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Modeling of Very Luminous Stars in the M33

Russian version

    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)
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)