Calibration of the CH and CN Variations Among Main-Sequence Stars in M71 and in M13

Abstract

An analysis of the CN and CH band strengths measured in a large sample of M71 and M13 main-sequence stars by Cohen is undertaken using synthetic spectra to quantify the underlying C and N abundances. In the case of M71 it is found that the observed CN and CH band strengths are best matched by the identical C/N/O abundances which fit the bright giants, implying: (1) little if any mixing is taking place during red giant branch ascent in M71, and (2) a substantial component of the C and N abundance inhomogeneities is in place before the main-sequence turn-off. The unlikelihood of mixing while on the main sequence requires an explanation for the abundance variations which lies outside the present stars (primordial inhomogeneities or intracluster self enrichment). For M13 it is shown that the 3883 Å CN bands are too weak to be measured in the spectra for any reasonable set of expected compositions. A similar situation exists for CH as well. However, two of the more luminous program stars do appear to have C abundances considerably greater than those found among the bright giants, thereby suggesting deep mixing has taken place on the M13 red giant branch.