New luminous blue variable candidates in NGC 4736

Abstract

We have found three new LBV candidates in the star-forming galaxy NGC 4736. They show typical well-known LBV spectra, broad and strong hydrogen lines, He i lines, many Fe ii lines, and forbidden [Fe ii] and [Fe iii]. Using archival Hubble Space Telescope and ground-based telescope data, we have estimated the bolometric magnitudes of these objects from -8.4 to -11.5, temperatures, and reddening. Source NGC 4736 1 (M V = −10.2 ± 0.1 mag) demonstrated variability between 2005 and 2018 as ∆V ≈ 1.1 mag and ∆B ≈ 0.82 mag, the object belongs to LBV stars. NGC 4736 2 (M V < −8.6 mag) shows P Cyg profiles and its spectrum has changed from 2015 to 2018. The brightness variability of NGC 4736 2 is ∆V ≈ 0.5 mag and ∆B ≈ 0.4 mag. In NGC 4736 3 (M V = −8.2 ± 0.2 mag), we found strong nebular lines, broad wings of hydrogen; the brightness variation is only ≈ 0.2 mag. Therefore, the last two objects may reside to LBV candidates.

A Long-term Photometric Variability and Spectroscopic Study of Luminous Blue Variable AF And in M31

“…In the figure, the variation in the (R − I) colour with respect to R magnitude may be represented by a quadratic polynomial equation of the following type (R − I) = −0.08 (R − 15) 2 + 0.11 (R − 15) + 0.04 (2) ±0.03 ± 0.10 ± 0.07 A best fit shown by a continuous line in Figure 3 yields a χ 2 /ndf of 0.04. A similar trend was also shown by Sholukhova et al (2011) and Solovyeva et al (2019) in their analysis of LBVs in the galaxies M33 and NGC 4736, respectively. Our study further ascertains the characteristic RWB nature during the LBV phase that suggests the colour of the LBV star becomes bluer as a consequence of increase in the temperature of stellar photosphere in its quiescence phase.…”

New data for the LBV in NGC 4736

“…New photometric data were obtained with the BTA (2020) of SAO RAS simultaneously with spectroscopy and Zeiss-1000 (2019/04/09) of SAO RAS and also with 2.5-m telescope of the Caucasian Mountain Observatory of SAI MSU (2020/03/07). We added these data to light curve from previous work (Solovyeva et al, 2019) (see Fig. 2, left).…”

“…The object NGC 4736 1 showed significant photometric variability: ∆V = 1.18 m ±0.12 m and ∆B = 0.90 m ±0.12 m in V and B bands (Solovyeva et al, 2019). The measurements with minimal errors were used for calculations.…”

“…The figure 2 (right panel) shows the position of NGC 4736 1 on the temperature -luminosity diagram. The positions of NGC 4736 2, NGC 4736 3 from (Solovyeva et al, 2019) are also shown. We used stellar evolutionary tracks from Tang et al (2014) with Z = 0.01 for NGC 4736 (Pilyugin et al, 2014).…”

“…Gray areas show the range of possible temperature and luminosity values of objects. The photosphere temperatures and bolometric luminosities estimates, given in Solovyeva et al (2019), were used.…”