Superwind models for the dust shells around infrared carbon stars

Abstract

We investigate the observational effect of a superwind mass-loss phase driven by a thennal pulse in the interior of infrared carbon stars. We modify the dust density distribution by adding dust to fonn a region of enhanced density which proceeds outward. Depending on the position and the degree of the enhancement, the emergent model spectral energy distributions can be significantly different from those with conventional power-law density distributions. These new results fit the observations of some infrared carbon stars better. In particular, the deficiency in 30-100 f.Lm observed fluxes of many infrared carbon stars, compared with conventional model results, can be explained by the superwind models with standard dust grains. Our superwind models also cover a much wider range in observed lRAS 12-, 25- and 60-f.Lm colours than is possible with conventional models. The time evolution of the spectral energy distribution after a superwind can explain some observations of infrared carbon stars. We find that most of the observed infrared carbon stars are in the early history of our superwind model. This may be due to a gradual increase in the mass-loss rate on the AGB or to the selection effect of infrared carbon stars identified by the 11-f.Lm SiC feature.