New LBV Stars in the Andromeda Galaxy

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Russian version

Two new LBV stars have been discovered in the Andromeda Galaxy. Since the middle of the last century, only four such objects had been known in this galaxy, they were discovered by American scientists Edwin Hubble and Allan Sandage with colleagues. Astronomers from SAO added two more stars to this list in 2015. LBV stars (Luminous Blue Variables) are very massive, they are born with masses greater than 50 solar masses. These stars are very rare: for each such a star with a mass of more than 50 solar masses, over 30000 less massive stars are born. Once the hydrogen thermonuclear fuel comes to the end in the center of very massive stars, the stars strongly change their brightness, and after a few ten thousand years they blow up as supernovae.
We have found a new method to search for LBV stars, which is based on their intrinsic property: when such a star decreases in size (the star's photosphere contracts), its photospheric temperature gets higher, the star's brightness dims; when the star expands, it cools and becomes brighter. In Figure 1 the spectral energy distributions for six stars are shown: upper row - the newly discovered LBV stars, middle row - two well-known LBV stars Var A-1 and AE And, the lower row - two new B[e]-supergiant stars in Andromeda. The two classical LBVs and our LBVs do change their brightness strongly in accordance with our models. The two stars in the lower row do not change their brightness, but they have strong infrared excesses. The B[e]-supergiants are probably the same as LBVs, but they may be located in close binaries. Therefore they cannot change their brightness but have instead strong disk-like dust envelopes. The optical spectra were obtained with the BTA telescope, the infrared spectra were taken (simultaneously) in the USA (the Apache Point Observatory). The infrared spectra are shown in Figure 2, strong emission lines of hydrogen (Pa and Br series), helium, iron, and nitrogen are seen in the spectra.
Published:
Sholukhova, O., Bizyaev, D., Fabrika, S., Sarkisyan, A., Malanushenko, V., Valeev, A.,
Monthly Notices of the Royal Astronomical Society, 447, 2459-2467, 2015

Contact - S.N.Fabrika

Fig.1. Spectral energy distributions of the two discovered LBV stars (top), two known classical LBVs stars (middle), and two new stars of the B[e]-supergiant type (bottom). In the B[e]-supergiants strong infrared excesses are observed because of the presence of disk-like dust envelopes. The optical spectra were obtained at wavelengths below 7000 A, the infrared spectra - at wavelengths greater than 10000 A. The model lines are the fits of the spectra and photometric data with different temperatures. It is obvious that once LBV stars get brighter, they become hotter. The red dashed lines indicate the contribution of the dust envelopes surrounding the stars.

Fig.2. Infrared spectra of the stars in the Andromeda Galaxy: two classical LBVs Var A-1 and AE And, two new LBVs J004526.62 and J004051.59, and two new B[e]-supergiants J004417.10 and J004444.52. The strong emission lines of the Paschen (Pa) and Brackett (Br) hydrogen series, and the helium (HeI), iron, oxygen, and nitrogen lines are observed. The B[e]-supergiants have a special feature in the infrared spectra: molecular lines 12CO. They are formed in cool disk-like envelopes.