CH, CN, and NH band strengths of metal-poor cluster and field red giants

The strengths of the CN, NH, and CH bands in the spectra of Population II red giants are discussed based on observations of a sample of both globular cluster and field giants. We confirm an anticorrelation between the oxygen abundance and the CN indices and a correlation between the 3883 Angstrom CN band and the 3360 Angstrom NH band strengths among giants in the globular clusters M15 and M13. We find one M15 star (IV-38) with a strong 3883 Angstrom CN band and detectable 4215 Angstrom CN and 3360 Angstrom NH bands. Abundance analysis of M15 IV-38 reveals a comparable carbon abundance to a comparison star that has much weaker CN bands. One possible reason for this may be that IV-38 is an insipid CH star that has also undergone some deep mixing. Alternatively, the atmosphere of IV-38 may have been extensively processed through the O --> N cycle of hydrogen burning such that carbon has been affected by O --> C conversion. By combining our data set with material from the literature and high-quality B-V photometry, we find that the CN band strength differences among the M5 red giants do not appear to broaden the giant branch of that cluster within the photometric errors. Using the CN, CH, and NH bands, we are able to confirm that the metal-poor field giant HD 135148 is a CH-strong star and has not undergone deep mixing.