The Radio and Infrared Spectrum of Early-type Stars Undergoing Mass Loss

A unified model is presented for the radio and infrared spectrum of early-type stars surrounded by a gaseous ionized envelope resulting from mass loss. The cases of uniform mass loss (i) at constant velocity, and (ii) with accelerative effects taken into account, are treated. It is shown that the radio and infrared free–free spectra are predicted to be of the form $$S_\nu \propto \nu^{0.6}$$ except (typically) in the near infrared where the spectrum will become flatter. Various effects which may cause a deviation from a 0.6 spectral index are considered. Finally, it is shown how radio and infrared flux densities may be used to derive mass loss rates and to obtain lower limits to the number of ionizing photons emitted per second by the underlying stars.