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 <SUP>12</SUP>C/<SUP>13</SUP>C ratio is between 20 and 90 except for a few <SUP>13</SUP>C-rich stars. When line blanketing by HCN and C<SUB>2</SUB>H<SUB>2</SUB> is included, models predict the observed intensity of the H<SUB>2</SUB> quadrupole lines. This and other evidence shows that the carbon stars are not hydrogen-deficient.