Astronomy and Astrophysics, volume 300, 237-258 (1995/8-1)
Chemical composition of Wolf-Rayet stars. II. Hydrogen-to-helium ratio.
NUGIS T. and NIEDZIELSKI A.
Abstract (from CDS):
The formation of HeII, HeI and HI lines in the winds of some representative WN stars of different spectral subtypes has been modeled. Two different types of models were studied: the clumped winds and smooth winds for both of which the standard velocity law v=v0+(v∞-v0) (1-R*/r)β with β=1 was assumed. The smooth winds model predicts about two times lower IR fluxes than observed if one determines the matter density in the winds through the observed values of radio fluxes. The increased density smooth wind models with 1.5 times higher density as compared to the radio-flux scaling are in reasonable agreement with IR fluxes and HeII and HeI line fluxes in the case of WN 5, WN 6 and WN 8 stars but for other subtypes these models predict discrepant line fluxes. The clumped wind models agree quite well with the most important observational data whereas somewhat lower mass loss rates are now derived as compared to the smooth wind models. Theoretical line fluxes were found by summing of the contributions from different layers of the wind. The statistical equilibria equations for level populations were solved in the Sobolev approximation by taking into account the overlap of HeII and HI lines in the expanding medium. We used 40, 20 and 52 level atomic models for HeII, HI and HeI respectively, whereas the influence of higher levels was taken into account through the correction terms. On the basis of our modelling study we derived a simple formula for the determination of hydrogen-to-helium ratios for WN stars which was used for concrete estimates for 28 stars. In all cases the hydrogen-to-helium ratios are lower than the mean cosmic value. By inspection of the line fluxes of the neighboring HeII lines of (n-4), (n-6) and (n-8) series in the spectra of two WC stars we concluded that no hydrogen seems to be present in their winds. The hydrogen-to-helium ratio is decreasing when going from late to early WN subtypes with strong scatter existing among the stars of WN 6 and later subtypes.