New luminous blue variables (LBV) in star-forming galaxy NGC4736

Luminous Blue Variables (LBV) are found both in our Galaxy and in the galaxies of the Local Group. In outer galaxies, farther away than 3 Mpc, one may see the galaxies at big distances (from 18 to 21 magnitude), while there will be a lot of objects in spiral and dwarf galaxies, and these targets will be very massive. The luminosities of these stars range from 5.3 to 6.5 in logarithms of the solar luminosity. On the Main Sequence (MS), the masses range from 30 to 120 solar masses, as for their lifetime (on the MS) they are from 3 to 5 million years old. With a strong stellar wind, the masses of the stars sharply decrease, reaching 10-40 solar. These objects may turn into Wolf-Rayet (WR) stars, but at any moment these massive stars may explode like Supernovae.

In the star-forming galaxy NGC4736, at a distance of 4.6 Mpc, the LBV star and two LBV candidates were discovered (Fig. 1). The first star is fully consistent with eta Car by luminosity. In Fig. 2, the color of the star is B-V, the brightness is B and V. As far as stellar magnitudes become weaker, the temperature of the photosphere increases similar to that in the Romano star (V532) in the galaxy M33. In the first LBV star, the brightness variability is 1.6 in the V band (according to the data obtained with the Hubble Space Telescope and the BTA telescope of SAO RAS), in the other two LBV candidates the variabilities are 0.5 and 0.2 respectively. These are very bright stars, their bolometric luminosities from 5.3 to 6.5 in logarithms. But they remain the LBV candidates due to weak variabilities. We must check both spectral and photometric variabilities for these two candidates.

Figure 3 shows three LBV spectra. The first object is in the middle due to the bright nebula. All the lines are emission lines, these are hydrogen and HeI lines. They show the explicit or hidden P Cyg profiles. All three stars are very bright; for several years according to the spectroscopic data (emission lines), the spectra can change significantly.

Fig.1. According to the Hubble Space Telescope (HST), as well as superimposed images obtained at the BTA telescope: (a) the first new LBV star, (b) the second LBV candidat, (c) the third LBV candidate.

Published: Solovyeva, Y., Vinokurov, A., Fabrika, S., Kostenkov, A., Sholukhova, O., Sarkisyan, A., Valeev, A., Atapin, K., Spiridonova, O., Moskvitin, A., Nikolaeva, E.; 2019, Monthly Notices of the Royal Astronomical Society: Letters, Volume 484, Issue 1, p.L24-L28