Chemical abundances and winds of massive stars in M31: a B-type supergiant and a WC star in OB 10

Abstract

We present high quality spectroscopic data for two massive stars in the OB 10 association of M31, OB 10-64 (B0 Ia) and OB 10-WR1 (WC6). Medium resolution spectra of both stars were obtained using the ISIS spectrograph on the William Herschel Telescope. This is supplemented with Hubble Space Telescope STIS UV spectroscopy and Keck I HIRES data for OB 10-64. A non-local thermodynamic equilibrium (LTE) model atmosphere and abundance analysis for OB 10-64 is presented, indicating that this star has similar photospheric CNO, Mg and Si abundances to solar neighbourhood massive stars. A wind analysis of this early B-type supergiant reveals a mass-loss rate of M˙=1.6×10−6 M⊙ yr−1, and v∞=1650 km s−1. The corresponding wind momentum is in good agreement with the wind momentum–luminosity relationship found for Galactic early-B supergiants. Observations of OB 10-WR1 are analysed using a non-LTE, line-blanketed code, to reveal approximate stellar parameters of log L/L⊙∼5.7, T∗∼75 kK, v∞∼3000 km s−1, M˙/(M⊙ yr−1)∼10−4.3 adopting a clumped wind with a filling factor of 10 per cent. Quantitative comparisons are made with the Galactic WC6 star HD 92809 (WR23) revealing that OB 10-WR1 is 0.4 dex more luminous, though it has a much lower C/He ratio (∼0.1 versus 0.3 for HD 92809). Our study represents the first detailed, chemical model atmosphere analysis for either a B-type supergiant or a Wolf–Rayet (WR) star in Andromeda, and shows the potential of how such studies can provide new information on the chemical evolution of galaxies and the evolution of massive stars in the local Universe.