CARBON IN RED GIANTS IN GLOBULAR CLUSTERS AND DWARF SPHEROIDAL GALAXIES

CARBON IN RED GIANTS IN GLOBULAR CLUSTERS AND DWARF SPHEROIDAL GALAXIES

We present carbon abundances of red giants in Milky Way (MW) 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...

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Bibliographic Details
Published in:The Astrophysical journal 2015-03, Vol.801 (2), p.1-19
Main Authors: Kirby, Evan N, Guo, Michelle, Zhang, Andrew J, Deng, Michelle, Cohen, Judith G, Guhathakurta, Puragra, Shetrone, Matthew D, Lee, Young Sun, Rizzi, Luca
Format: Article
Language:eng
Subjects:
Carbon
Clusters
Dwarf galaxies
Halos
Metallicity
Red giant stars
Spheroidal galaxies
Stars
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Summary:We present carbon abundances of red giants in Milky Way (MW) 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 sub([middot in circle])) [Asymptotically = to] 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 SNe Ia at higher metallicities. We also identified 11 very carbon-rich giants (eight previously known) in three dSphs. However, our selection biases preclude a detailed comparison to the carbon-enhanced fraction of the MW 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 [ alpha /Fe] distributions that the progenitors of the halo had a shorter timescale for chemical enrichment than the surviving dSphs.
ISSN:0004-637X
1538-4357