The Chemical Inhomogeneity of Faint M13 Stars: Carbon and Nitrogen Abundances

Building upon earlier observations which demonstrate substantial star-to-stardifferences in the carbon abundances of M13 subgiants, we present new Keck LRISspectra reaching more that 1.5 mag below the M13 main-sequence turn-off (to V ~20). Our analysis reveals a distribution of C abundances similar to that foundamong the subgiants, implying little change in the compositions of the M13stars at least through the main-sequence turn-off. We presume these differencesto be the result of some process operating early in the cluster history. Additional spectra of previously studied bright M13 giants have been obtainedwith the Hale 5-m. A comparison of C abundances derived using the presentmethods and those from the literature yield a mean difference of 0.03+-0.14 dexfor four stars in common with Smith et al. (1996) and 0.14+-0.07 dex for starsalso observed by Suntzeff (1981) (if one extreme case is removed). We concludethat the lower surface C abundances of these luminous giants as compared to thesubgiants and main-sequence stars are likely the result of mixing rather than adifference in our abundance scales. NH band strengths have also been measured for a handful of the most luminousM13 turn-off stars. While molecular band formation in such stars is weak,significant star-to-star NH band strength differences are present. Moreover,for the stars with both C and N measurements, differences between stars inthese two elements appear to be anticorrelated. Finally, the most recent C and N abundances for main-sequence, main-sequenceturn-off, and subgiant stars in 47 Tuc, M71, M5, and the present M13 data arecompared.Comment: To be published in The Astronomical Journal (March 2004