STELLAR WINDS DRIVEN BY RADIATION PRESSURE

The interaction of radiation with matter results in momentum transfer from the radiation field to the intervening medium. The resulting force points in the direction of the net radiation flux and is proportional to both the flux and the optical depth of the medium. In non-astronomical environments, such forces are usually negligible. However, when the luminosity of a star is about 10000 solar luminosities, the radiation pressure force at the top of its atmosphere can become larger than the gravitational force and the outer layers of the star are blown away. A continuous process of such mass-loss results in a stellar wind and an expanding envelope surrounding the star. We present a detailed, self-consistent model of the radiatively driven winds which couples the radiative transfer and hydrodynamics equations. The circumstellar envelope, which consists of gas and dust, is described as a two-component fluid to account for relative drifts. The radiative transfer equation is treated in the moment form. Our results show that steady-state outflows driven by radiation pressure on dust grains adequately describe the surroundings of late-type stars. Thanks to scaling properties, both the dynamics and the radiative transfer are fully characterized by τF, the flux averaged optical depth of the wind. The region of parameter space where radiation pressure can support a given mass-loss rate is identified, and it shows that radiatively driven winds can explain the highest mass-loss rates observed to date. A new procedure to derive mass-loss rates from the observational data is introduced, and its results agree with other determinations. Theoretical predictions for the dust emission are in good agreement with observations. Observed spectra are associated with different τF and various grain materials, and a new method to determine τF from infrared observations is presented. We show that analysis of infrared spectral signatures provides constraints on the grains chemical composition and find that, in carbonaceous grains, the abundance of SiC grains is limited to < 20-30%. Similarly, in mixtures of astronomical silicate and crystalline olivine, the abundance of olivine is limited to < 20-30%.