N. Fabrika,1A.V. Zharova,2A. F. Valeev,1and V.P. Goranskij2
1Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia
2Sternberg State Astronomical Institute, Moscow University, Moscow, 119992 Russia
(Received December 2, 2010; Revised December 18, 2010)
Wepresenttheresultsofstudyingthespectralandphotometric vari-
abilityoftheluminousbluevariablestar V532inM33.Thephotomet-
ricvariationsaretracedfrom1960to2010,spectralvariations—from1992
to2009.Thestarhasrevealedanabsolutemaximum of visualbrightness
(1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state)
with a brightness difference of ∆B ≈ 2.3m. The temperature estimates in the absolute
maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, re-
spectively. The variability of the spectrum of V532 is fully consistent with the temperature
variations in its photosphere, while both permitted and forbidden lines are formed in an
extended stellar atmosphere.Broad components of the brightest lines were found, the
broadening of these components is due to electron scattering in the wind parts closest to
the photosphere. We measured the wind velocity as a difference between the emission and
absorption peaks in the PCyg type profiles. The wind velocity clearly depends on the size
of the stellar photosphere or on the visual brightness, when brightness declines, the wind
velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere
was detected. The wind velocity increases and its temperature declines with distance from
the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in
the high/cold state—to WN11. We studied the evolution of V532 along with the evolution
of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all
the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997).
However, when visual brightness increases, all the three stars form a separate sequence. It
is possible that this reflects a new property of LBV stars, namely, in the high/cold states
they do not pertain to the bona fide WNL stars.
1.INTRODUCTION
Luminous blue variables (LBVs) represent one
of the least understood stages in the evolution of
massive stars. Difficulties in the understanding
of these objects are dictated both by the paucity
of well-known LBV stars, their diversity, strong
spectral and photometric variability [1, 2], as well
as by our inaccurate knowledge of their funda-
mental parameters. Evidently, a massive star can
manifest itself as an LBV after the O star stage in
the Main Sequence, and before the late nitrogen
WR star (WNL). However, a more accurate po-
sition of LBV stars in the OV → WNL transition
is yet unclear [3]. It is very likely that the LBV
stage and the wind instabilities, corresponding to
this stage are determined by the hydrogen abun-

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dance in the stellar atmosphere. However, it is
not an easy matter to both measure the hydro-
gen abundance, and to precisely determine the
luminosity and temperature of the star. Mea-
surement of fundamental parameters of massive
stars in nearby galaxies, such as the M33 galaxy,
rich in massive stars, may prove to be more con-
fident than in our Galaxy, since the distances to
galaxies are measured with sufficient accuracy.
LBV stars reveal strong spectral and bright-
ness variations at a roughly constant bolomet-
ric luminosity.Characteristic times of such a
global variability amount to months and years.
When visual brightness of an LBV star increases,
the temperature of the photosphere drops to
9000 − 10000K, the size of the stellar photo-
sphere increases, and the star enters its high/cold
state. From here on, when we mention the pho-
tosphere, we refer to the “pseoudophotosphere”,
i.e. the place in stellar wind, where the observed
continuum radiation is formed. With a decrease
in visual brightness the temperature notably in-
creases up to 35000K or higher [1, 4, 5], the size
of the star decreases, it enters its low/hot state.
In the hot state the LBV spectra are very sim-
ilar to the spectra of WNL stars. However, the
bona fide WN7/8/9-type stars differ from the
LBVs in the low/hot state by a number of spec-
tral features [3, 6] and low hydrogen abundance.
The so-called transitional (or “slash”) stars of
the Ofpe/WN9-type, isolated in a separate class
by Walborn [7] are similar to LBV stars in the
low/hot state. Ofpe/WN9 stars are character-
ized by a mixed spectrum with emission lines,
typical of the Of and WNL stars.Taking an
example of two galactic LBV-type stars AGCar
and He 3–519, which in the low/hot states cor-
respond the Ofpe/WN9-type, Smith, Crowther
& Prinja [6] proposed the extension of the WNL
classification up to WN10 and WN11.
Nevertheless, the question whether the bona
fide LBV stars in the low/hot states differ from
the latest WN9/10/11 stars is still open. In other
words, is it possible that all the latest WNL-
type stars are in fact LBVs, which did not show
any strong brightness variability over the time of
their observations (only a few dozens of years).
Such stars, which may be suspected to belong
to the LBV type, are commonly called dormant
LBVs. Note that dormant LBV stars can be
found not only among the WNL stars.Two
classical LBV stars, η Car and PCyg now have
a spectrum of OB-hypergiants, their brightness
variability is low. However, a few hundred years
ago they had giant eruptions [1]. If it were not
for these giant brightenings of η Car and PCyg
and the famous nebula around ηCar, we would
not be able to suspect them in belonging to the
LBV class.
There are two LBV objects known that re-
vealed LBV ↔ WNL-type transitions in recent
years.This is an LBV star AGCar in our
Galaxy (see, for example, [8]) and an LBV star
V532 in M33, to which we devote this pa-
per.A reverse transition is as well known,
WN3 → WN11(LBV) → WN4/5, this is a mas-
sive WR binary HD5980 in SMC (e.g., [9]). The
study of such stars is particularly important. Ob-
viously, the parameters of these stars can be de-
termined more accurately, since during the dras-

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tic variations in their spectra neither the chemi-
cal composition, nor the value of interstellar ab-
sorption (in some approximation, the bolometric
luminosity as well) would vary.The study of
LBV stars during such transitions might give us
a chance to conclude about the relation between
the bona fide LBVs and WNL stars.
In this paper, we study the star V532 in
the M33 galaxy from the spectra obtained from
1992 to 2009. During this period it revealed its
highest/cold (1992) and lowest/hot (1997–1998)
states, registered from the observations carried
out since 1960. The visual brightness difference
between these two extreme states amounted to
2.3m.
The star V532 ([10]1, other names—Romano’s
Star, GR290) was discovered by G. Romano in
1978 [11]. He assumed that this blue star belongs
to the Hubble-Sandage-type variables.How-
ever, without spectral data no definite conclu-
sions could be done. Furthermore, this star was
studied photometrically in [12–16]. Photometric
behavior of this star is quite typical for LBVs, it
is very similar to the observed brightness varia-
tion in AGCar [17]. Even the total amplitude of
visual brightness variation of these two stars is
equal, it amounts to 2.3 − 2.5m.
The first spectrum of V532 was apparently
obtained by T. Szeifert in 1992, and published
in 1996 [4]. In his paper, this star was classified
as an LBV candidate. The second spectrum was
1in
is
http://cdsarc.u-strasbg.fr/viz-bin/Cat?II/205;
http://cdsarc.u-strasbg.fr/viz-bin/Cat?B/gcvs
(GCVS B/gcvs).
the
M33
catalog
V0532,
its
GCVS
designation
II/205,
obtained in 1994 [18] in a spectral survey of LBV
candidate stars in M33. The spectrum did not
have a very good quality, however, the authors
suggested that this is a WN-type star.Until
2000, V532 remained an LBV candidate, in a
well-known survey by Humphreys [1] it is also
described as a candidate. Explicit spectral vari-
ability of V532 was found from the spectra, ob-
tained from 1992 to 1999 [19]. Photometric vari-
ability, followed by spectral variability, did not
leave any doubt that V532 is an LBV [19].
Later, V532 has been spectrally studied as
an LBV star in [20–25], where new evidences
of spectral variability were presented. From the
1998-2001 spectra, when the star had an inter-
mediate brightness, Fabrika et al.[21] found
that its spectrum corresponds to the Ofpe/WN9-
type. Using quantitative spectral criteria intro-
duced in [26], they classified the star as WN10-
11. In the same paper [21] a nebula of low sur-
face brightness was found around V532.The
stellar parameters were found in [24] from the
spectral energy distribution (SED), constructed
based on the optical observations [27], performed
between autumn 2000 and autumn 2001, when
the star was in its intermediate brightness state
(spectral class WN10-11 [21]).The tempera-
ture of its photosphere was estimated in [24] as
T ∼ 35000 K, luminosity lg(L/L⊙) = 6.24, and
the value of interstellar absorption AV ∼ 0.8.
Since 2006 the apparent brightness of V532 has
heavily dropped, hence, the temperature of the
photosphere has increased.In this brightness
minimum the spectrum of the star was classified
as WN8-9 [25].

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In this paper, we present the longest series of
spectral and photometric observations of V532,
based on which we trace the variations in the
stellar spectrum.
2.OBSERVATIONS
2.1.Photometric Observations
For the photometric estimates of the V532
we used the initial data by Romano [11]. These
brightness estimates were obtained based on
104 photographic plates by the 67-cm Schmidt
telescope of the Asiago Observatory in 1960–
1977. We as well used the photo materials
from the Sternberg Astronomical Institute (SAI)
of Moscow State University, represented by 678
records, obtained on the 50-cm Maksutov tele-
scope (AZT-5) in 1973–2005. Photographic ob-
servations were carried out in a system, similar
to the Johnson B filter. The SAI photographic
materials and the method of data reduction are
described in [15, 28]. All the photographic es-
timates were reduced to the B system. The ac-
curacy of these estimates is approximately 0.1m,
but it decreases and the error reaches 0.2mwhen
the star gets weaker, near a plate edge the error
can reach 0.5m. A more detailed description of
all the photographic data is presented in [16].
In 2001 we started the CCD observations
of V532.They were carried out on the 1-
m Zeiss telescope of the Special Astrophysical
Observatory (SAO) and the two telescopes of
the Crimean Observatory of the SAI MSU—
AZT-5 and 60-cm Zeiss-2 telescope. Errors of
the CCD observations amount to 0.01 − 0.05m,
they are not worse than 0.05m. New BVRc
standards were obtained at the 1-m Zeiss tele-
scope of the SAO RAS. They were also used
for the photographic observations [16]. Hence,
we finally have 104 estimates by Romano [11],
645 estimates on the AZT-5, and multicolor
CCD BVRc photometry (65 B, 79 V, 46 Rc)
in 2001–2010. We also use several esti-
mations by Viotti et al [22], and one by
Humphreys [29]. They are well consistent with
our data.
2.2.Spectral Observations
The spectra of V532 were obtained mainly
with the 6-m BTA telescope using the UAGS,
MPFS [21] and SCORPIO [30] instruments. We
also use the spectrum of V532, courtesy of
T. Szeifert [4], obtained in October 1992 on the
3.5-m Calar Alto telescope with the TWIN spec-
trograph, and the spectrum obtained at our re-
quest by A. Knyazev on the 2.2-m Calar Alto
telescope with the CAFOS spectrograph in June
1999. We also present here the spectrum we ob-
tained on the SUBARU telescope with the FO-
CAS spectrograph. It has been taken within the
SS433 observational program [31] to make a com-
parison of spectra of these two objects. SS433
spectrum (hot wind of the supercritical accretion
disk) is very similar to the spectra of WN10-11-
type stars [32]. The log of spectroscopic obser-
vations is given in the Table, which also presents
the spectral ranges and resolution for each obser-
vation. All the spectra were processed by stan-
dard techniques, used for the corresponding spec-

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trographs in the MIDAS and IDL environments.
3.RESULTS
3.1.Photometric Variability
Figure1 presents all the photometric mea-
surements of V532 from 1960 to 2010 in
the B-band.The arrows indicate the tim-
ings of the spectra.Over 50 years of obser-
vations the star revealed four strong bright-
ness increases, one of which, occurring be-
tween 1990–1992, was the greatest.Conse-
quently, there were some brightness minima as
well, the deepest of which was in 2007–2008.
The maximum brightness difference amounted to
∆B ≈ 2.3m. Spectral observations cover both
these extreme states of V532. Such a brightness
variability with an amplitude of 2.1min the time
interval of years is typical for SDor-type stars
[2]. The light curve of V532 is very similar to
that of AGCar [8], which also revealed several
brightness extrema over the past 30 years with a
total amplitude of ∆V ≈ 2.4m[8, 17].
The scatter of the photometric estimates
within one season of observations is about 0.4m.
It is determined not only by the measurement
errors, but by real brightness variability as well.
In the early photographic observations the error
of one measurement amounted to 0.1−0.2m, but
later, in the photoelectric and CCD observations
it was several times lower, still, the scatter of
brightness estimates for one season is as great,
about 0.2 − 0.3m. Sholukhova et al. [15] studied
the light curve of V532 in the B-band. A number
of quasi-periods were found with characteristic
times of dozens of days both in the high and low
states. Some of them were quite stable and were
present in both states. The authors [15] sug-
gested that periods in the range of 20 − 30 days
and shorter are linked with the stellar pulsations,
and longer periods are related to the wind pulsa-
tions driven by the wind instabilities. In hydro-
dynamic simulations of nonlinear radial oscilla-
tions of hot LBV stars Fadeev [33] has found that
the periodic brightness variations in the range of
6 − 31 days are largely of pulsational nature.
Figure2a demonstrates a fragment of the light
curve of V532 in the B, V, R filters along with
the photographic measurements. Spectral ob-
servations cover the local minimum of visual
brightness observed in 2000–2002, the local max-
imum of 2004–2005, and the absolute minimum
of 2007–2008 quite well. In the late 2009 (from
November) the brightness of the star began to
somewhat rise, and by November 2010 it in-
creased by 0.7min the V-band. Consequently, the
minimum of 2007–2008 was the deepest during
the entire epoch of observations of V532. The
same figure shows the variation of B−V color
during the epoch of photoelectric and CCD ob-
servations. We can see that the color variations
are typical of LBVs: when the star’s brightness
decreases, it grows bluer, however, the variations
reveal some irregularity.
Figure3 demonstrates the B−V color depen-
dence on the B-band brightness variations. In
the low state at B >17.9 the color–magnitude
dependency is very irregular, the variation ampli-
tude reaches ∆(B − V ) ≈ 0.15m. The B bright-
ness of about 17.9mis isolated for another reason
as well, when the brightness is weaker, the color–