Bibcode
Venn, Kim A.; Shetrone, Matthew D.; Irwin, Mike J.; Hill, Vanessa; Jablonka, Pascale; Tolstoy, Eline; Lemasle, Bertrand; Divell, Mike; Starkenburg, Else; Letarte, Bruno; Baldner, Charles; Battaglia, Giuseppina; Helmi, Amina; Kaufer, Andreas; Primas, Francesca
Bibliographical reference
The Astrophysical Journal, Volume 751, Issue 2, article id. 102, 25 pp. (2012).
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6
2012
Journal
Citations
138
Refereed citations
124
Description
The detailed abundances of 23 chemical elements in nine bright red giant
branch stars in the Carina dwarf spheroidal galaxy are presented based
on high-resolution spectra gathered at the Very Large Telescope (VLT)
and Magellan telescopes. A spherical model atmospheres analysis is
applied using standard methods (local thermodynamic equilibrium and
plane-parallel radiative transfer) to spectra ranging from 380 to 680
nm. Stellar parameters are found to be consistent between photometric
and spectroscopic analyses, both at moderate and high resolution. The
stars in this analysis range in metallicity from -2.9 < [Fe/H]
<-1.3, and adopting the ages determined by Lemasle et al., we are
able to examine the chemical evolution of Carina's old and
intermediate-aged populations. One of the main results from this work is
the evidence for inhomogeneous mixing in Carina and therefore for a poor
statistical sampling of the supernova contributions when forming stars;
a large dispersion in [Mg/Fe] indicates poor mixing in the old
population, an offset in the [α/Fe] ratios between the old and
intermediate-aged populations (when examined with previously published
results) suggests that the second star formation event occurred in
α-enriched gas, and one star, Car-612, seems to have formed in a
pocket enhanced in SN Ia/II products. This latter star provides the
first direct link between the formation of stars with enhanced SN Ia/II
ratios in dwarf galaxies to those found in the outer Galactic halo
(Ivans et al.). Another important result is the potential evidence for
SN II driven winds. We show that the very metal-poor stars in Carina
have not been enhanced in asymptotic giant branch or SN Ia products, and
therefore their very low ratios of [Sr/Ba] suggests the loss of
contributions from the early SNe II. Low ratios of [Na/Fe], [Mn/Fe], and
[Cr/Fe] in two of these stars support this scenario, with additional
evidence from the low [Zn/Fe] upper limit for one star. It is
interesting that the chemistry of the metal-poor stars in Carina is not
similar to those in the Galaxy, most of the other dwarf spheroidal
galaxies, or the ultra faint dwarfs, and suggests that Carina may be at
the critical mass where some chemical enrichments are lost through SN II
driven winds.
This project is partially based on VLT FLAMES spectroscopic observations
obtained at the European Southern Observatory, proposals 074.B-0415 and
076.B-0146, and partially based on MIKE spectra gathered at the 6.5 m
Magellan Telescopes, in Chile.