Red giant model atmospheres. III. Carbon-enriched models

We have calculated a series of model stellar atmospheres, for both solar-like and carbon-enriched compositions, based on the standard assumptions of LTE, flux constancy, and hydrostatic equilibrium in a plane-parallel geometry. Besides the usual hydrogenic opacities, we include many lines of the molecules CN, CO, C2, TiO, NH, CH, OH, and MgH as well as atomic lines, all treated by the opacity sampling method. H2O is included as a straight mean opacity. Three series of models are computed, each with log g = 0.0, having T/sub eff/ = 2500, 3000, and 3500 K. In each series we increment the carbon abundance in the atmosphere to produce ratios of C/O = 0.60 (solar), 0.90, 0.95, 0.98, 1.00, 1.02, 1.05, 1.20, and 2.00. A few additional models are computed to study the effect of other parameters. Twenty theoretical models are tabulated. Changes in the opacity and thermal structure as carbon is added to the atmosphere are examined. Compared to an M star atmosphere, opacity is lower in the model with C/O = 1.00, for which value the model resembles an atmosphere based on continuous opacities alone, whereas the opacity in a C stasr is greater than an M star. The influence of changes in turbulent pressure and surface gravity are illustrated. Changing the abundance of N in C-rich models has little effect