CN AND CH VARIATIONS ON THE M5 SUBGIANT BRANCH

Moderate-resolution (FWHM congruent-to 3 angstrom) blue spectra have been obtained using the Palomar 5 m telescope for a sample of 14 subgiants and six giants in the Galactic globular cluster M5. These data have been combined with MMT spectra of subgiants to yield 3883 angstrom CN band strength indices for 20 stars with M(V) > +1. A G-band (CH) index was also measured for the stars observed at Palomar. The bimodality in 3883 angstrom CN band strengths, seen among the more luminous giants by Smith & Norris and Langer, Kraft, & Friel, is clearly present. That large C and N abundance variations are found among such relatively unevolved stars suggests that the CN distribution found among the brighter stars is at least in part due to a preexisting variation. A decrease in C abundance with a corresponding increase in N is seen between the subgiant branch (SGB) and red giant branch (RGB) stars, as would be expected from mixing accompanying the RGB ascent (although the differences are on the order of the size of the error bars). Similar trends have been seen in RGB stars of many other intermediate metallicity clusters, and it is suggested that two abundance altering mechanisms are responsible for the C and N abundance variations in M5-one having produced early abundance inhomogeneities on the SGB or main sequence, and the second being RGB ascent mixing.