S. N. MILAM, SETI Institute, NASA Ames Research Center, Moffett Field, CA 94035; N. J. WOOLF, L. M. ZIURYS, University of Arizona, Steward Observatory, Department of Chemistry, Arizona Radio Observatory, Tucson, AZ 85721.
A survey of the 12C/13C ratio towards circumstellar envelopes has been conducted at millimeter wavelengths using the telescopes of the Arizona Radio Observatory (ARO). These ratios were obtained for a sample of local AGB, Red Giant, and Supergiant stars from observations of the 12C and 13C isotopologues of CO and CN. The 1 0 transitions of both molecules were measured at 3 mm using the 12m telescope, while the 2 1 lines were obtained using the Submillimeter Telescope (SMT) at 1 mm. Ratios were established from CO using radiative transfer modeling, while the hyperfine structure was used to evaluate opacity effects in CN. For C-rich shells, the ratios fall in the range 12C/13C 25-90, while the oxygen-rich AGB objects have values of 10-35. Ratios of 12C/13C 3-14 are found for the Supergiant stars. A qualitative model has been constructed based on first and third dredge-up convective mixing that can reproduce the observed ratios. Substantial mixing of H-burning (i.e. CNO) products must occur to explain the ratios in the O-rich envelopes, while a wide range of 12C/13C values can be generated by only a few percent mixing of He-burning ashes in the carbon-rich case.