ABUNDANCES IN RED GIANT STARS - NITROGEN ISOTOPES IN CARBON-RICH MOLECULAR ENVELOPES

Millimeter- and submillimeter-wave observations of HCN and HCCCN have been made of the circumstellar envelopes of eight carbon stars, including the two protoplanetary nebulae: CRL 618 and CRL 2688. The observations yield a measure of the double ratio: N-14-C-13/N-15-C-12. By using measured C-12/C-13 ratios, we estimate the N-14/N-15 abundance ratio, with resulting lower limits in all eight envelopes and possible direct determinations in two envelopes. The two determinations, and four of the remaining six lower limits are in excess of the terrestrial value of N-14/N-15 = 272, indicating an evolution of the nitrogen isotope ratio, consistent with stellar CNO processing. Observations of thermal SiO (v = 0, J = 2-1) emission show that the Si-29/Si-28 ratio can be determined in carbon stars, and further observations are indicated.