The Chemical Inhomogeneity of Faint M13 Stars: Carbon and Nitrogen Abundances
Building upon earlier observations that 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 turnoff (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 turnoff. 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 5 m Hale Telescope. 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 the 1996 study by Smith et al. and 0.14 ± 0.07 dex for stars also observed in Suntzeff's 1981 survey (if one extreme case is removed). We conclude that the lower surface C abundances of these luminous giants as compared with 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 turnoff 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.
Additional Information© 2004 The American Astronomical Society. Received 2003 October 14; accepted 2003 December 11. Based in large part on observations obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronatics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The entire Keck/LRIS user community owes a huge debt to Jerry Nelson, Gerry Smith, Bev Oke, and many other people who have worked to make the Keck Telescope and LRIS a reality. We are grateful to the W. M. Keck Foundation, and particularly its late president, Howard Keck, for the vision to fund the construction of the W. M. Keck Observatory. We also wish to express our thanks to Roger Bell, whose SSG code was instrumental in this project, and the anonymous referee for suggestions. Partial support was provided by the National Science Foundation under grant AST 00-98489 to M. M. B. and grant AST 02-05951 to J. G. C. and by the F. John Barlow Professorship and University of Wisconsin Oshkosh Faculty Development Program (M. M. B.).
Published - Briley_2004_AJ_127_1579.pdf
Accepted Version - 0312315.pdf