SIMBAD references

We investigate the influence of stellar rotation on the H_α line formation in O-star winds. The 2-D wind model used is based on the kinematical approach by Bjorkman & Cassinelli (1993, BC), adapted to the parameter space considered in this paper. We discuss only those cases where the rotational rates are well below those that would induce an onset of disk formation. The influence of gravity darkening on the line formation is shown to be negligible, as long as appropriate averaged photospheric parameters (which then are a function of the rotational rate) are used. The distortion of the stellar radius from sphericity can likewise be neglected in most cases. Our investigations show that the H_α line formation is strongly affected by two processes which we call the resonance zone and the ρ²-effect. The former process diminishes the emission near the line core and enhances the emission in both wings due to a twist in the resonance zones induced by differential rotation. The latter process leads to an increase in the overall emission due to the density contrast between the polar and the equatorial zones caused by the deflection of material towards the equator in the BC-model. We compare the line profiles from our 2-D models with those resulting from the conventional 1-D approach, as a function of absolute or projected rotational velocity, and inclination angle and mass-loss rate. It is shown that in all cases independent of inclination angle and rotational rate, the 1-D method - for a given mass-loss rate - yields the smallest wind emission. This in turn means that all mass-loss rates presently derived from H_α are overestimated, with typical errors of 20...30%. The maximum error introduced by this simplified approach is of the order 50...70% for O-Supergiants and occurs for stars with small v_rotsini and observed nearly pole on. Moreover, our theoretical line shapes show a number of features actually found in the observations of rapidly rotating stars. Finally, the specific influence of the rotational rate and inclination angle which both, independently modify the profiles in a distinctive way may provide us with a method for the determination of sini from H_α line fits (in connection with the analysis of other spectral regions) in future investigations.