Abstract
The authors present an analysis of high resolution (R ∼ 17000) echelle spectra in the wavelength range 3200 to 6800 Å obtained at the Observatorio Astronómico Nacional at San Pedro Mártir, of HDE 226868, the optical component of the Cyg X-1 binary system. Empirical radial velocity vs. excitation (RV vs. E) relations are obtained from the spectra at 7 orbital phases and the slope (A) of these relations is found to vary, having two extrema: -0.30 km s-1eV-1 near phase 0.5 (collapsed object "in front") and -0.22 km s-1eV-1 near phase 0.0. At other phases the value is A = -0.13 km s-1eV-1. This variability in the slope of the RV vs. E relation is in very good agreement with the theoretically predicted anisotropic mass-loss rates resulting from the modified gravitational potential of the system (Friend and Castor, 1982) for the case in which the primary star fills its Roche lobe. Variations at the 25% level in the shapes and strengths of Hβ and He I photospheric absorption lines are detected. The variability of the absorption line profiles appears to correlate with the Hα and the He II 4686 Å emission line variations, suggesting that the absorption lines are contaminated by variable emission components. From a classical curve-of-growth analysis HDE 226868 is found to have Texc = 32100±1600° K, and to be overabundant in He and underabundant in C with respect to the B0 Ia supergiant ɛ Ori.
