On the Carbon and Nitrogen Abundances of 47 Tucanae's Main-Sequence Stars

We report the results of an analysis of CN and CH band strengths among a large sample of 47 Tucanae main-sequence and turnoff stars presented earlier by Harbeck et al. The resulting C and N abundances derived from synthetic spectra demonstrate the following: (1) A strongly anticorrelated relationship is seen between [C/Fe] and [N/Fe], with the CN-strong stars exhibiting depleted carbon and enhanced nitrogen. (2) The abundances of both elements agree remarkably well with those found among the evolved red giants of the cluster, implying little change in surface abundances from at least MV ≈ +6.5 mag to the tip of the red giant branch. (3) The pattern of C depletions and N enhancements is quite similar to that seen among the turnoff stars of M71, a cluster of almost identical metallicity but lower central concentration and escape velocity. At the same time, similar if not smaller N enhancements and larger C depletions are evident among like stars in the more metal-poor cluster M5. We interpret these results, as did Harbeck et al., as evidence of the operation of some pollution or accretion event early in the cluster's history—the most likely source being asymptotic giant branch ejecta. However, the present results rule out simple surface pollution and suggest that a substantial fraction of the present stars' masses must be involved.