The chemical composition of carbon stars. I. Carbon, nitrogen, and oxygen in 30 cool carbon stars in the Galactic disk

The chemical compositions of 30 Galactic carbon stars are determined from high-resolution infrared (1.5-2.5 ¡im) spectra and new model atmospheres. Elemental C, N, and O abundances (with respect to hydrogen) are obtained from lines of the C2 Phillips system’s Ay = -2 sequence, the CN red system’s Ay = -2 sequence, and the CO X '2 vibration-rotation second-overtone bands. The C/C ratios are extracted from the CN Ay = —2 sequence and, in two semi-independent ways, from the CO firstand second-overtone bands. Metal abundances are determined from a handful of atomic fines. Spectra at 3-4 jüim for a few stars provide additional fines which are used as checks. These include the HCN 3 fim bands, the CN red system’s Af = 3 sequence, and the NH X 2 and the CH X U fundamental vibration-rotation bands. The H2 1-0 S(0) fine near 2.22 ^m is also available as a check. Model atmospheres accounting for the fine blanketing by the poly atomics HCN and C2H2 were computed for a grid of effective temperatures and surface gravities. Selected models for differing opacities and assumptions (e.g., surface heating) were computed. Effective temperatures are on the scale established by lunar occultation angular diameters. The surface gravity is based on a mean stellar mass and absolute luminosity. The principal results are as follows: 1. The dispersion in C, N, and O abundances for the 30 program stars is small. 2. The stars show only modest carbon enrichments; the most carbon-rich star has C/O -1.6, and about 50% of the stars in the sample have C/O <1.1. 3. The C abundance is, in general, low, with 30<C/C < 70 encompassing the larger part of our sample, and the very C-rich (C/C 4) stars are the exceptions, not the rule. 4. The oxygen abundances are, in general, slightly subsolar and in good agreement with results for M and MS giants (progenitors) and carbon-rich planetary nebulae (descendants). 5. The nitrogen abundances are subsolar and below the values measured for M and MS giants and planetary nebulae. This difference may reflect an error in the CN dissociation energy or possibly in the effective temperature scale. 6. The metal abundances are approximately solar. 7. The weak H2 fines are not evidence for hydrogen-deficient atmospheres. It is shown that the observed CH fine strengths are less than the predicted strengths. This discrepancy is discussed but not resolved. Possible explanations include non-LTE effects in the excitation and/or formation of CH, strengthened by thermal inhomogeneities, and a significant underestimate of the effective temperature scale. For the typical star with a low C content, conversion to a carbon star was achieved by addition of pure C, a product of helium burning, to the envelope of a typical G, K, or M giant. The C-rich stars are not nitrogen-rich and, therefore, may not have undergone severe exposures to the hydrogen-burning CNO cycles. It is shown that the abundances may be used to calibrate published spectrophotometric indices of C2, CO, and CN bands. Such a calibration exposes fainter carbon stars to quantitative analysis. Subject headings: stars: abundances — stars: carbon 1 Visiting Observer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.