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_sun) ~= 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 [alpha/Fe]
distributions that the progenitors of the halo had a shorter timescale for
chemical enrichment than the surviving dSphs.
