Graphite particle model for NML Cygnus

Abstract

The strong infrared source NML Cygnus is typical of a wide class of infrared stars which have been identified with M-type giants or supergiants, or with Mira variables. The relatively low effective temperatures associated with such stars make them very likely candidates for the production of solid particles. It has recently been shown by one of us that graphite particles tend to form in the atmospheres of Mira variables which have only a very small excess of O over C (ref. 4). There is some evidence to suggest that an O/C ratio near unity may be appropriate to NML Cygnus as well; the formation of graphite particles in the stellar atmosphere would thus seem entirely plausible. In this communication we discuss the possibility that the infrared radiation from NML Cygnus comes from a circumstellar shell of graphite particles.