The detailed abundances of 23 elements in nine bright RGB stars in the Carina dSph are presented based on high resolution spectra gathered at the VLT and Magellan telescopes. A spherical model ...atmospheres analysis is applied using standard methods to spectra ranging from 380 to 680 nm. The stars in this analysis range from -2.9 < Fe/H < -1.3, and adopting the ages determined by Lemasle et al. (2012), 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; a large dispersion in Mg/Fe indicates poor mixing in the old population, an offset in the alpha/Fe ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in alpha-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. 2003). 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 AGB 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 dSphs, or the UFDs, and suggests that Carina may be at the critical mass where some chemical enrichment events are lost through SN II driven winds.
The NIR Ca II triplet absorption lines have proven to be an important tool for quantitative spectroscopy of individual red giant branch stars in the Local Group, providing a better understanding of ...metallicities of stars in the Milky Way and dwarf galaxies and thereby an opportunity to constrain their chemical evolution processes. An interesting puzzle in this field is the significant lack of extremely metal-poor stars, below Fe/H=-3, found in classical dwarf galaxies around the Milky Way using this technique. The question arises whether these stars are really absent, or if the empirical Ca II triplet method used to study these systems is biased in the low-metallicity regime. Here we present results of synthetic spectral analysis of the Ca II triplet, that is focused on a better understanding of spectroscopic measurements of low-metallicity giant stars. Our results start to deviate strongly from the widely-used and linear empirical calibrations at Fe/H<-2. We provide a new calibration for Ca II triplet studies which is valid for -0.5<Fe/H<-4. We subsequently apply this new calibration to current data sets and suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought.
Carnegie Obs. Astrophy. Series, Vol. 4, Origin and Evolution of
the Elements, 2003 Dwarf irregular galaxies appear to have undergone very slow chemical
evolution since they have low nebular ...abundances, but have had ongoing star
formation over the past 15 Gyr. They are too distant for red giant abundance
analyses to examine the details of their chemical evolution, however the
isolated, bright blue supergiants do allow us to determine their present-day
iron abundances to compare with both stellar and nebular alpha-element results.
The alpha/Fe ratios in four Local Group dwarf irregular galaxies (NGC6822,
WLM, Sextans A, and GR 8) all appear to have solar ratios regardless of the
differences in their metallicities and star formation histories. Surprisingly,
WLM's stellar metallicity is three times higher than the nebular oxygen
abundance. We compare the alpha/Fe ratios in the dwarf irregulars to those
from recent analyses of red giant branch stars in dwarf spheroidal galaxies,
and also to damped Ly-alpha systems, and discuss these in the context of model
predictions.
Dwarf irregular galaxies appear to have undergone very slow chemical evolution since they have low nebular abundances, but have had ongoing star formation over the past 15 Gyr. They are too distant ...for red giant abundance analyses to examine the details of their chemical evolution, however the isolated, bright blue supergiants do allow us to determine their present-day iron abundances to compare with both stellar and nebular alpha-element results. The alpha/Fe ratios in four Local Group dwarf irregular galaxies (NGC6822, WLM, Sextans A, and GR 8) all appear to have solar ratios regardless of the differences in their metallicities and star formation histories. Surprisingly, WLM's stellar metallicity is three times higher than the nebular oxygen abundance. We compare the alpha/Fe ratios in the dwarf irregulars to those from recent analyses of red giant branch stars in dwarf spheroidal galaxies, and also to damped Ly-alpha systems, and discuss these in the context of model predictions.
Astrophys.J.655:212-232,2007 We present the initial results from the Spitzer Survey of the Small
Magellanic Cloud (S3MC), which imaged the star-forming body of the Small
Magellanic Cloud (SMC) in all ...seven MIPS and IRAC wavebands. We find that the
F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and
takes a wide range of values that are unrelated to metallicity but
anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that
photodestruction is primarily responsible for the low abundance of PAHs
observed in star-forming low-metallicity galaxies. We use the S3MC images to
compile a photometric catalog of ~400,000 mid- and far-infrared point sources
in the SMC. The sources detected at the longest wavelengths fall into four main
categories: 1) bright 5.8 um sources with very faint optical counterparts and
very red mid-infrared colors (5.8-8.0>1.2), which we identify as YSOs. 2)
Bright mid-infrared sources with mildly red colors (0.16<5.8-8.0<0.6),
identified as carbon stars. 3) Bright mid-infrared sources with neutral colors
and bright optical counterparts, corresponding to oxygen-rich evolved stars.
And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This
excess is reminiscent of debris disks, and is detected in only a small fraction
of these stars (<5%). The majority of the brightest infrared point sources in
the SMC fall into groups one to three. We use this photometric information to
produce a catalog of 282 bright YSOs in the SMC with a very low level of
contamination (~7%).
We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S3MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC ...wavebands. We find that the F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 um sources with very faint optical counterparts and very red mid-infrared colors (5.8-8.0>1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colors (0.16<5.8-8.0<0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars (<5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%).
PDF