79 research outputs found
Local stellar kinematics from RAVE data: III. Radial and Vertical Metallicity Gradients based on Red Clump Stars
We investigate radial and vertical metallicity gradients for a sample of red
clump stars from the RAdial Velocity Experiment (RAVE) Data Release 3. We
select a total of 6781 stars, using a selection of colour, surface gravity and
uncertainty in the derived space motion, and calculate for each star a
probabilistic (kinematic) population assignment to a thin or thick disc using
space motion and additionally another (dynamical) assignment using stellar
vertical orbital eccentricity. We derive almost equal metallicity gradients as
a function of Galactocentric distance for the high probability thin disc stars
and for stars with vertical orbital eccentricities consistent with being
dynamically young, e_v<=0.07, i.e. d[M/H]/dR_m = -0.041(0.003) and d[M/H]/dR_m
= -0.041(0.007) dex/kpc. Metallicity gradients as a function of distance from
the Galactic plane for the same populations are steeper, i.e. d[M/H]/dz_{max} =
-0.109(0.008) and d[M/H]/dz_{max} = -0.260(0.031) dex/kpc, respectively. R_m
and z_{max} are the arithmetic mean of the perigalactic and apogalactic
distances, and the maximum distance to the Galactic plane, respectively.
Samples including more thick disc red clump giant stars show systematically
shallower abundance gradients. These findings can be used to distinguish
between different formation scenarios of the thick and thin discs.Comment: 27 pages, including 15 figures and 4 tables, accepted for publication
in MNRA
Kinematics, ages and metallicities for F and G type stars in the solar neighbourhood
A new metallicity distribution and an age-metallicity relation are presented
for 437 nearby F and G turn-off and sub-giant stars selected from radial
velocity data of Nidever et al. Photometric metallicities are derived from
uvby-H\beta photometry, and the stellar ages from the isochrones of Bergbusch &
VandenBerg as transformed to photometry using the methods of Clem et al.
The X (stellar-population) criterion of Schuster et al., which combines both
kinematic and metallicity information, provides 22 thick-disk stars.
\sigma_{\rm W} = 32 \pm 5 km s^{-1}, = 154 \pm 6 km s^{-1} and
= -0.55 \pm 0.03 dex for these thick-disk stars, which is in agreement
with values from previous studies of the thick disk. \alpha -element abundances
which are available for some of these thick-disk stars show the typical
alpha-element signatures of the thick disk, supporting the classification
procedure based on the criteria. Both the scatter in metallicity at a given
age and the presence of old, metal-rich stars in the age-metallicity relation
make it difficult to decide whether or not an age-metallicity relation exists
for the older thin-disk stars. For ages greater than 3 Gyr, our results agree
with the other recent studies that there is almost no correlation between age
and metallicity, \Delta ([M/Fe])/\Delta(age) = -0.01 \pm 0.005 dex Gyr^{-1}.
For the 22 thick-disk stars there is a range in ages of 7-8 Gyr, but again
almost no correlation between age and metallicity.Comment: 11 pages, including 10 figures and 3 tables, accepted for publication
in MNRA
Formation & evolution of the Galactic bulge: constraints from stellar abundances
We compute the chemical evolution of the Galactic bulge in the context of an
inside-out model for the formation of the Milky Way. The model contains updated
stellar yields from massive stars. The main purpose of the paper is to compare
the predictions of this model with new observations of chemical abundance
ratios and metallicity distributions in order to put constraints on the
formation and evolution of the bulge. We computed the evolution of several
alpha-elements and Fe and performed several tests by varying different
parameters such as star formation efficiency, slope of the initial mass
function and infall timescale. We also tested the effect of adopting a primary
nitrogen contribution from massive stars. The [alpha/Fe] abundance ratios in
the Bulge are predicted to be supersolar for a very large range in [Fe/H], each
element having a different slope. These predictions are in very good agreement
with most recent accurate abundance determinations. We also find a good fit of
the most recent Bulge stellar metallicity distributions. We conclude that the
Bulge formed on a very short timescale (even though timescales much shorter
than about 0.1 Gyr are excluded) with a quite high star formation efficiency of
about 20 Gyr and with an initial mass function more skewed toward high
masses (i.e. x <= 0.95) than the solar neighbourhood and rest of the disk. The
results obtained here are more robust than previous ones since they are based
on very accurate abundance measurements.Comment: 26 pages, 9 figures, accepted for publication in A&
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S
AIMS. Our aims are twofold. First we aim to evaluate the robustness and
accuracy of stellar parameters and detailed elemental abundances that can be
derived from high-resolution spectroscopic observations of microlensed dwarf
and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to
investigate the abundance structure and chemical evolution of the Milky Way
Bulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysis
of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the
Bulge, for which we obtained a high-resolution spectrum with the MIKE
spectrograph on the Magellan Clay telescope. We have performed four different
analyses of OGLE-2008-BLG-209S. [ABRIDGED] We have also re-analysed three
previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and
OGLE-2007-BLG-349S with the same method. This homogeneous data set, although
small, enables a direct comparison between the different stellar populations.
RESULTS. We find that OGLE-2008-BLG-209S is a subgiant star that has a
metallicity of [Fe/H] ~-0.33. It possesses [alpha/Fe] enhancements similar to
what is found for Bulge giant stars at the same metallicity, and what also is
found for nearby thick disc stars at the same metallicity. In contrast, the
previous three microlensing dwarf stars have very high metallicities,
[Fe/H]>+0.4, and more solar-like abundance ratios, i.e. [alpha/Fe]~0. The
decrease in the [alpha/Fe] ratio with [Fe/H] is the typical signature of
enrichment from low and intermediate mass stars. We furthermore find that the
results for the four Bulge stars, in combination with results from studies of
giant stars in the Bulge, seem to favour a secular formation scenario for the
Bulge.Comment: Accepted for publication in A&A, 17 pages, online table will be
available in published version, or by contacting the first autho
Overlapping abundance gradients and azimuthal gradients related to the spiral structure of the Galaxy
The connection between some features of the metallicity gradient in the
Galactic disk, best revealed by Open Clusters and Cepheids, and the spiral
structure, is explored. The step-like abrupt decrease in metallicity at 8.5 kpc
(with R_0= 7.5 kpc, or at 9.5 kpc if R_0 = 8.5 kpc is adopted) is well
explained by the corotation ring-shaped gap in the density of gas, which
isolates the internal and external regions of the disk one from the other. This
solves a long standing problem of understanding the different chemical
characteristics of the inner and outer parts of the disk. The time required to
build up the metallicity difference between the two sides of the step is a
measure of the minimal life-time of the present grand-design spiral pattern
structure, of the order of 3 Gyr. The plateaux observed on each side of the
step are interpreted in terms of the large scale radial motion of the stars and
of the gas flow induced by the spiral structure. The star-formation rate
revealed by the density of open clusters is maximum in the Galactic radial
range from 6 to 12 kpc (with an exception of a narrow gap at corotation),
coinciding with the region where the 4-arms mode is allowed to exist. We argue
that most of the old open clusters situated at large galactocentric radii were
born in this inner region where conditions more favorable to star-formation are
found. The ratio of -elements to Fe of the sample of Cepheids does not
vary appreciably with the Galactic radius, which reveals an homogeneous history
of star formation. Different arguments are given showing that usual
approximations of chemical evolution models, which assume fast mixing of
metallicity in the azimuthal direction and ignore the existence of the spiral
arms, are a poor ones.Comment: 13 pages, 12 figures, paper accepted by MNRAS main journa
Abundance ratios and IMF slopes in the dwarf elliptical galaxy NGC 1396 with MUSE
Deep observations of the dwarf elliptical (dE) galaxy NGC 1396 (MV = â16.60, Mass ~4 Ă 10^8 Mïżœ), located in the Fornax cluster, have been performed with the VLT/ MUSE spectrograph in the wavelength region from 4750 â 9350 Ă
. In this paper we present a stellar population analysis studying chemical abundances, the star formation history (SFH) and the stellar initial mass function (IMF) as a function of galacto-centric distance. Different, independent ways to analyse the stellar populations result in a luminosity-weighted age of ⌠6 Gyr and a metallicity [Fe/H]⌠â0.4, similar to other dEs of similar mass. We find unusually overabundant values of [Ca/Fe] ⌠+0.1, and under-abundant Sodium, with [Na/Fe] values around â0.1, while [Mg/Fe] is overabundant at all radii, increasing from ⌠+0.1 in the centre to ⌠+0.2 dex. We notice a significant metallicity and age gradient within this dwarf galaxy.
To constrain the stellar IMF of NGC 1396, we find that the IMF of NGC 1396 is consistent with either a Kroupa-like or a top-heavy distribution, while a bottom-heavy IMF is firmly ruled out.
An analysis of the abundance ratios, and a comparison with galaxies in the Local Group, shows that the chemical enrichment history of NGC 1396 is similar to the Galactic disc, with an extended star formation history. This would be the case if the galaxy originated from a LMC-sized dwarf galaxy progenitor, which would lose its gas while falling into the Fornax cluster
Oxygen trends in the Galactic thin and thick disks
We present oxygen abundances for 72 F and G dwarf stars in the solar
neighbourhood. Using the kinematics of the stars we divide them into two
sub-samples with space velocities that are typical for the thick and thin
disks, respectively. The metallicities of the stars range from [Fe/H] = -0.9 to
+0.4 and we use the derived oxygen abundances of the stars to: (1) perform a
differential study of the oxygen trends in the thin and the thick disk; (2) to
follow the trend of oxygen in the thin disk to the highest metallicities. We
analyze the forbidden oxygen lines at 6300 {\AA} and 6363 {\AA} as well as the
(NLTE afflicted) triplet lines around 7774 {\AA}. For the forbidden line at
6300 A we have spectra of very high S/N (>400) and resolution (R=215 000). This
has enabled a very accurate modeling of the oxygen line and the blending Ni
lines. The high internal accuracy in our determination of the oxygen abundances
from this line is reflected in the very tight trends we find for oxygen
relative to iron........Comment: Accepted for publication in A&A, 16 page
- âŠ