EDP Sciences
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Volume 481, Number 2, April II 2008
Page(s) 499 - 506
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20078991
Published online 04 February 2008

A&A 481, 499-506 (2008)
DOI: 10.1051/0004-6361:20078991

On the importance of the wind emission to the optical continuum of OB supergiants

M. Kraus1, J. Kubát1, and J. Krticka2

1  Astronomický ústav, Akademie ved Ceské republiky, Fricova 298, 251 65 Ondrejov, Czech Republic
    e-mail: kraus@sunstel.asu.cas.cz; kubat@sunstel.asu.cas.cz
2  Ústav teoretické fyziky a astrofyziky PrF MU, 611 37 Brno, Czech Republic
    e-mail: krticka@physics.muni.cz

(Received 5 November 2007 / Accepted 15 January 2008)

Context. Thermal wind emission in the form of free-free and free-bound emission is known to show up in the infrared and radio continuum of hot and massive stars. For OB supergiants with moderate mass loss rates and a wind velocity distribution with $\beta$$\simeq$ 0.8 ... 1.0, no influence of the wind to the optical continuum, i.e. for $\lambda \la$ 1.0 $\mu$m, is expected. Investigations of stellar and wind parameters of OB supergiants over the last few years suggest, however, that for many objects $\beta$ is much higher than 1.0, reaching values up to 3.5.
Aims. We investigate the influence of the free-free and free-bound emission on the emerging radiation, especially at optical wavelengths, from OB supergiants having wind velocity distributions with $\beta \ge$ 1.0.
Methods. For the case of a spherically symmetric, isothermal wind in local thermodynamical equilibrium (LTE) we calculate the free-free and free-bound processes and the emerging wind and total continuum spectra. We localize the generation region of the optical wind continuum and especially focus on the influence of a $\beta$-type wind velocity distribution with $\beta$ > 1 on the formation of the wind continuum at optical wavelengths.
Results. The optical wind continuum is found to be generated within about 2 $R_{\rm *}$ which is exactly the wind region where $\beta$ strongly influences the density distribution. We find that for $\beta$ > 1, the continuum of a typical OB supergiant can indeed be contaminated with thermal wind emission, even at optical wavelengths. The strong increase in the optical wind emission is dominantly produced by free-bound processes.

Key words: stars: early-type -- stars: supergiants -- stars: winds, outflows -- stars: mass-loss -- stars: circumstellar matter

© ESO 2008