Evolution of infrared carbon stars.

The paper presents model calculations on the evolution from visual carbon stars to infrared carbon stars by using a sample of 472 infrared carbon stars from the IRAS low resolution spectra. The selection of the sample based on the presence of an SiC feature is discussed. A new empirical opacity function for the SiC grain is derived, and it is noted that since many infrared carbon stars have optically-thick dust envelopes, a full radiative transfer calculation is necessary. The new SiC opacity function is used to calculate the emergent spectra of infrared carbon stars as they ascend the asymptotic giant branch (AGB) with increasing mass-loss rates. The observed energy distribution of 395 infrared carbon stars are then fitted with the model. It is suggested that there is a branching of M and C stars on the AGB with each branch evolving independently to the planetary nebula stage.