Amorphous Carbon and Silicon Carbide Grains Around Carbon Stars

Abstract

It is well known that a large amount of dust (~ 40%) is produced in the envelopes of late-type stars (Jura, 1985). Moreover, according to Knapp and Morris (1985) a very important contribution to this amount (about 50%) comes from C-rich giant stars. Therefore, it is very interesting to study this kind of circumstellar material in order to get information on its physical, chemical and morphological properties. It is widely accepted that, in a carbon-rich stellar atmosphere, solid carbon is the dominant condensate. It is still open, however, the discussion about the structure of this material: amorphous (amorphous, glassy carbon) or crystalline (graphite). Graphite grains have been considered for quite some time (Hoyle and Wickramasinghe, 1962; Mathis et al., 1977; Mitchell and Robinson, 1980; Greenberg and Chlewicki, 1983). However the presence of this material in space has been strongly questioned because of its mechanical, thermal, optical and chemical properties, while amorphous carbon grains are more likely to exist (Czyzak and Santiago, 1973; Donn et al., 1981; Czyzak et al., 1982).