A Lifetime of Influence: Abstracts, Talks, and Posters
 

UV Spectrum and Properties of the Mass Donor Star in HD 226868 = Cygnus X-1

Saida Caballero
Georgia State University

Douglas Gies
Georgia State University

We present continued results of high resolution ultraviolet spectroscopy from Hubble Space Telescope of the massive X-ray binary HD 226868 = Cyg X-1 which consists of an O supergiant with a black hole (BH) companion. The spectra were used in conjunction with International Ultraviolet Explorer (IUE) and ground-based optical spectra to determine effective temperature and gravity of the O9.7 Iab supergiant. Using non-LTE, line blanketed, plane parallel TLUSTY models, we obtain an approximate temperature of 26 kK and gravity of log g = 2.8 dex for the supergiant, both lower than in previous studies. We fit a spectral energy distribution for this temperature and gravity to the UV, optical and IR fluxes to determine the angular size and extinction towards the binary. The angular size we used to constrain the lower mass limits of each of the components as a function of distance following the argument proposed by Paczynski (1974). Our results agree with a distance <2 kpc and masses ranging from 18-32 solar masses for the supergiant and 8-16 solar masses for the BH, slightly lower values than in previous estimates (Ziolkowski 2005).