ON THE NATURE OF THE EXCESS 100 MICRON FLUX ASSOCIATED WITH CARBON STARS

Infrared Astronomical Satellite (IRAS) data indicates that many carbon stars, especially those with optically thin dust shells, have large fluxes at 60 and 100-mu-m. It has been suggested that a remnant dust shell from an earlier mass-loss episode can explain the excess fluxes. Two hypotheses have been proposed to explain the origin of the remnant dust shell. First, that O-rich asymptotic giant branch stars may undergo a transformation to C-rich by carbon dredge-up, resulting in the formation of an inner C-rich dust shell in 10(4) yr and a remnant O-rich dust shell. In the second scenario, a C-rich remnant dust shell is formed when helium shell flashes stop the mass-loss process which resumes in 10(4) yr. We have constructed radiation transport models of dust around carbon stars: models with either a single C-rich dust shell or double shells. We find that single-shell models cannot explain the observed color distribution, while two-shell models with either a C-rich or an O-rich remnant shell can. Typically the remnant shell must be about 1 pc thick and lie at a distance of 0.1 pc from the inner dust shell. To differentiate between the above two scenarios, we examined the relation between mass-loss rate and the flux ratio at long wavelengths (60/25 and 100/60-mu-m flux ratios). We compared the model results with observations using the relation between mass-loss rate and the strength of the SiC feature at 11.2-mu-m. We find that models with a single dust shell do not reproduce the observed relationship between mass-loss rate and flux ratio (i.e., objects with a low-mass loss rate have a larger flux ratio at long wavelengths), but the two-shell models do. The time scales required by the models and the necessity of a prior O-rich phase are consistent with the hypothesis that some O-rich stars evolve to form carbon stars. While we cannot rule out the existence of C-rich remnant shells, we conclude that they can only be formed after a phase of O-rich mass loss has occurred.