Astronomy & Astrophysics

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Issue		A&A Volume 415, Number 1, February III 2004
Page(s)		349 - 376
Section		Stellar atmospheres
DOI		10.1051/0004-6361:20034594

A&A 415, 349-376 (2004)
DOI: 10.1051/0004-6361:20034594

Stellar and wind parameters of Galactic O-stars

The influence of line-blocking/blanketing

T. Repolust¹, J. Puls¹ and A. Herrero^{2, 3}

¹ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, s/n, 38071 La Laguna, Spain

(Received 20 May 2003 / Accepted 17 October 2003 )

Abstract
We have re-analyzed the Galactic O-star sample from Puls et al. (1996) 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 $_{\rm\gamma}$ and H $_{\rm\beta}$ in supergiants (the synthetic profiles are too weak when the mass-loss rate is determined by matching H $_{\rm\alpha}$ ) 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 $T_{\rm eff}$ and higher $\log g$ . Consequently, any line-blanketed analysis based on the He ionization equilibrium results in lower $T_{\rm eff}$ -values along with a reduction of either $\log g$ or helium abundance (if the reduction of $\log g$ 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 (Herrero et al. 1992) becomes significantly reduced, a systematic trend for masses below 50 ${M}_{\odot}$ seems to remain: The spectroscopically derived values are smaller than the "evolutionary masses" by roughly 10 ${M}_{\odot}$ . 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 $_{\rm\alpha}$ 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 $<\rho^2>/<\rho>^2\, \approx 5$ should be derived by "unifying" the different WLRs.

Key words: stars: atmospheres -- stars: distances -- stars: early-type -- stars: fundamental parameters -- stars: mass loss -- stars: winds, outflows

Offprint request: T. Repolust, repo@usm.uni-muenchen.de

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