Ultraviolet and optical spectroscopy of a B supergiant star in M 31.

Abstract

We present and discuss the UV spectrum of the supergiant star in NGC206-277 in M 31, obtained with the Faint Object Spectrograph (FOS) on Hubble Space Telescope. Customised extraction of the data was used to properly correct for the background light in the FOS. An optical blue spectrum taken at KPNO is also shown. From the optical and UV line spectra we classify the star B1.5Ia. We fit the continuum far-UV flux distribution, deriving T_eff_=20000K, and with this temperature we find log g=2.5 by comparing the H_gamma_ line and HeI lines with predictions from NLTE models. We analyse the stellar UV wind line profiles with the SEI method and derive a terminal velocity of v_infinity_=~700km/s. We fit the Hα emission line obtained at the WHT with a newly developed (by H.J.G.L.M.L.) code similar to the SEI method for UV lines, and derive ˙(M)=1.3+/-0.5x10^-6^M_sun_/yr. We also compare the UV lines with those of Galactic stars of the same type and we find that the P Cygni profiles of the M 31 star have slightly weaker and narrower absorptions, and no emission component. We correct our previously published mass-loss estimate for an O star (NGC206-231) in the same association. The correct value is ˙(M)=~10^-6^M_sun_/yr, assuming solar abundances.