Nitrogen Variations Among Faint Globular Cluster Stars via ubvy Photometry

Recent deep ubvy photometry in 18 Galactic globular clusters has revealed surprisingly large and significant scatters in the c_{1 = (u-v) - (v-b)} index for red giant branch (RGB) stars as faint as the base of the sub--giant branch. We present modeling with synthetic spectra and ubvy filter curves which indicates that there is ``contamination'' in the u filter by the NH band at 3360{ Angstroms} and that the spread in c<sub>1</sub> colors is due to large (factor 10) variations in the N abundances of these stars. We also find that for all clusters the c<sub>1</sub> scatter remains relatively constant from the base of the RGB to the luminosity of the RGB bump. At luminosities higher than the RGB bump the scatter decreases. We interpret this narrowing of the c<sub>1</sub> scatter as the onset of mixing and the subsequent transport of N-enriched material into the stellar envelopes. This has the effect of reducing the relative spread in N abundance between the ``N--rich'' and ``N--poor'' groups. We note that if, as theory predicts, there is no significant mixing at luminosities lower than the RGB bump, then we conclude (from our 18 cluster sample) that all metal--poor clusters are likely to have strong N abundance variations, caused by self- and/or pre--enrichment. This work was supported in part by a grant from the National Science Foundation (AST-9624680 to MMB).