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: ¹⁴N¹³C/¹⁵N¹²C. By using measured ¹²C/¹³C ratios, we estimate the ¹⁴N/¹⁵N 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 ¹⁴N/¹⁵N = 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 ²⁹Si/²⁸Si ratio can be determined in carbon stars, and further observations are indicated.

Duke Scholars

Author Ken H Young Pathology