Carbon Isotopic Abundances in the Red Giants of Centauri (NGC 5139)
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abstract
12C and 13C abundances have been measured in 11 bright giant members of the globular cluster Centauri via observations of the first-overtone CO bands near 2.3 m. The stars in this sample were selected to span a substantial fraction of the range of iron abundances found in this cluster. In addition, the sample covers a range of [O/Fe], [Na/Fe], and [Al/Fe] abundance ratios derived in previous studies. In all Cen giants the 12C/13 C abundance ratio is found to be quite low, indicating deep mixing in these red giants. The mean value for the entire sample is 12C/13C = 4.3 0.4 ( = 1.3), with nine stars equal, within the errors, to the equilibrium ratio 12C/13 C = 3.5 and two stars having slightly higher values. There is no correlation between the 12C/13C and the abundance of iron. In addition, no correlation of 12C/ 13C with [12C/Fe] is found (all giants are deeply mixed), although the derived abundances of [12C/Fe] show a positive correlation with [O/Fe] and an anticorrelation with [Na/Fe] (with the oxygen and sodium abundances taken from previous studies in the literature). A comparison of the isotopic carbon ratios in Cen with those from other globular clusters (M4, M71, NGC 6752, and 47 Tue) and with literature oxygen abundances, may reveal a slight trend of decreasing 12C/ 13C ratios with decreasing [O/Fe] in the entire globular cluster sample of red giants, A comparison between 12C/13C and both [Na/Fe] and [Al/Fe], however, reveals no trend.
