The Chemical Inhomogeneity of Faint M13 Stars: C and N Abundances

Building upon earlier observations which demonstrate substantial star-to-star differences in the carbon abundances of M13 subgiants, we present new Keck LRIS spectra 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 found among the subgiants, implying little change in the compositions of the M13 stars at least through the main-sequence turn-off. We presume these differences to be the result of some process operating early in the cluster history. Additional spectra of previously studied bright M13 giants have been obtained with the Hale 5-m. A comparison of C abundances derived using the present methods and those from the literature yield a mean difference of 0.03±0.14 dex for four stars in common with Smith et al. (1996) and 0.14±0.07 dex for stars also observed by Suntzeff (1981) (if one extreme case is removed). We conclude that the lower surface C abundances of these luminous giants as compared to the subgiants and main-sequence stars are likely the result of mixing rather than a difference in our abundance scales. NH band strengths have also been measured for a handful of the most luminous M13 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 in these two elements appear to be anticorrelated. Finally, the most recent C and N abundances for main-sequence, main-sequence turnoff, and subgiant stars in 47 Tuc, M71, M5, and the present M13 data are compared. Subject headings: globular clusters: general — globular clusters: individual (M13) — stars: evolution – stars:abundances Based in large part on observations obtained at the W.M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California and NASA. Department of Physics and Astronomy, University of Wisconsin Oshkosh, 800 Algoma Boulevard, Oshkosh, WI 54901 Palomar Observatory, MS 105-24, California Institute of Technology, Pasadena, CA 91125 Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada