C and N Abundances in Stars At the Base of the Red Giant Branch in M15
  We present an analysis of a large sample of moderate resolution Keck LRIS spectra of subgiants and stars at the base of the RGB in M15, most within the range 16.5 < V < 19.5 (1.2 < M_V < 4.2), with the goal of deriving C abundances (from the G band of CH) and N abundances (from the NH band at 3360 A). Star-to-star stochastic variations with significant range in both [C/Fe] and [N/Fe] are found at all luminosities extending to the subgiants at M_V ~ +3. The C and N abundances appear anti-correlated, as would be expected from the CN-cycle processing of stellar material. Yet these M15 stars are considerably fainter than the RGB bump, the point at which deep mixing is believed to set in. On this basis, while the observed abundance pattern is consistent with proton capture nucleosynthesis, we infer that the site of the reactions is likely not within the present sample. The range of variation of the N abundances is very large and the sum of C+N increases as C decreases. To reproduce this requires the incorporation not only of CN but also of ON-processed material. Combining our work with that of Trefzger et al (1983) for the brighter giants in M15, we find strong evidence for additional depletion of C among the most luminous giants. This presumably represents the first dredge up (with enhanced deep mixing) expected for such luminous cluster RGB stars in the course of normal stellar evolution as they cross the RGB bump. We compare the behavior of these patterns for C and N in globular clusters covering a wide range of metallicity and of current mass and discuss possible scenarios to reproduce the observed behavior of these key elements. (abridged) 
Judith G. Cohen, Michael M. Briley and Peter B. Stetson