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Sz. Mészáros,
C. Allende Prieto,
B. Edvardsson,
F. Castelli,
A. E. García Pérez,
B. Gustafsson,
S. R. Majewski,
B. Plez,
R. Schiavon, M. Shetrone,
A. de Vicente
[show abstract]
[hide abstract]
ABSTRACT: We present a new grid of model photospheres for the SDSS-III/APOGEE survey of
stellar populations of the Galaxy, calculated using the ATLAS9 and MARCS codes.
New opacity distribution functions were generated to calculate ATLAS9 model
photospheres. MARCS models were calculated based on opacity sampling
techniques. The metallicity ([M/H]) spans from -5 to 1.5 for ATLAS and -2.5 to
0.5 for MARCS models. There are three main differences with respect to previous
ATLAS9 model grids: a new corrected H2O linelist, a wide range of carbon
([C/M]) and alpha element [alpha/M] variations, and solar reference abundances
from Asplund et al. 2005. The added range of varying carbon and alpha element
abundances also extends the previously calculated MARCS model grids. Altogether
1980 chemical compositions were used for the ATLAS9 grid, and 175 for the MARCS
grid. Over 808 thousand ATLAS9 models were computed spanning temperatures from
3500K to 30000K and log g from 0 to 5, where larger temperatures only have high
gravities. The MARCS models span from 3500K to 5500K, and log g from 0 to 5.
All model atmospheres are publically available online.
08/2012;
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P. North,
G. Cescutti,
P. Jablonka,
V. Hill, M. Shetrone,
B. Letarte,
B. Lemasle,
K. A. Venn,
G. Battaglia,
E. Tolstoy,
M. J. Irwin,
F. Primas,
P. François
[show abstract]
[hide abstract]
ABSTRACT: We provide manganese abundances (corrected for the effect of the hyperfine
structure) for a large number of stars in the dwarf spheroidal galaxies
Sculptor and Fornax, and for a smaller number in the Carina and Sextans dSph
galaxies. Abundances had already been determined for a number of other elements
in these galaxies, including alpha and iron-peak ones, which allowed us to
build [Mn/Fe] and [Mn/alpha] versus [Fe/H] diagrams. The Mn abundances imply
sub-solar [Mn/Fe] ratios for the stars in all four galaxies examined. In
Sculptor, [Mn/Fe] stays roughly constant between [Fe/H]\sim -1.8 and -1.4 and
decreases at higher iron abundance. In Fornax, [Mn/Fe] does not vary in any
significant way with [Fe/H]. The relation between [Mn/alpha] and [Fe/H] for the
dSph galaxies is clearly systematically offset from that for the Milky Way,
which reflects the different star formation histories of the respective
galaxies. The [Mn/alpha] behavior can be interpreted as a result of the
metal-dependent Mn yields of type II and type Ia supernovae. We also computed
chemical evolution models for star formation histories matching those
determined empirically for Sculptor, Fornax, and Carina, and for the Mn yields
of SNe Ia, which were assumed to be either constant or variable with
metallicity. The observed [Mn/Fe] versus [Fe/H] relation in Sculptor, Fornax,
and Carina can be reproduced only by the chemical evolution models that include
a metallicity-dependent Mn yield from the SNe Ia.
03/2012;
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M. Tafelmeyer,
P. Jablonka,
V. Hill, M. Shetrone,
E. Tolstoy,
M. J. Irwin,
G. Battaglia,
A. Helmi,
E. Starkenburg,
K. A. Venn,
T. Abel,
P. Francois,
A. Kaufer,
P. North,
F. Primas,
T. Szeifert
[show abstract]
[hide abstract]
ABSTRACT: We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have [Fe/H] <= -3 and three stars are below [Fe/H]~-3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at [Fe/H]=-3.96 +- 0.10 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the alpha, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis. Comment: Accepted for publication in A&A
Astronomy and Astrophysics 02/2011; 527:1. · 4.59 Impact Factor
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E. Starkenburg,
V. Hill,
E. Tolstoy,
J. I. González Hernández,
M. Irwin,
A. Helmi,
L. Boschman,
P. Francois,
G. Battaglia,
P. Jablonka,
M. Tafelmeyer, M. Shetrone,
K. Venn,
T. de Boer
[show abstract]
[hide abstract]
ABSTRACT: The NIR Ca II triplet has proven to be an important tool for quantitative spectroscopy. Here we present results of synthetic spectral analysis for the Ca II triplet for low-metallicity red giant stars, combined with observational data. Our results start to deviate strongly from the widely-used and linear empirical calibrations below [Fe/H] = −2. We provide a new calibration for Ca II triplet studies which is valid down until [Fe/H] = −4 and apply this new calibration to current data sets. We suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought and discuss preliminary results and possibilities for follow-up observations of these extremely low-metallicity candidates.
EAS Publications Series 12/2010; 48:13 - 18.
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W. Aoki,
N. Arimoto,
K. Sadakane,
E. Tolstoy,
G. Battaglia,
P. Jablonka, M. Shetrone,
B. Letarte,
M. Irwin,
V. Hill,
P. Francois,
K. Venn,
F. Primas,
A. Helmi,
A. Kaufer,
M. Tafelmeyer,
T. Szeifert,
C. Babusiaux
[show abstract]
[hide abstract]
ABSTRACT: Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph. (1) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by previous estimates from the Ca triplet, is confirmed by our measurements, although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity. (2) While one object shows an overabundance of Mg (similar to Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar to the solar value. This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy. No evidence for over-abundances of Ca and Ti are found in these five stars, though the measurements for these elements are less certain. Possible mechanisms to produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are discussed. (3) Ba is under-abundant in four objects, while the remaining two stars exhibit large and moderate excesses of this element. The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo, indicating that the enrichment of heavy elements, probably by the r-process, started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.
05/2009;
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C. Allende Prieto,
S.R. Majewski,
R. Schiavon,
K. Cunha,
P. Frinchaboy,
J. Holtzman,
K. Johnston, M. Shetrone,
M. Skrutskie,
V. Smith,
J. Wilson
[show abstract]
[hide abstract]
ABSTRACT: APOGEE is a large-scale, NIR, high-resolution (R ∼ 20 000) spectroscopic survey of Galactic stars. It is one of the four experiments in SDSS-III. Because APOGEE will observe in the H band, where the extinction is six times smaller than in V, it will be the first survey to pierce through Galactic dust and provide a vast, uniform database of chemical abundances and radial velocities for stars across all Galactic populations (bulge, disk, and halo). The survey will be conducted with a dedicated, 300-fiber, cryogenic, spectrograph that is being built at the University of Virginia, coupled to the ARC 2.5-m telescope at Apache Point Observatory. APOGEE will use a significant fraction of the SDSS-III bright time during a three-year period to observe, at high signal-to-noise ratio (S /N > 100), about 100 000 giant stars selected directly from 2MASS down to a typical flux limit ofH < 13. The main scientific objectives of APOGEE are: (1) measuring unbiased metallicity distributions and abundance patterns for the different Galactic stellar populations, (2) studying the processes of star formation, feedback, and chemical mixing in theMilkyWay, (3) surveying the dynamics of the bulge and disk, placing constraints on the nature and influence of the Galactic bar and spiral arms, and (4) using extensive chemodynamical data, particularly in the inner Galaxy, to unravel its formation and evolution. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Astronomische Nachrichten 11/2008; 329(9‐10):1018 - 1021. · 1.01 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present preliminary results obtained from the first year of
observations of a new, long-term project of the University of Texas, the
Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH)
Project.
07/2008; 393:203.
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C. Allende Prieto,
T. Sivarani,
T. C. Beers,
Y. S. Lee,
L. Koesterke, M. Shetrone,
C. Sneden,
D. L. Lambert,
R. Wilhelm,
C. M. Rockosi,
D. Lai,
B. Yanny,
I. I. Ivans,
J. A. Johnson,
W. Aoki,
C. A. L. Bailer-Jones,
P. Re Fiorentin
[show abstract]
[hide abstract]
ABSTRACT: We report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which we compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R = 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), we empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km/s, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. We estimate random errors for lower S/N spectra based on numerical simulations.
11/2007;
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T. Sivarani,
T. C. Beers,
Y. Lee,
J. Krugler,
R. Wilhelm,
C. Allende Prieto,
C. Sneden,
D. L. Lambert, M. Shetrone,
J. Johnson,
I. Ivans,
C. Rockosi,
D. Lai,
H. Morrison,
W. Aoki
[show abstract]
[hide abstract]
ABSTRACT: We present a discussion of efforts to obtain external validation of the
estimated atmospheric parameters (Teff, log g, [Fe/H]) obtained from
medium-resolution (R = 2000) SDSS spectroscopy and ugriz photometry,
which are being employed for both the completed SDSS-I and the ongoing
SEGUE survey. The SDSS/SEGUE spectroscopic pipeline makes use of a
number of methods for the estimation of each parameter, with estimated
internal errors on the order of σ(Teff) = 150 K, σ(log g) =
0.4 dex, and σ([Fe/H]) = 0.3 dex. Over the course of the past two
years, we have obtained over 100 high-resolution optical spectra of
SDSS/SEGUE stars using the HET, KECK and SUBARU telescopes. For the
KECK/HIRES spectra, which have R = 40000, we have performed standard
high-resolution analyses to estimate the stellar parameters. For the HET
and KECK-ESI data, which have R = 15000 and R = 5000, respectively, we
have performed synthetic spectra matching in order to to estimate the
stellar parameters. We find that the derived stellar parameters agree
well with the SDSS/SEGUE pipeline estimates for the temperature range
5000 K < Teff < 6500K; the errors are of the order of the internal
errors expected from the SDSS/SEGUE pipeline. For effective temperatures
in the range 4000 K to 5000 K the estimated parameters from the
high-resolution spectroscopy exhibit offsets relative to the SDSS/SEGUE
pipeline values on the order of ΔTeff = 200 K, Δlogg = 0.8
dex, and Δ[Fe/H] = 0.4 dex. Similar offsets exist for stars with T
> 6500 K. The main reason for these offsets appears to arise due to
varying microturbulence, for which the medium-resolution SDSS spectra
are not sensitive. We also have performed external checks on pipline
radial velocities. We find that the errors in radial velocities are on
the order of 7 km/s for stars, which is at the expected level.
11/2006; 38:1139.
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Amina Helmi,
M. J. Irwin,
E. Tolstoy,
G. Battaglia,
V. Hill,
P. Jablonka,
K. Venn, M. Shetrone,
B. Letarte,
N. Arimoto,
T. Abel,
P. Francois,
A. Kaufer,
F. Primas,
K. Sadakane,
T. Szeifert
[show abstract]
[hide abstract]
ABSTRACT: As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART)
program, we have measured the metallicities of a large sample of stars in four
nearby dwarf spheroidal galaxies (dSph's): Sculptor, Sextans, Fornax, and
Carina. The low mean metal abundances and the presence of very old stellar
populations in these galaxies have supported the view that they are fossils
from the early universe. However, contrary to naive expectations, we find a
significant lack of stars with metallicities below [Fe/H] ~ -3 dex in all four
systems. This suggests that the gas that made up the stars in these systems had
been uniformly enriched prior to their formation. Furthermore, the metal-poor
tail of the dSph metallicity distribution is significantly different from that
of the Galactic halo. These findings show that the progenitors of nearby dSph's
appear to have been fundamentally different from the building blocks of the
Milky Way, even at the earliest epochs.
11/2006;
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[show abstract]
[hide abstract]
ABSTRACT: We have used UVES on VLT-UT2 to take spectra of 15 red giants in the
Sculptor, Fornax, Carina and Leo I dwarf spheroidal galaxies. We measure the
abundances of alpha, iron peak, s and r-process elements. No dSph giants in our
sample show the deep mixing abundance pattern seen in nearly all globular
clusters. At a given metallicity, the dSph giants exhibit lower [el/Fe]
abundance ratios for the alpha elements than stars in the Galactic halo. This
can be caused by a slow star formation rate and contribution from Type Ia SN,
and/or a small star formation event (low total mass) and mass dependent Type II
SN yields. Differences in the even-Z [el/Fe] ratios between these galaxies, as
well as differences in the evolution of the s&r-process elements are
interpreted in terms of their star formation histories. Comparison of the dSph
abundances with those of the Galactic halo reveals some consistencies. In
particular, we find stars that mimic the abundance pattern found by Nissen &
Shuster (1997) for metal-rich, high R_max, high z_max halo stars, supporting
their suggestion that disrupted dSph's may explain up to 50% of the metal-rich
halo. A comparison with the metal-poor Galactic halo reveal no consistencies
suggesting that the majority of the metal-poor Galactic halo could not have
been formed from objects similar to these dSph. Finally, we use the dSph
abundances to place new constraints on the nucleosynthetic origins, including
Cu and Mn which are more consistent with metallicity dependent SN yields than
the more commonly assumed SN Ia production.
11/2002;
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[show abstract]
[hide abstract]
ABSTRACT: We have used UVES on VLT-UT2 to take spectra of 15 individual red giant stars
in the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax,
Carina and Leo I. We measure the abundance variations of numerous elements in
these low mass stars with a range of ages (1-15Gyr old). This means that we can
effectively measure the chemical evolution of these galaxies WITH TIME. Our
results show a significant spread in metallicity with age, but an overall trend
consistent with what might be expected from a closed (or perhaps leaky) box
chemical evolution scenario over the last 10-15Gyr. We notice that each of
these galaxies show broadly similar abundance patterns for all elements
measured. This suggests a fairly uniform progression of chemical evolution with
time, despite quite a large range of star formation histories. It seems likely
that these galaxies had similar initial conditions, and evolve in a similar
manner with star formation occurring at a uniformly low rate, even if at
different times. With our accurate measurements we find evidence for small
variations in abundances which are correlated to variations in star formation
histories. The alpha-elements suggest that dSph chemical evolution has not been
affected by very high mass stars (>15-20 Msun). The abundance patterns we
measure for stars in dwarf spheroidal galaxies are significantly different from
those typically observed in the disk, bulge and inner-halo of our Galaxy. This
suggests that it is NOT possible to construct a significant fraction of our
Galaxy from STARS formed in these dwarf spheroidal galaxies which subsequently
merged into our own. Any merger scenario involving dSph has to occur in the
very early Universe whilst they are still gas rich, so the majority of mass
transfer is gas, and few stars.
11/2002;
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E. Starkenburg,
V. Hill,
E. Tolstoy,
J. I. González Hernández,
M. Irwin,
A. Helmi,
G. Battaglia,
P. Jablonka,
M. Tafelmeyer, M. Shetrone,
K. Venn,
T. de Boer
http://dx.doi.org/10.1051/0004-6361/200913759.
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B. Letarte,
V. Hill,
E. Tolstoy,
P. Jablonka, M. Shetrone,
K. A. Venn,
M. Spite,
M. J. Irwin,
G. Battaglia,
A. Helmi,
F. Primas,
P. François,
A. Kaufer,
T. Szeifert,
N. Arimoto,
K. Sadakane
http://dx.doi.org/10.1051/0004-6361/200913413.
