Carbon abundances of M92 red giant branch stars

Using CCD imaging, Fusi Pecci et al. have identified a local maximum at M-v = -0.4 in the giant branch luminosity function of three co- added globular clusters having metallicities [Fe/H]similar to 22.2, including M92 (NGC 6341). Theories of deep mixing predict that surface carbon abundance depletions should be produced only within stars brighter than this luminosity function peak. Only in such red giants should the circulation-inhibiting molecular weight discontinuity between the base of the convective envelope and the hydrogen-burning shell be absent. However, spectroscopic analyses of M92 giants by Carbon et al. and Langer et al. indicate that surface carbon depletions may set in as faint as M-v > 2.0. In order to further test this potential discord between theory and observations, KPNO 4 m telescope Hydra spectra were obtained of a sample of 50 red giants in M92 covering the absolute magnitude range -2.4 < M-v < 1.4. Carbon abundances were determined by comparing a G-band index measured for the program stars to index values computed from synthetic stellar spectra. Analysis of these index values confirms a pattern of decreasing carbon abundance as a function of increasing evolutionary state among cluster members, in accord with the results of Carbon et al. and Langer et al. Our analysis indicates that carbon depletion sets in at absolute magnitudes at least as faint as M-v = 0.5-1.0, well below that of the luminosity function peak at M-v = -0.4 for metal-poor clusters.