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 (WISE) 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/ L ⊙ ) ≈ 5.4, star of temperature of 25 ± 2 kK, with a mass-loss rate of ≈ 3 × 10 −5 M ⊙ yr −1 , a wind velocity of 280 ± 50 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 ≈ 25 M ⊙ 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 (SALT) 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 ± 0.5 M ⊙ 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).