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

Abstract

Carbon isotopic abundances ( 12 C and 13 C) 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, indicating that all of these low-mass, low-metallicity giants have undergone extensive mixing of their outer envelopes with material 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