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 gamma 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 Delta M_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 ifmmode T_eff else T_efffi(O) = 35 000 K, L(O) = 2.1x10(5) L_sun and cal M(O) = 30 M_sun. An age of 3.59x10(6) 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 dot{M}(O) = 1.8x10(-7) Msun yr(-1) and v_infty(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, cal M(WR) = 9 M_sun. The mass-luminosity relation for Wolf-Rayet stars then leads to L(WR) = 1.5x10(5) L_sun. We finally present the gamma Vel Wolf-Rayet spectrum de-convolved from the O star in the range 3800-6700 Angstroms.