Carbon isotopic abundances in giant stars in the CN-bimodal globular clusters NGC 6752 and M4

Carbon isotopic abundances (C-12 and C-13) are determined for 45 red giants in the CN-bimodal globular clusters M4 and NGC 6752 through low-resolution infrared spectroscopy of the first-overtone CO bands. All of the red giants are found to have quite low isotope ratios about 3-10), indicating that all of these low-mass, low-metallicity giants have undergone extensive mixing of their outer envelopes with materials processed through the CN cycle. It is also found that those giants with the strongest CN bands have the lowest C abundances; this implies an anticorrelation of the C and N abundances and again suggests the presence of various fractions of CN-processed material mixed with unprocessed material in the outer envelopes of these giants. It is argued that, with all of the observations taken together, the CN-bimodal globular clusters most probably have 'primordial' abundance variations of C and N, as evidenced by the main-sequence stars, while all of the low-mass giants undergo extensive mixing as they ascend the red giant branch, as evidenced by low C-12/C-13 ratios.