New LBV Stars in the Andromeda Galaxy
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.
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.