CARBON AND NITROGEN ABUNDANCES IN OLD, VERY METAL-POOR STARS: A REPRISE BASED ON THE LOW-RESOLUTION LICK SURVEY

A short review and summary are given of the carbon and nitrogen abundances found in the Lick survey of nearly 300 old, very metal-poor stars ([Fe/H] < — 1.3). The survey consists of population samples drawn from various stellar subsystems of the Galaxy as well as different stages of stellar evolution, and includes halo field subdwarfs (Sandage), halo field giants (Bond), and subgiants and giants of six metal-poor globular clusters. The work of a single group is emphasized to avoid systematic errors introduced by differences in observational technique and analysis; results of related investigations are integrated into the discussion. We explore whether a systematic pattern of carbon depletion and nitrogen enhancement emerges when these data taken together are examined. Three main conclusions emerge: (1) The average value of [(C + N)/Fe] lies within rather narrow limits for all stellar subsystems. This strongly suggests that the large nitrogen enhancements found in many evolved metal-poor stars are the result of conversion of interior C - N and mixing to the surface. (2) Typical large nitrogen abundances found in evolved metal-poor stars can be accounted for without recourse to either large primordial nitrogen abundances or O - N processing. The degree of carbon depletion and corresponding nitrogen enhancement is typically much larger than predicted by classical evolutionary theory. (3) The extensive -N processing that occurs in evolved, metal-poor stars proceeds further in some stellar subsystems than others, even for stars having the same metallicity and evolutionary state. This is particularly true of the most metal-poor halo field giants vis-a-vis giants in M15 and M 92. The third conclusion may place interesting constraints on models for the early history and chemical evolution of the Galaxy. Explanations such as a range in age, X(CNO), and stellar rotation are explored, the last-named being favored, but all have difficulties.