The γ Velorum binary system I. O star parameters and light ratio

In this paper we demonstrate how previous determinations of the light ratio between the O and Wolf-Rayet stellar components of the γ Vel system are affected by large uncertainties. This is due, amongst other things, to the difficulty of measuring the equivalent widths of emission and absorption lines. We then present a new technique to de-blend and measure spectral lines, in which we compensate for the observed absorption features with synthetic profiles. From the new values of the diagnostic line strengths we determine a hotter spectral type for the O star companion (O7.5) than previously published. The light ratio is then determined, together with the stellar parameters, via a spectroscopic analysis. We obtain ΔM_V=1.47±0.13 mag. From the light ratio and the system's luminosity we find M_V(O) = -5.14 mag and M_V(WR) = -3.67 mag. Simultaneously we determine T_eff(O) = 35000 K, L(O) = 2.1×10⁵ L_⊙ and M(O) = 30M_⊙. An age of 3.59 × 10⁶ yr is derived from these parameters and evolutionary tracks. We find that the H/He abundance ratio is solar. From a hydro-dynamical calculation of the radiation-driven wind we obtain M(O) = 1.8×10^-7 M⊙ yr^-1 and v_∞(O) = 2500 km s^-1. From the O star mass derived here and the mass ratio from the literature we derive the mass of the Wolf-Rayet star, M(WR) = 9 M_⊙. The mass-luminosity relation for Wolf-Rayet stars then leads to L(WR) = 1.5×10⁵ L_⊙. We finally present the γ Vel Wolf-Rayet spectrum de-convolved from the O star in the range 3800-6700 Å.