The Circumstellar (CO)-C-12/(CO)-C-13 Ratio in AGB Stars of Different Chemical Type

Paper in proceeding, 2015

We have investigated the evolution of the C-12/C-13 ratio along the AGB through the circumstellar isotopologue (CO)-C-12/(CO)-C-13 abundance ratio for a significant number of M-, S-, and C-type AGB stars. The isotopologue ratios are estimated through a detailed radiative transfer analysis of single-dish radio-line observations. The circumstellar (CO)-C-12/(CO)-C-13 ratio differs between the three spectral types; the median values are 13, 26, and 34 for the M-, S-type, and carbon AGB stars, respectively, consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence. The spread of the abundance ratio among carbon stars is much larger than for the other chemical types. We find no correlation between the isotopologue ratio and the mass-loss rate.