2002PASP..114.1097S

Measurements of the λ3883 CN band absorption strength in spectra of red giant members of the globular cluster M3 have been compiled from several sources in the literature, including the pioneering survey of N. B. Suntzeff, and transformed onto a common system. The homogenized indices give a picture of the behavior of the CN band strength over a range in stellar magnitude of ∼3.6 mag, from the tip of the red giant branch (RGB) to M_V~+1.0. As with other well-studied globular clusters, the red giant branch stars of M3 exhibit a bimodal distribution of CN band strengths that extends to the limit of the available survey. The asymptotic giant branch stars also show evidence of an intrinsic spread in CN content. Anticorrelations exist between the CN band strength and both carbon and oxygen abundances, suggesting that the high nitrogen abundances of the CN-strong giants have derived from a conversion of C and/or O into nitrogen, either within the cluster stars themselves or in some primordial site. Among both CN-strong and CN-weak stars, the [C/Fe] abundance decreases with increasing luminosity on the RGB. Stellar atmosphere models indicate that in the magnitude range -1.5<M_V<0, the CxN abundance product is roughly constant among the CN-weak giants, while it decreases toward the upper RGB among CN-strong stars. This decrease in the product of CxN may indicate that carbon-to-nitrogen conversion plus deep mixing is occurring within the CN-strong giants to a greater degree than in the CN-weak giants. These results, when interpreted with additional constraints from the giant branch luminosity function of M3, suggest that the C and N abundance pattern requires a hybrid scenario involving interior mixing within red giants as well as primordial abundance inhomogeneities that precede the RGB phase of evolution.