Publication details
On the importance of the wind emission to the optical continuum of OB supergiants
| Authors | |
|---|---|
| Year of publication | 2008 |
| Type | Article in Periodical |
| Magazine / Source | Astronomy and Astrophysics |
| MU Faculty or unit | |
| Citation | |
| web | http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/abs/2008/14/aa8991-07/aa8991-07.html |
| Field | Astronomy and astrophysics |
| Keywords | stars: early-type -- stars: supergiants -- stars: winds outflows -- stars: mass-loss -- stars: circumstellar matter |
| Description | 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. |