Dust formation and evolution around carbon stars

Evolved intermediate-mass stars with carbon-to-oxygen ratios (C/O) above unity are known as carbon stars. Carbon stars are surrounded by dust shells dominated by carbon (C) and silicon carbide (SiC) grains. These SiC grains have a diagnostic spectral feature at [about]11 [mu]m. We have selected a sample of 9 carbon stars with low mass-loss rates such that their dust shells are sufficiently optically thin to allow abundance analysis of the stars' photospheres. This allows the study of how atomic abundances affect dust formation around carbons stars. We present the result of radiative transfer modeling for these stars, and compare the resulting dust shell parameters to published abundance measurements. To constrain model parameters, we use published mass-loss rates, expansion velocities, and theoretical dust condensation models to estimate the dust condensation temperature, and spectral types to constrain stellar effective temperatures. We found significant correlations for the single-shell modeling with graphite/iron grains and amorphous carbon (AmC)/iron grains: (1) [subscript]0.55[mu]m and gas-to-dust ratio, (2) iron grains and graphite or AmC grains, (3) graphite or AmC grains and Fe/H, and (4) iron grains and Fe/H. For the collated data the significant correlations we found were: (1) for dust formation temperature and the change of temperature from the formation of graphite grains to the formation of SiC grains (2) C/O and the change of temperature from the formation of graphite grains to the formation of SiC grains. Lastly, between the abundance of SiC grains when compared to the abundances of SiC grains in graphite, AmC, graphite and iron and AmC and iron grains models. This shows that there is no sensitivity in the continuum when choosing the type of carbon to model with.