DEEP MIXING AND METALLICITY: CARBON DEPLETION IN GLOBULAR CLUSTER GIANTS

We present the results of an observational study of the efficiency of deep mixing in globular cluster red giants as a function of stellar metallicity. We determine [C/Fe] abundances based on low-resolution spectra taken with the Kast spectrograph on the 3 m Shane telescope at Lick Observatory. Spectra centered on the 4300 A CH absorption band were taken for 42 bright red giants in 11 Galactic globular clusters ranging in metallicity from M92 ([Fe/H] = –2.29) to NGC 6712 ([Fe/H] = –1.01). Carbon abundances were derived by comparing values of the CH-band strength index S 2(CH) measured from the data with values measured from a large grid of SSG synthetic spectra. Present-day abundances are combined with theoretical calculations of the time since the onset of mixing, which is also a function of stellar metallicity, to calculate the carbon depletion rate across our metallicity range. We find that the carbon depletion rate is twice as high at a metallicity of [Fe/H] = –2.3 than at [Fe/H] = –1.3, which is a result qualitatively predicted by some theoretical explanations of the deep mixing process.