The γ Velorum binary system. II. WR stellar parameters and the photon loss mechanism

In this paper we derive stellar parameters for the Wolf-Rayet star in the gamma Velorum binary system (WR11), from a detailed non-LTE model of its optical and infrared spectra. Compared to the study of Schaerer et al., the parameters of the WC8 star are revised to a hotter effective temperature (T_eff ~ 57 kK), a higher luminosity (log (L/L_sun) = 5.00), and a lower mass-loss rate (log(dot {M} / M_sun/yr) = -5.0, using a 10% clumping filling factor). These changes lead to a significant decrease in wind efficiency number, from 144 to 7, so that the driving mechanism of the wind of this WR star may be simply radiation pressure on lines. The derived spectroscopic luminosity is found to be 40% lower than that derived by De Marco & Schmutz through the mass-luminosity relationship for WR stars (log (L/L_sun) = 5.2). The paper furthermore presents a comparison of the independently-developed modelling programs, cmfgen and isa-wind. Overall, there seems to be very reasonable agreement between the derived parameters for WR11, except for the carbon content, which is 2 times higher for cmfgen (C/He=0.15 vs. 0.06, by number). The comparison also confirms a disparity in the predicted flux at $lambda mbda$<400 Ä, found by Crowther et al., which will have effects on several nebular line strengths. The paper also presents the first independent check of the photon loss mechanism proposed by Schmutz. We conclude that, not only is it important to include very many lines to realistically model line blanketing, but in particular those ones that critically interact with strong resonance lines (e.g. He ii\ lambda 303.78). The inclusion of these latter lines may significantly alter the wind ionization structure. Based on observations collected at the European Southern Observatory at La Silla, Chile. ESO proposals Nrs. 56.D-327, 57.D-517 and 56.D-0700}