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Sputtering in the outflows of cool stars
Authors:Carl Covatto  Per A Aannestad
Institution:Dept. of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA
Abstract:Radiation pressure acts to accelerate dust grains and, by transfer of momentum through collisions with the gas, drives the outflows of late-type stars. Some of these dust–gas collisions may be energetic enough to remove atoms from the dust grains. From an assumed initial size distribution for the dust grains, the method of Krüger et al. is used to study the evolution of a sample of spherical amorphous carbon grains under conditions typical of a late-type star. The size distribution of dust grains is presented for various sets of model parameters. One set of models assumes an initial Mathis, Rumpl & Nordsieck (MRN) distribution for the dust grains. The high-luminosity ( L ?), high-effective temperature ( T eff) set of parameters has a terminal velocity ( v term) that is near, but above , the upper limit of observed outflow velocities for carbon stars (~30 km s?1 for the assumed ? of 5×10?6 M yr?1). The low L ?, T eff model has a v term that lies near, but below , the upper limit of observed velocities. A significant amount of sputtering occurs in the high L ?, T eff model with ~40 per cent of the grain mass sputtered. About ~1 per cent of the dust mass is sputtered in the low L ?, T eff. Another set of models assumes that the dust forms with a log-normal distribution. Here, v term is nearly the same for the high L ?, T eff model as for the low L ?, T eff model. This is a result of the large amount of dust mass loss (~75 per cent) by sputtering in the high L ?, T eff model.
Keywords:stars: AGB and post-AGB  circumstellar matter
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