NLTE models of line-driven stellar winds II. O stars in SMC

Abstract

We calculate NLTE line-driven wind models of selected O stars in the spectral range of O4 to O9 in the Small Magellanic Cloud (SMC). We compare predicted basic wind properties, i.e. the terminal velocity and the mass-loss rate with values derived from observation. We found relatively good agreement between theoretical and observed terminal velocities. On the other hand, predicted mass-loss rates and mass-loss rates derived from observation are in a good agreement only for higher mass-loss rates. Theoretical mass-loss rates lower than approximately 10^{-7} M_o/year are significantly higher than those derived from observation. These results confirm the previously reported problem of weak winds, since our calculated mass-loss rates are in a fair agreement with predictions of Vink et al. (2001). We study multicomponent models for these winds. For this purpose we develop a more detailed description of wind decoupling. We show that the instability connected with the decoupling of individual wind elements may occur for low-density winds. In the case of winds with very low observed mass-loss rate the multicomponent effects are important for the wind structure, however this is not able to consistently explain the difference between predicted mass-loss rate and mass-loss rate derived from observation for these stars. Similar to previous studies, we found the dependence of wind parameters on the metallicity. We conclude that the wind mass-loss rate significantly increases with metallicity as dM/dt ~ Z^0.67, whereas the wind terminal velocity on average depends on metallicity only slightly, namely v_infty ~ Z^0.06 (for studied stars).