Carbon abundances in red giant stars in the globular clusters M92 and NGC 6397

Abstract

Results of carbon-abundance determinations based on synthetic spectrum analyses are presented for the two metal-poor globular clusters, M92 and NGC 6397. Representative color-magnitude diagrams are examined, and synthetic spectra, synthetic colors, and model atmospheres are calculated. Observed and computed spectra are compared, and photometric data are also analyzed. The results indicate a weakening of the G-band strength for most stars along the upper giant branches in both clusters, including even the coolest stars at the tip of the giant branch. This weakening of the G band is shown to correspond to a carbon depletion such that the logarithmic carbon abundance relative to all other elements except H and He lies between -0.5 and -1.0.