Issue |
A&A
Volume 499, Number 1, May III 2009
|
|
---|---|---|
Page(s) | 291 - 299 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200810319 | |
Published online | 01 April 2009 |
Parameters of galactic early B supergiants
The influence of the wind on the interstellar extinction determination
1
Astronomický ústav, Akademie věd České republiky, Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: [kraus;kubat]@sunstel.asu.cas.cz
2
UMR 6525 H. Fizeau, Univ. Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, Av. Copernic, 06130 Grasse, France e-mail: Marcelo.Borges@obs-azur.fr
3
Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
Received:
3
June
2008
Accepted:
16
February
2009
Context. The interstellar extinction, , of OB supergiants is usually
derived from the observed color index,
, in comparison with the
intrinsic one. This method works properly only if the stellar wind does
not influence the optical continuum emission. Over the past years, many OB
supergiants have been found to have wind-velocity distributions with rather
high β values; i.e., their winds are only slowly accelerating, resulting
in relative density enhancements within the wind-continuum forming region.
It has been found that these kinds of winds might indeed influence the total
continuum emission even at optical wavelengths.
Aims. We investigate the influence of the wind on the observed color indices of OB supergiants, in order to distinguish between interstellar and circumstellar extinction contributions.
Methods. We first tested the influence of the wind on the UBV band fluxes of a model O
supergiant star with artificial winds of different β values and a fixed
interstellar extinction value. From the UBV magnitudes of these systems,
the apparent interstellar extinction was then derived by calculating the
values of , from the
as well as from the
color index.
Then we turned to a sample of galactic early-type B supergiants for which the
stellar and wind parameters are known. All except one of these stars have
β values higher than 1.0, which makes this set the most suitable one for
our investigation. We calculated each star's wind contribution to the BV
band fluxes. The observed magnitudes were corrected for the wind contributions,
and the interstellar extinctions were derived and compared with those
derived purely from the
color index.
Results. From our model supergiant, we find that with increasing β the wind
starts to influence the observable color indices. The wavelength dependence
of the wind contribution is thereby different from that of the interstellar
extinction contribution. Thus, the interstellar extinction values derived from
the two color indices of our reddened star plus wind systems disagree. This
effect is stronger for higher β. In addition, derived from the
color index always overestimates the real interstellar
extinction. This trend is also found for the investigated B supergiant sample.
Consequently, the luminosities of these stars are systematically overestimated.
Our wind model always computes lower limits to the real wind contributions.
This means that the real interstellar extinction values, hence the stellar
luminosities of the studied B supergiant sample, might well be lower.
Key words: stars: early-type / stars: fundamental parameters / supergiants / stars: winds, outflows
© ESO, 2009
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