2003PASP..115.1211S

The behavior of carbon abundance as a function of luminosity is used to compare the rates of deep mixing within red giants of four globular clusters and the Galactic halo field population. Measurements of [C/Fe] for the clusters M92, NGC 6397, M3, and M13 have been compiled from the literature, together with the Gratton et al. data for halo field stars. Plots of [C/Fe] versus absolute visual magnitude show that for M_V<+1.6 the rate of decline of carbon abundance with increasing luminosity on the red giant branch is d[C/Fe]/dM_V~0.22±0.03 among the field stars, as well as in M92, NGC 6397, and M3. Among giants fainter than M_V=+1.6 the variation of [C/Fe] with absolute magnitude is much less. The data indicate that the rate at which deep mixing introduces carbon-depleted material into the convective envelopes of field halo stars during the upper red giant branch phase of evolution is similar to that of many globular cluster giants. The notable exception appears to be M13, in which stars exhibit deep mixing at a greater rate; this may account for the high incidence of very low oxygen abundances among the most luminous giants in M13 in comparison to M3.