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

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 β ≃ 0.8 . . . 1.0, no influence of the wind to the optical continuum, i.e. for λ <∼ 1.0 μm, is expected. Investigations of stellar and wind parameters of OB supergiants over the last few years suggest, however, that for many objects β 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 β ≥ 1.0. Methods. For the case of a spherically symmetric, isothermal wind in local thermodynamical equilibrium (LTE) we calculate the freefree 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 β-type wind velocity distribution with β > 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∗ which is exactly the wind region where β strongly influences the density distribution. We find that for β > 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.