Carbon and Nitrogen Abundances in Stars at the Base of the Red Giant Branch in M5

Abstract

We present an analysis of a large sample of moderate resolution Keck LRIS spectra of subgiant (V ~ 17.2) and fainter stars in the Galactic globular cluster M5 (NGC 5904) with the goal of deriving C and N abundances. Star-to-star stochastic variations with significant range in both [C/Fe] and [N/Fe] are found at all luminosities extending to the bottom of the red giant branch at M_V ~ +3. Similar variations in CH appear to be present in the main-sequence turnoff spectra, but the signal in the current sample is too low for a detailed analysis. The variations seen among the M5 subgiants are consistent with the abundances found earlier by Briley et al. for brighter giants in this cluster. There is thus no sign of a change in the behavior of C and N with evolutionary stage over the full range in luminosity of the red giant and subgiant branches, although a systematic decrease with luminosity in the mean [C/H] smaller than a factor of 2 cannot be ruled out with confidence at present. The C and N abundances appear strongly anticorrelated, as would be expected from the CN-cycle processing of stellar material. Yet the present stars are considerably fainter than the red giant branch 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, but rather in a population of more massive (2–5 M⊙), now-defunct stars. 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. Furthermore, the existence of this correlation is quite difficult to reproduce with an external mechanism such as "pollution" with material processed in a more massive asymptotic giant branch star, which mechanism is fundamentally stochastic in nature. We therefore suggest that although the internal mixing hypothesis has serious flaws, new theoretical insights are needed and it should not yet be ruled out.

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