30,940 research outputs found
More pieces of the puzzle: Chemistry and substructures in the Galactic thick disk
We present a study of the chemical abundances of Solar neighbourhood stars
associated to dynamical structures in the Milky Way's (thick) disk. These stars
were identified as overdensity in the eccentricity range 0.3< ecc < 0.5 in the
Copenhagen-Geneva Survey by Helmi et al. (2006). We find that the stars with
these dynamical characteristics do not constitute a homogeneous population. A
relatively sharp transition in dynamical and chemical properties appears to
occur at a metallicity of [Fe/H] ~ -0.4. Stars with [Fe/H] > -0.4 have mostly
lower eccentricities, smaller vertical velocity dispersions, are alpha-enhanced
and define a rather narrow sequence in [alpha/Fe] vs [Fe/H], clearly distinct
from that of the thin disk. Stars with [Fe/H] < -0.4 have a range of
eccentricities, are hotter vertically, and depict a larger spread in
[alpha/Fe]. We have also found tentative evidence of substructure possibly
associated to the disruption of a metal-rich star cluster. The differences
between these populations of stars is also present in e.g. [Zn/Fe], [Ni/Fe] and
[SmII/Fe], suggesting a real physical distinction.Comment: Astrophysical Journal in press. 5 pages, 4 figure
Characterization and history of the Helmi streams with Gaia DR2
The halo of the Milky Way has long been hypothesized to harbour significant
amounts of merger debris. This view has been supported over more than a decade
by wide-field photometric surveys which have revealed the outer halo to be
lumpy. The recent release of Gaia DR2 is allowing us to establish that mergers
also have been important and possibly built up the majority of the inner halo.
In this work we focus on the Helmi streams, a group of streams crossing the
Solar vicinity and known for almost two decades. We characterize their
properties and relevance for the build-up of the Milky Way's halo. We identify
new members of the Helmi streams in an unprecedented dataset with full
phase-space information combining Gaia DR2, and the APOGEE DR2, RAVE DR5 and
LAMOST DR4 spectroscopic surveys. Based on the orbital properties of the stars,
we find new stream members up to a distance of 5 kpc from the Sun, which we
characterize using photometry and metallicity information. We also perform
N-body experiments to constrain the time of accretion and properties of the
progenitor of the streams. We find nearly 600 new members of the Helmi streams.
Their HR diagram reveals a broad age range, from approximately 11 to 13 Gyr,
while their metallicity distribution goes from 2.3 to 1.0, and
peaks at [Fe/H] 1.5. These findings confirm that the streams originate in
a dwarf galaxy. Furthermore, we find 7 globular clusters to be likely
associated, and which follow a well-defined age-metallicity sequence whose
properties suggest a relatively massive progenitor object. Our N-body
simulations favour a system with a stellar mass of accreted Gyr ago. The debris from the Helmi
streams is an important donor to the MilkyWay halo, contributing approximately
15\% of its mass in field stars and 10\% of its globular clusters.Comment: 14 pages, 17 figures, submitte
The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?
Multi-zone chemical evolution models (CEMs), differing in the nucleosynthesis
prescriptions (yields) and prescriptions of star formation, have been computed
for the Milky Way. All models fit the observed O/H and Fe/H gradients well and
reproduce the main characteristics of the gas distribution, but they are also
designed to do so. For the C/H gradient the results are inconclusive with
regards to yields and star formation. The C/Fe and O/Fe vs. Fe/H, as well as
C/O vs. O/H trends predicted by the models for the solar neighbourhood zone
were compared with stellar abundances from the literature. For O/Fe vs. Fe/H
all models fit the data, but for C/O vs. O/H, only models with increased carbon
yields for zero-metallicity stars or an evolving initial mass function provide
good fits. Furthermore, a steep star formation threshold in the disc can be
ruled out since it predicts a steep fall-off in all abundance gradients beyond
a certain galactocentric distance (~ 13 kpc) and cannot explain the possible
flattening of the C/H and Fe/H gradients in the outer disc seen in
observations. Since in the best-fit models the enrichment scenario is such that
carbon is primarily produced in low-mass stars, it is suggested that in every
environment where the peak of star formation happened a few Gyr back in time,
winds of carbon-stars are responsible for most of the carbon enrichment.
However, a significant contribution by zero-metallicity stars, especially at
very early stages, and by winds of high-mass stars, which are increasing in
strength with metallicity, cannot be ruled out by the CEMs presented here. In
the solar neighbourhood, as much as 80%, or as little as 40% of the carbon may
have been injected to the interstellar medium by low- and intermediate-mass
stars. The stellar origin of carbon remains an open question, although
production in low- and intermediate-mass stars appears to be the simplest
explanation of observed carbon abundance trends.Comment: 13 pages, 9 figures. Accepted by Astronomy & Astrophysic
Multi-Element Abundance Measurements from Medium-Resolution Spectra. I. The Sculptor Dwarf Spheroidal Galaxy
We present measurements of Fe, Mg, Si, Ca, and Ti abundances for 388 radial
velocity member stars in the Sculptor dwarf spheroidal galaxy (dSph), a
satellite of the Milky Way. This is the largest sample of individual alpha
element (Mg, Si, Ca, Ti) abundance measurements in any single dSph. The
measurements are made from Keck/DEIMOS medium-resolution spectra (6400-9000 A,
R ~ 6500). Based on comparisons to published high-resolution (R >~ 20000)
spectroscopic measurements, our measurements have uncertainties of
sigma([Fe/H]) = 0.14 and sigma([alpha/Fe]) = 0.13. The Sculptor [Fe/H]
distribution has a mean = -1.58 and is asymmetric with a long,
metal-poor tail, indicative of a history of extended star formation. Sculptor
has a larger fraction of stars with [Fe/H] < -2 than the Milky Way halo. We
have discovered one star with [Fe/H] = -3.80 +/- 0.28, which is the most
metal-poor star known anywhere except the Milky Way halo, but high-resolution
spectroscopy is needed to measure this star's detailed abundances. As has been
previously reported based on high-resolution spectroscopy, [alpha/Fe] in
Sculptor falls as [Fe/H] increases. The metal-rich stars ([Fe/H] ~ -1.5) have
lower [alpha/Fe] than Galactic halo field stars of comparable metallicity. This
indicates that star formation proceeded more gradually in Sculptor than in the
Galactic halo. We also observe radial abundance gradients of -0.030 +/- 0.003
dex per arcmin in [Fe/H] and +0.013 +/- 0.003 dex per arcmin in [alpha/Fe] out
to 11 arcmin (275 pc). Together, these measurements cast Sculptor and possibly
other surviving dSphs as representative of the dwarf galaxies from which the
metal-poor tail of the Galactic halo formed.Comment: Accepted to ApJ on 2009 Sep 15, 22 pages, 23 figure
The Ital-FLAMES survey of the Sagittarius dwarf Spheroidal galaxy. I. Chemical abundances of bright RGB stars
We present iron and element (Mg, Ca, Ti) abundances for a sample of
15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf
Spheroidal galaxy. Abundances have been obtained from spectra collected using
the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our
sample have a mean metallicity of [Fe/H]=-0.410.20 with a metal poor tail
extending to [Fe/H]=-1.52. The element abundance ratios are slightly
subsolar for metallicities higher than [Fe/H]\gtsima-1, suggesting a slow star
formation rate. The [/Fe] of stars having [Fe/H]-1 are compatible to
what observed in Milky Way stars of comparable metallicity.Comment: 16 pages, 10 figures, 6 tables. Accepted for publication in A&A.
Minor changes in the tex
The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
We present the first chemical evolution model for Enceladus, alias the Gaia
Sausage, to investigate the star formation history of one of the most massive
satellites accreted by the Milky Way during a major merger event. Our best
chemical evolution model for Enceladus nicely fits the observed stellar
[/Fe]-[Fe/H] chemical abundance trends, and reproduces the observed
stellar metallicity distribution function, by assuming low star formation
efficiency, fast infall time scale, and mild outflow intensity. We predict a
median age for Enceladus stars Gyr, and - at the time
of the merger with our Galaxy ( Gyr ago from Helmi et al.) - we
predict for Enceladus a total stellar mass . By looking at the predictions of our best model, we
discuss that merger events between the Galaxy and systems like Enceladus may
have inhibited the gas accretion onto the Galaxy disc at high redshifts,
heating up the gas in the halo. This scenario could explain the extended period
of quenching in the star formation activity of our Galaxy about 10 Gyr ago,
which is predicted by Milky Way chemical evolution models, in order to
reproduce the observed bimodality in [/Fe]-[Fe/H] between thick- and
thin-disc stars.Comment: Accepted for publication in MNRAS Letter
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&
Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultra-Faint Dwarf Spheroidal Satellite Galaxies
We present new metallicity measurements for 298 individual red giant branch
stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the
Milky Way (MW) system. Our technique is based on medium resolution Keck/DEIMOS
spectroscopy coupled with spectral synthesis. We present the first
spectroscopic metallicities at [Fe/H] < -3.0 of stars in a dwarf galaxy, with
individual stellar metallicities as low as [Fe/H] = -3.3. Because our [Fe/H]
measurements are not tied to empirical metallicity calibrators and are
sensitive to arbitrarily low metallicities, we are able to probe this extremely
metal-poor regime accurately. The metallicity distribution of stars in these
dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that
the luminosity-metallicity relation previously seen in more luminous dSph
galaxies (M_V = -13.4 to -8.8) extends smoothly down to an absolute magnitude
of M_V = -3.7. The discovery of extremely metal-poor stars in dSphs lends
support to the LCDM galaxy assembly paradigm wherein dwarf galaxies dissolve to
form the stellar halo of the MW.Comment: 5 pages, 5 figures, accepted for publication in ApJ
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