Carbon Abundances of M92 Red Giant Branch Stars

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Abstract
Using CCD imaging, Fusi Pecci et al. have identified a local maximum at MV = -0.4 in the giant branch luminosity function of three co‐added globular clusters having metallicities [Fe/H]~ -2.2, 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 MV>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 - 2.4VV = 0.5–1.0, well below that of the luminosity function peak at MV = -0.4 for metal‐poor clusters.