NH-BAND, CH-BAND, AND CN-BAND STRENGTHS IN M5 AND M13 BRIGHT RED GIANTS

The Lick 3-m telescope has been used with the Kast spectrograph to obtain moderate-resolution spectra of eight of the brighter red giants in the globular cluster M5 and four in M13. The spectra cover the region of the 3360 angstrom NH band, the 3883 angstrom CN band, and the 4300 angstrom G band. Indices sensitive to the amount of absorption caused by these molecules have been measured and combined with oxygen and sodium abundances derived by Kraft et al. (1992, AJ, 104, 645) and Sneden et al. (1992, AJ, 104, 2121) for the program stars in order to explore the relationships between C, N, O, and Na among the CN-strong and CN-weak cluster stars. Among the four M13 stars, NH appears to be anticorrelated with both [O/Fe] and CH, as well as correlated with [Na/Fe]. The NH (CH) bands are strongest (weakest) in the two stars having the strongest CN, implying that the N abundance is regulating the CN-band strength. The observations are consistent with the suggestion by Langer et al. (1993) that the envelopes of the CN-enhanced giants in M13 have been cycled through the interior ON-processing shell, where in addition to the synthesis of oxygen into nitrogen, sodium has also been produced by proton-addition reactions. Similar abundance trends appear to be evident among the majority of the stars investigated in M5. Two significant results obtained for the M5 stars are that (a) a CN versus Na correlation and a CN versus O anticorrelation is discernible even within the CN-weak group of giants in this cluster, and (b) two of the five CN-strong giants also have relatively high oxygen abundances. It appears that while interior nucleosynthesis within the ON-processing shell might account for the element peculiarities exhibited by the CN-strong, O-poor giants as well as the CN-weak giants in M5, there are some CN-strong giants in this cluster (i,e., those with relatively high [O/Fe]) that perhaps owe their CN enhancements to initial excesses of nitrogen or carbon.