Stellar and wind parameters of Galactic O-stars: The influence of line-blocking/blanketing

Abstract

We have re-analyzed the Galactic O-star sample from [CITE] by means of line-blanketed NLTE model atmospheres in order to investigate the influence of line-blocking/blanketing on the derived parameters. The analysis has been carried out by fitting the photospheric and wind lines from H and He. In most cases we obtained a good fit, but we have also found certain inconsistencies which are probably related to a still inadequate treatment of the wind structure. These inconsistencies comprise the line cores of H\gamma and H\beta in supergiants (the synthetic profiles are too weak when the mass-loss rate is determined by matching H\alpha) and the “generalized dilution effect” (cf. Voels et al. 1989) which is still present in He i 4471 of cooler supergiants and giants. Compared to pure H/He plane-parallel models we found a decrease in effective temperatures which is largest at earliest spectral types and for supergiants (with a maximum shift of roughly 8000 K). This finding is explained by the fact that line-blanketed models of hot stars have photospheric He ionization fractions similar to those from unblanketed models at higher Teff and higher . Consequently, any line-blanketed analysis based on the He ionization equilibrium results in lower Teff-values along with a reduction of either or helium abundance (if the reduction of is prohibited by the Balmer line wings). Stellar radii and mass-loss rates, on the other hand, remain more or less unaffected by line-blanketing. We have calculated “new” spectroscopic masses and compared them with previous results. Although the former mass discrepancy [CITE] becomes significantly reduced, a systematic trend for masses below 50 seems to remain: The spectroscopically derived values are smaller than the “evolutionary masses” by roughly 10 . Additionally, a significant fraction of our sample stars stays over-abundant in He, although the actual values were found to be lower than previously determined. Also the wind-momentum luminosity relation (WLR) changes because of lower luminosities and almost unmodified wind-momentum rates. Compared to previous results, the separation of the WLR as a function of luminosity class is still present but now the WLR for giants/dwarfs is consistent with theoretical predictions. We argue that the derived mass-loss rates of stars with H\alpha in emission are affected by clumping in the lower wind region. If the predictions from different and independent theoretical simulations (Vink et al. 2000; Pauldrach et al. 2003; Puls et al. 2003a) that the WLR should be independent of luminosity class were correct, a typical clumping factor should be derived by “unifying” the different WLRs.