Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies
We present carbon abundances of red giants in Milky Way globular clusters and dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the clusters decreases with increasing luminosity above log(L/L_⊙)≃1.6, which can be explained by deep mixing in evolved giants. The same decrease is observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform. Stars in dSphs at lower metallicities have larger [C/Fe] ratios. We hypothesize that [C/Fe] (corrected to the initial carbon abundance) declines with increasing [Fe/H] due to the metallicity dependence of the carbon yield of asymptotic giant branch stars and due to the increasing importance of Type Ia supernovae at higher metallicities. We also identified 11 very carbon-rich giants (8 previously known) in three dSphs. However, our selection biases preclude a detailed comparison to the carbon-enhanced fraction of the Milky Way stellar halo. Nonetheless, the stars with [C/Fe] < +1 in dSphs follow a different [C/Fe] track with [Fe/H] than the halo stars. Specifically, [C/Fe] in dSphs begins to decline at lower [Fe/H] than in the halo. The difference in the metallicity of the [C/Fe] "knee" adds to the evidence from [α/Fe] distributions that the progenitors of the halo had a shorter timescale for chemical enrichment than the surviving dSphs.
© 2015 American Astronomical Society. Received 2014 December 5; accepted 2015 January 27; published 2015 March 12. Submitted on Jan. 27 2015. We thank Vinicius Placco for kindly computing astra- tion corrections to carbon abundances. We also thank Chris Sneden for constructive comments on the structure of this paper and the anonymous referee for an insightful report. M.G., A.J.Z., and M.D. carried out their work through UCSC's Science Internship Program for highschool students. P.G. acknowledges support from NSF grants AST-1010039 and AST-1412648. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. Facility: Keck:II (DEIMOS
Published - 0004-637X_801_2_125.pdf
Submitted - 1501.06908v1.pdf