22 research outputs found
The Chemical and Dynamical Evolution of Isolated Dwarf Galaxies
Using a suite of simulations (Governato et al. 2010) which successfully
produce bulgeless (dwarf) disk galaxies, we provide an analysis of their
associated cold interstellar media (ISM) and stellar chemical abundance
patterns. A preliminary comparison with observations is undertaken, in order to
assess whether the properties of the cold gas and chemistry of the stellar
components are recovered successfully. To this end, we have extracted the
radial and vertical gas density profiles, neutral hydrogen velocity dispersion,
and the power spectrum of structure within the ISM. We complement this analysis
of the cold gas with a brief examination of the simulations' metallicity
distribution functions and the distribution of alpha-elements-to-iron.Comment: To appear in the proceedings of the JENAM 2010 Symposium "Dwarf
Galaxies: Keys to Galaxy Formation and Evolution" (Lisbon, 9-10 September
2010), P. Papaderos, S. Recchi, G. Hensler (eds.), Springer Verlag (2011), in
pres
The stellar content of the Hamburg/ESO survey VI. The metallicity distribution of main-sequence turnoff stars in the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactic halo
based on metal-poor main-sequence turnoff-stars (MSTO) which were selected from
the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up
moderateresolution observations (R ~ 2000) of some 682 stars (among which 617
were accepted program stars) were carried out with the 2.3m telescope at the
Siding Spring Observatory (SSO). Corrections for the survey volume covered by
the sample stars were quantitatively estimated and applied to the observed MDF.
The corrections are quite small, when compared with those for a previously
studied sample of metal-poor giants. The corrected observational MDF of the
turnoff sample was then compared with that of the giants, as well as with a
number of theoretical predictions of Galactic chemical evolution, including the
mass-loss modified Simple Model. Although the survey-volume corrected MDFs of
the metal-poor turnoff and the halo giants notably differ in the region of
[Fe/H] > -2.0, below [Fe/H] ~ -2.0, (the region we scientifically focus on
most) both MDFs show a sharp drop at [Fe/H] ~ -3.6 and present rather similar
distributions in the low-metallicity tail. Theoretical models can fit some
parts of the observed MDF, but none is found to simultaneously reproduce the
peak as well as the features in the metal-poor region with [Fe/H] between -2.0
to -3.6. Among the tested models only the GAMETE model, when normalized to the
tail of the observed MDF below [Fe/H] ~ -3.0, and with Z_{cr} =
10^{-3.4}Z_{\odot}, is able to predict the sharp drop at [Fe/H] ~ -3.6.Comment: 10 pages, 11 figures, accepted for publication in A&
Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature with
detailed chemical abundance information (spanning -4.2 < [Fe/H] < +0.3), and we
compute their space velocities and Galactic orbital parameters. We identify
members of the inner and outer stellar halo populations in our sample based
only on their kinematic properties and then compare the abundance ratios of
these populations as a function of [Fe/H]. In the metallicity range where the
two populations overlap (-2.5 < [Fe/H] < -1.5), the mean [Mg/Fe] of the outer
halo is lower than the inner halo by ~0.1 dex. For [Ni/Fe] and [Ba/Fe], the
star-to-star abundance scatter of the inner halo is consistently smaller than
in the outer halo. The [Na/Fe], [Y/Fe], [Ca/Fe], and [Ti/Fe] ratios of both
populations show similar means and levels of scatter. Our inner halo population
is chemically homogeneous, suggesting that a significant fraction of the Milky
Way stellar halo originated from a well-mixed ISM. In contrast, our outer halo
population is chemically diverse, suggesting that another significant fraction
of the Milky Way stellar halo formed in remote regions where chemical
enrichment was dominated by local supernova events. We find no abundance trends
with maximum radial distance from the Galactic center or maximum vertical
distance from the Galactic disk. We also find no common kinematic signature for
groups of metal-poor stars with peculiar abundance patters, such as the
alpha-poor stars or stars showing unique neutron-capture enrichment patterns.
Several stars and dSph systems with unique abundance patterns spend the
majority of their time in the distant regions of the Milky Way stellar halo,
suggesting that the true outer halo of the Galaxy may have little resemblance
to the local stellar halo.Comment: Accepted for publication in AJ. Full tables available upon reques
The stellar content of the Hamburg/ESO survey. IV. Selection of candidate metal-poor stars
We present the quantitative methods used for selecting candidate metal-poor
stars in the Hamburg/ESO objective-prism survey (HES). The selection is based
on the strength of the Ca II K line, B-V colors (both measured directly from
the digital HES spectra), as well as J-K colors from the 2 Micron All Sky
Survey. The KP index for Ca II K can be measured from the HES spectra with an
accuracy of 1.0 Angstrom, and a calibration of the HES B-V colors, using CCD
photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color
range 0.3 < B-V < 1.4. These accuracies make it possible to reliably reject
stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest
metallicities. A test of the selection using 1121 stars of the HK survey of
Beers, Preston, and Shectman present on HES plates suggests that the
completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the
contamination of the candidate sample with false positives is low: 50% of all
stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected.
The selection was applied to 379 HES fields, covering a nominal area of 8853
square degrees of the southern high Galactic latitude sky. The candidate sample
consists of 20,271 stars in the magnitude range 10 < B < 18. A comparison of
the magnitude distribution with that of the HK survey shows that the magnitude
limit of the HES sample is about 2 mag fainter. Taking the overlap of the sky
areas covered by both surveys into account, it follows that the survey volume
for metal-poor stars has been increased by the HES by about a factor of 10 with
respect to the HK survey. We have already identified several very rare objects
with the HES, including, e.g., the three most heavy-element deficient stars
currently known.Comment: 11 pages, 10 figures, accepted for publication in A&
Chemical abundances of distant extremely metal-poor unevolved stars
Aims: The purpose of our study is to determine the chemical composition of a
sample of 16 candidate Extremely Metal-Poor (EMP) dwarf stars, extracted from
the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first
place to verify the reliability of the metallicity estimates derived from the
SDSS spectra; in the second place to see if the abundance trends found for the
brighter nearer stars studied previously also hold for this sample of fainter,
more distant stars. Methods: We used the UVES at the VLT to obtain
high-resolution spectra of the programme stars. The abundances were determined
by an automatic analysis with the MyGIsFOS code, with the exception of lithium,
for which the abundances were determined from the measured equivalent widths of
the Li I resonance doublet. Results: All candidates are confirmed to be EMP
stars, with [Fe/H]<= -3.0. The chemical composition of the sample of stars is
similar to that of brighter and nearer samples. We measured the lithium
abundance for 12 stars and provide stringent upper limits for three other
stars, for a fourth star the upper limit is not significant, owing to the low
signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is
confirmed, but some of the lowest metallicity stars of the sample lie on the
plateau. Conclusions: The concordance of the metallicities derived from
high-resolution spectra and those estimated from the SDSS spectra suggests that
the latter may be used to study the metallicity distribution of the halo. The
abundance pattern suggests that the halo was well mixed for all probed
metallicities and distances. The fact that at the lowest metallicities we find
stars on the Spite plateau suggests that the meltdown depends on at least
another parameter, besides metallicity. (abridged)Comment: A&A in pres
The Hamburg/ESO R-process Enhanced Star survey (HERES) VI. The Galactic Chemical Evolution of Silicon
We determined the silicon abundances of 253 metal-poor stars in the
metallicity range , based on non-local thermodynamic
equilibrium (NLTE) line formation calculations of neutral silicon and
high-resolution spectra obtained with VLT-UT2/UVES. The
dependence of [Si/Fe] noticed in previous investigation is diminished in our
abundance analysis due to the inclusion of NLTE effects. An increasing slope of
[Si/Fe] towards decreasing metallicity is present in our results, in agreement
with Galactic chemical evolution models. The small intrinsic scatter of [Si/Fe]
in our sample may imply that these stars formed in a region where the yields of
type II supernovae were mixed into a large volume, or that the formation of
these stars was strongly clustered, even if the ISM was enriched by single SNa
II in a small mixing volume. We identified two dwarfs with
: HE 01313953, and HE 14301123. These
main-sequence turnoff stars are also carbon-enhanced. They might have been
pre-enriched by sub-luminous supernovae.Comment: 12 pages, 9 figures, 1 electronical table. Accepted by A &
An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy
Using high-resolution spectroscopy, we provide a determination of [Fe/H] and
[Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf
spheroidal galaxy. Based on this we explore the ages of the prevailing stellar
populations in Hercules, and the enrichment history from supernovae.
Additionally, we provide a new simple metallicity calibration for Stromgren
photometry for metal-poor, red giant branch stars. We find that the red-giant
branch stars of the Hercules dSph galaxy are more metal-poor than estimated in
our previous study that was based on photometry alone. Additionally, we find an
abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and
lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe]
trend suggests an early rapid chemical enrichment through supernovae of type
II, followed by a phase of slow star formation dominated by enrichment through
supernovae of type Ia. A comparison with isochrones indicates that the red
giants in Hercules are older than 10 Gyr.Comment: 12 pages, 11 figures. Accepted for publication in A&
Stellar Archaeology -- Exploring the Universe with Metal-Poor Stars
The abundance patterns of the most metal-poor stars in the Galactic halo and
small dwarf galaxies provide us with a wealth of information about the early
Universe. In particular, these old survivors allow us to study the nature of
the first stars and supernovae, the relevant nucleosynthesis processes
responsible for the formation and evolution of the elements, early star- and
galaxy formation processes, as well as the assembly process of the stellar halo
from dwarf galaxies a long time ago. This review presents the current state of
the field of "stellar archaeology" -- the diverse use of metal-poor stars to
explore the high-redshift Universe and its constituents. In particular, the
conditions for early star formation are discussed, how these ultimately led to
a chemical evolution, and what the role of the most iron-poor stars is for
learning about Population III supernovae yields. Rapid neutron-capture
signatures found in metal-poor stars can be used to obtain stellar ages, but
also to constrain this complex nucleosynthesis process with observational
measurements. Moreover, chemical abundances of extremely metal-poor stars in
different types of dwarf galaxies can be used to infer details on the formation
scenario of the halo. and the role of dwarf galaxies as Galactic building
blocks. I conclude with an outlook as to where this field may be heading within
the next decade. A table of ~1000 metal-poor stars and their abundances as
collected from the literature is provided in electronic format.Comment: Ludwig Biermann Award Lecture 2009; data table can be found at
http://www.cfa.harvard.edu/~afrebel/abundances/abund.html; published in AN
331, 47
X-Shooter GTO: Chemical analysis of a sample of EMP candidates
Context: Extremely metal-poor stars (EMP) are very rare objects that hold in
their atmospheres the fossil record of the chemical composition of the early
phases of Galactic evolution. Finding these objects and determining their
chemical composition provides important constraints on these early phases.
Aims: Using a carefully designed selection method, we chose a sample of
candidate EMP stars from the low resolution spectra of the Sloan Digital Sky
Survey and observed them with X-Shooter at the VLT to confirm their
metallicities and determine abundances for as many elements as possible.
Method: The X-Shooter spectra are analysed by means of one-dimensional,
plane-parallel, hydrostatic model atmospheres. Corrections for the granulation
effects are computed using CO5BOLD hydrodynamical simulations. Results: All the
candidates are confirmed to be EMP stars, proving the efficiency of our
selection method within about 0.5 dex. The chemical composition of this sample
is compatible with those of brighter samples, suggesting that the stars in the
Galactic halo are well mixed. Conclusions These observations show that it is
feasible to observe, in a limited amount of time, a large sample of about one
hundred stars among EMP candidates selected from the SDSS. Such a size of
sample will allow us, in particular, to confirm or refute the existence of a
vertical drop in the Galactic Halo metallicity distribution function around
[Fe/H] ~ -3.5.Comment: To be published on A&
The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactic halo
by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO
objective-prism survey (HES). The sample was corrected for minor biases
introduced by the strategy for spectroscopic follow-up observations of the
metal-poor candidates, namely "best and brightest stars first". [...] We
determined the selection function of the HES, which must be taken into account
for a proper comparison between the HES MDF with MDFs of other stellar
populations or those predicted by models of Galactic chemical evolution. The
latter show a reasonable agreement with the overall shape of the HES MDF for
[Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical
metallicity for low-mass star formation of Z_cr = 10^{-3.4} * Z_Sun reproduces
the sharp drop at [Fe/H] ~-3.6 present in the HES MDF. [...] A comparison of
the MDF of Galactic globular clusters and of dSph satellites to the Galaxy
shows qualitative agreement with the halo MDF, derived from the HES, once the
selection function of the latter is included. However, statistical tests show
that the differences between these are still highly significant. [ABSTRACT
ABRIDGED]Comment: Accepted for publication in A&