The gamma 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 ar e affected by large uncertainties. This is due, amongst other things, to th e difficulty of measuring the equivalent widths of emission and absorption lines. We then present a new technique to de-blend and measure spectral lin es, in which we compensate for the observed absorption features with synthe tic profiles. From the new values of the diagnostic line strengths we deter mine a hotter spectral type for the O star companion (07.5) than previously published. The light ratio is then determined, together with the stellar parameters, v ia 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 MV(W R) = 3.67 mag. Simultaneously we determine T-eff(O) = 35 000 K, L(O)= 2.1 x 10(5) L. and M(O) = 30 M.. An age of 3.59 x 10(6) yr is derived from these parameters and evolutionary tracks. We find that the H/He abundance ratio is solar. From a hydrodynamical calculation of the radiation-driven wind we obtain (M) over dot (O)= 1.8 x 10(-7) M. yr(-1) and v(infinity)(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 x 10(5) L.. We final ly present the gamma Vel Wolf-Rayet spectrum de-convolved from the O star i n the range 3800-6700 Angstrom.