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Atmospheres

The fundamental properties of RCBs may be deduced from their spectra at maximum light. A spectrum represents the overall distribution of flux with energy (or wavelength). The properties of a stellar atmosphere may be measured from both the large-scale features - the broad-band flux distribution - and the detailed spectrum of absorption lines. Properties of interest are the effective temperature and the gravity at the stellar surface (these also provide a measure of the luminosity to mass ratio) and the chemical composition. The former can be compared with models for stellar structure and evolution, whilst the composition may provide clues to the past evolution of the star. RCB effective temperatures fall mostly around 7000K, although a small number are as cool as 5500K ([Kilkenny & Whittet 1984],[Asplund et al. 1996]). The surface gravities are also low, confirming that RCBs have high $L/M$ ratios, as suspected from the LMC observations. The spectra of most RCBs have now been analyzed in detail ([Asplund et al. 2000]). They show that, in general, hydrogen makes up less than one part per thousand down to less than one part per million of the stellar atmosphere. Most of the atmosphere is neutral helium. Although carbon is enriched, it is not possible to establish its abundance reliably. All RCB atmospheres show products of hydrogen- and helium-burning nucleosynthesis in several different episodes. They also show mild enhancements of s-process elements; these are elements formed as a result of certain nuclei being bombarded by neutrons while the star was a red giant.

The broad-band flux distributions of RCBs show an excess amount of flux at low energies. This `infrared excess' corresponds to cool material with a temperature of a few hundred degrees Kelvin which surrounds the star ([Kilkenny & Whittet 1984], [Walker 1985]).