The Chemical Composition of Carbon Stars. I. Carbon, Nitrogen, and Oxygen in 30 Cool Carbon Stars in the Galactic Disk

Chemical compositions of 30 Galactic carbon stars are determined from high-resolution infrared spectra and new model atmospheres. Elemental C, N, and O abundances are quite similar to those seen in planetary nebulae and are consistent with those reported for G to M giants. Carbon stars show only modest carbon enrichments: C/O is less than 1.6, and half the sample have C/O less than 1.1. The ¹²C/¹³C ratio is between 20 and 90 except for a few ¹³C-rich stars. When line blanketing by HCN and C₂H₂ is included, models predict the observed intensity of the H₂ quadrupole lines. This and other evidence shows that the carbon stars are not hydrogen-deficient.