Carbon stars and circumstellar shells. II - A model for quantitative description of the shells

Details Carbon stars and circumstellar shells. II - A model for quantitative description of the shells Carbon stars and circumstellar shells. II - A model for quantitative description of the shells

Oct 1976

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976a%26a....52..245b&link_type=abstract

Astronomy and Astrophysics, vol. 52, no. 2, Oct. 1976, p. 245-261.

Astronomy and Astrophysics

Astrophysics

32

Carbon Stars, Infrared Stars, Stellar Envelopes, Stellar Models, Cosmic Dust, Stellar Spectrophotometry, Stellar Temperature

Scientific paper

A quantitative model of a spherically symmetric gray shell is developed for the circumstellar shells of carbon stars. It is suggested that large graphite grains are responsible for the grey thermal emission up to a wavelength of 8.5 microns. The absorption optical depth of the shell is derived along with its inner angular radius and the grain temperature. The true stellar angular radius is computed by taking grain extinction into account, and the outer angular radius of the shell is obtained for simple distributions of grain number density. The shell model is examined from a dynamic point of view, and it is proposed that grains are expelled by the stellar radiation field and are limited to subsonic drift velocities by viscous drag from atmospheric gas. The data are shown to confirm the extreme stratification of cool carbon-star atmospheres. It is concluded that grain thermal emission backscattered to an observer could be responsible for the emissivity increase longward of 8.5 microns, that small graphite or silicon carbide grains should contribute strongly to the 'blue-ultraviolet depression', and that both small and large graphite grains are less depleted locally than intermediate grains.

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