Astronomy and Astrophysics, volume 286, 523-534 (1994/6-2)
Calculations of the outflow velocity of envelopes of cool giants.
HABING H.J., TIGNON J. and TIELENS A.G.G.M.
Abstract (from CDS):
We study the transfer of momentum from photons to dust grains to (molecular) gas in the outflow around cool giants (carbon-stars, Mira variables and OH/IR stars) beyond the radius where the dust grains condense. The problem is circular: radiation pressure determines the outflow velocity of the dust and thus also the dust density; on the other hand the dust density determines, via radiative transfer effects, the spectrum of the photons and thus the effective radiation pressure. This circular problem is solved by a rapidly converging iterative procedure. The gas outflow velocity at large distances, vout, is a function of the stellar luminosity, L*, of the dust-to-gas ratio, δ, of the mass loss rate, dM/dt, and of the radius of the dust grain, a: vout∝L*0.3δ0.5 f, where f∝((dM/dt)/a)1.0 for dM/dt<10–7M☉/yr and f∝((dM/dt)/a)0.04 for dM/dt>10–5M☉/yr: for stars with large mass loss rates, such as OH/IR stars, vout does not depend on dM/dt nor on the grain radius, a. The fact that vout -a quantity that can be measured accurately and easily- depends rather strongly on δ opens the possibility to measure the dust-to-gas ratio in O-rich stars, such as OH/IR stars; this ratio is probably indicative of the primordial stellar abundance of silicium. The largest outflow velocities are predicted for carbon stars, and this is caused by the slower decrease in photon absorption cross section towards longer wavelengths. We compare our predictions with observed properties of a large sample of OH/IR stars and of Miras and find a good qualitative and quantitative agreement. We confirm a conclusion by Wood et al. (1993) that very luminous OH/IR stars in the Large Magellanic Cloud owe their low outflow velocity to the low dust-to-gas ratio, a consequence of the low metallicity of the LMC. Similarly we consider a sample of about 100 OH/IR stars within 200 pc from the galactic center that has an average AGB luminosity and an uncommonly high value of vout; we conclude that these stars are probably very metal rich, perhaps even more than the stars in the Baade window studied by Rich (1990).
stars: AGB, circumstellar matter, mass loss
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