Wray 15-906: a candidate luminous blue variable discovered with WISE, Herschel, and SALT

ABSTRACT We present the results of study of the Galactic candidate luminous blue variable Wray 15-906, revealed via detection of its infrared circumstellar shell (of ≈2 pc in diameter) with the Wide-field Infrared Survey Explorer and the Herschel Space Observatory. Using the stellar atmosphere code cmfgen and the Gaia parallax, we found that Wray 15-906 is a relatively low-luminosity, $\log (L/\rm \, L_\odot)\approx 5.4$, star of temperature of 25 ± 2 kK, with a mass-loss rate of ${\approx}3\times 10^{-5} \, \rm \, M_\odot \, {\rm yr}^{-1}$, a wind velocity of $280\pm 50 \, {\rm \, km\, s^{-1}}$, and a surface helium abundance of 65 ± 2 per cent (by mass). In the framework of single-star evolution, the obtained results suggest that Wray 15-906 is a post-red supergiant star with initial mass of ${\approx}25 \, \rm \, M_\odot$ and that before exploding as a supernova it could transform for a short time into a WN11h star. Our spectroscopic monitoring with the Southern African Large Telescope does not reveal significant changes in the spectrum of Wray 15-906 during the last 8 yr, while the V-band light curve of this star over years 1999–2019 shows quasi-periodic variability with a period of ≈1700 d and an amplitude of ≈0.2 mag. We estimated the mass of the shell to be $2.9\pm 0.5 \, \rm \, M_\odot$ assuming the gas-to-dust mass ratio of 200. The presence of such a shell indicates that Wray 15-906 has suffered substantial mass-loss in the recent past. We found that the open star cluster C1128−631 could be the birth place of Wray 15-906 provided that this star is a rejuvenated product of binary evolution (a blue straggler).