|
We find a transition of LBV star from the LBV instability strip to the
region of classical WN for the first time.
Luminous blue variables (LBVs) are rare objects the origin of which has
not yet been clearly ascertained. They are generally believed to be a
relatively short evolutionary stage in the life of a massive star,
marking the transition from the Main Sequence toward Wolf-Rayet (WR)
stars. LBV demonstrate pronounced photometrical and spectral variability.
Some LBVs at visual minimum resemble the late WN stars. Numerical modeling
of the atmospheres of these objects are very important for understanding
the evolutionary relationship WR and LBV stars, as well as of the
physical causes of LBVs variability.
We studied spectra of two very luminous stars in the M33 galaxy - LBV V532
and late WN star FSZ35. During 2004 and 2005, V532 becomes brighter and
reaches 17m in B band (Fig.1). Starting from the middle 2005,
Romano's star weakens in all bands. Using the radiative transfer code
CMFGEN
(J.D.Hillier), we determine the parameters of atmospheres of V532 in
2005 and 2007. The spectrum during the outburst corresponds to the
spectral class WN11 and is similar to the spectrum of P Cyg. All the
observed properties of the object in the minimum are well described
by a WN8 star model. V532 moves from LBV minimum instability strip to
area of classical WN. Probably, we observe final transition from LBV to WN.
Bolometric luminosity is shown to vary between the two states by a factor
of 1.5.
We classify FSZ35 as a WN8h star, estimate its parameters and compare
them to the corresponding parameters of other WN8 stars. Unlike V532,
FSZ35 does not demonstrate any prominent spectral variability. The lack
of photometrical variability may be ascribed to the more advanced
evolutionary status of FSZ35.
We use the data from
the archive
of the Special Astrophysical Observatory and data from
SMOKA.
O.Maryeva (Special Astrophysical Observatory RAS),
P. Abolmasov (Sternberg Astronomical Institute, MSU)
Published:
Maryeva O.V., Abolmasov P.K., 2012
MNRAS 419 1455-1464
Maryeva O.V., Abolmasov P.K., 2012
MNRAS 421 1189-1195
Contact - Olga Maryeva
|
Fig.1.
The B-band light curve of V532. Arrows indicate the dates of spectral
observations
Fig.2.
The normalized optical spectra (black line) compared with the best-fit
CMFGEN models (red line). Top panel: the spectrum obtained in February
2006 when V532 is 17m.27 in V band. Middle: the spectrum
obtained in October 2007, when V-band magnitude is V=18m.68.
Bottom: The spectrum of FSZ35 with the final model
Fig.3.
HR diagram is showing the positions of V532 and FSZ35, according to the
stellar parameters determined in this work. Position for the LBV minimum
instability strip is provided (dashed line).The location of the
Humphreys-Davidson limit is shown (solid line)
|
