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 M-V approximate to + 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.