The Dynamics of Stellar Outflows Dominated by Interaction of Dust and Radiation

Wind outflow around a late-type star driven by radiation pressure on dust grains is investigated in detail. The equation of motion for the outflow coupled with the equation of radiative transfer is solved treating the circumstellar envelope, which consists of gas and dust, as a two-component fluid. Because of the drift of the dust particles through the gas, the dust-to-gas ratio varies with distance even if grain formation is a prompt process. The coupling between dust and gas weakens as the mass-loss rate decreases until finally the rate of momentum transfer to the gas is insufficient to overcome gravity when the mass-loss rate is too low. This results in a lower limit, on the order of 10-7 M⊙ yr-1, to the possible mass-loss rates that can be driven by radiation pressure on dust. Radiative transfer considerations pose a bound on the capability of the radiation field to overcome gravity also in the opposite limit of large mass-loss rates. Due to the difference in optical properties between carbon stars and oxygen stars, these considerations produce a meaningful upper limit on the mass-loss rate, of several times 10-4 M⊙ yr-1, only for carbon stars