63 research outputs found
Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars
Using a sample of 31 main-sequence OB stars located between galactocentric
distances 8.4 - 15.6 kpc, we aim to probe the present-day radial abundance
gradients of the Galactic disk. The analysis is based on high-resolution
spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m
telescope on Las Campanas. We used a non-NLTE analysis in a self-consistent
semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric
parameters and chemical abundances. Stellar parameters (effective temperature,
surface gravity, projected rotational velocity, microturbulence, and
macroturbulence) and silicon and oxygen abundances are presented for 28 stars
located beyond 9 kpc from the Galactic centre plus three stars in the solar
neighborhood. The stars of our sample are mostly on the main-sequence, with
effective temperatures between 20800 - 31300 K, and surface gravities between
3.23 - 4.45 dex. The radial oxygen and silicon abundance gradients are negative
and have slopes of -0.07 dex/kpc and -0.09 dex/kpc, respectively, in the region
\,kpc. The obtained gradients are compatible with the
present-day oxygen and silicon abundances measured in the solar neighborhood
and are consistent with radial metallicity gradients predicted by
chemodynamical models of Galaxy Evolution for a subsample of young stars
located close to the Galactic plane.Comment: Accepted for publication in the A&
Chemical Abundances of OB Stars with High Projected Rotational Velocities
Elemental abundances of carbon, nitrogen, oxygen, magnesium, aluminum, and
silicon are presented for a sample of twelve rapidly rotating OB star (v sin i
> 60 km s^-1) members of the Cep OB2, Cyg OB3 and Cyg OB7 associations. The
abundances are derived from spectrum synthesis, using both LTE and non-LTE
calculations. As found in almost all previous studies of OB stars, the average
abundances are slightly below solar, by about 0.1 to 0.3 dex. In the case of
oxygen, even with the recently derived low solar abundances the OB stars are
closer to, but still below, the solar value. Results for the 9 Cep OB2 members
in this sample can be combined with results published previously for 8 Cep OB2
stars with low projected rotational velocities to yield the most complete set
of abundances, to date, for this particular association. These abundances
provide a clear picture of both the general chemical and individual stellar
evolution that has occurred within this association. By placing the Cep OB2
stars studied in an HR diagram we identify the presence of two distinct age
subgroups, with both subgroups having quite uniform chemical abundances. Two
stars are found in the older subgroup that show significant N/O overabundances,
with both stars being two of the most massive, the most evolved, and most
rapidly rotating of the members studied in Cep OB2. These characteristics of
increased N abundances being tied to high mass, rapid rotation, and an evolved
phase are those predicted from models of rotating stars which undergo
rotationally driven mixing.Comment: 5 Figures, 5 Tables, accepted for publication in the Astrophysical
Journa
On the "Galactic Habitable Zone"
The concept of Galactic Habitable Zone (GHZ) was introduced a few years ago
as an extension of the much older concept of Circumstellar Habitable Zone.
However, the physical processes underlying the former concept are hard to
identify and even harder to quantify. That difficulty does not allow us, at
present, to draw any significant conclusions about the extent of the GHZ: it
may well be that the entire Milky Way disk is suitable for complex life.Comment: 12 pages, 6 figures, Invited talk in "Strategies for Life Detection"
(ISSI Bern, 24-28 April 2006), Eds, J. Bada et al., to appear in Space
Science Review
Chemical abundances of the young inner-disk open cluster NGC 6705 observed by APOGEE: sodium-rich and not -enhanced
Previous results in the literature have found the young inner-disk open
cluster NGC 6705 to be mildly -enhanced. We examined this possibility
via an independent chemical abundance analysis for 11 red-giant members of NGC
6705. The analysis is based on near-infrared APOGEE spectra and relies on LTE
calculations using spherical model atmospheres and radiative transfer. We find
a mean cluster metallicity of , indicating that
NGC 6705 is metal-rich, as may be expected for a young inner-disk cluster. The
mean -element abundance relative to iron is , which is not at odds with expectations
from general Galactic abundance trends. NGC 6705 also provides important probes
for studying stellar mixing, given its turn-off mass of M3.3 M.
Its red giants have low C abundances ([C/Fe]=0.16) and
enhanced N abundances ([N/Fe]=+0.51), which are key signatures of
the first dredge-up on the red giant branch. An additional signature of
dredge-up was found in the Na abundances, which are enhanced by [Na/Fe]=+0.29,
with a very small non-LTE correction. The O and Al abundances are found
to be near-solar. All of the derived mixing-sensitive abundances are in
agreement with stellar models of approximately 3.3 M evolving along
the red giant branch and onto the red clump. As found in young open clusters
with similar metallicities, NGC 6705 exhibits a mild excess in the s-process
element cerium, with .Comment: 17 pages, 10 figures, 6 table
Open clusters towards the Galactic center: chemistry and dynamics. A VLT spectroscopic study of NGC6192, NGC6404, NGC6583
In the framework of the study of the Galactic metallicity gradient and its
time evolution, we present new high-resolution spectroscopic observations
obtained with FLAMES and the fiber link to UVES at VLT of three open clusters
(OCs) located within 7~kpc from the Galactic Center (GC): NGC~6192,
NGC~6404, NGC~6583. We also present new orbit determination for all OCs with
Galactocentric distances (R8~kpc and metallicity from
high-resolution spectroscopy. We aim to investigate the slope of the inner disk
metallicity gradient as traced by OCs and at discussing its implication on the
chemical evolution of our Galaxy. We have derived memberships of a group of
evolved stars for each clusters, obtaining a sample of 4, 4, and 2 member stars
in NGC~6192, NGC~6404, and NGC~6583, respectively. Using standard LTE analysis
we derived stellar parameters and abundance ratios for the iron-peak elements
Fe, Ni, Cr, and for the -elements Al, Mg, Si, Ti, Ca. We calculated the
orbits of the OCs currently located within 8~kpc from the GC, and discuss their
implication on the present-time radial location. {The average metallicities of
the three clusters are all oversolar: [Fe/H]= (NGC~6192),
(NGC 6404), (NGC 6583). They are in qualitative
agreement with their Galactocentric distances, being all internal OCs, and thus
expected to be metal richer than the solar neighborhood. The abundance ratios
of the other elements over iron [X/Fe] are consistent with solar values. The
clusters we have analysed, together with other OC and Cepheid data, confirm a
steep gradient in the inner disk, a signature of an evolutionary rate different
than in the outer disk.Comment: 17 pages, 13 figures, A&A accepted for publicatio
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
The Gaia-ESO Survey: homogenisation of stellar parameters and elemental abundances
The Gaia-ESO Survey is a public spectroscopic survey that has targeted
stars covering all major components of the Milky Way from the end
of 2011 to 2018, delivering its public final release in May 2022. Unlike other
spectroscopic surveys, Gaia-ESO is the only survey that observed stars across
all spectral types with dedicated, specialised analyses: from O
(~K) all the way to K-M (3,500~K).
The physics throughout these stellar regimes varies significantly, which has
previously prohibited any detailed comparisons between stars of significantly
different type. In the final data release (internal data release 6) of the
Gaia-ESO Survey, we provide the final database containing a large number of
products such as radial velocities, stellar parameters and elemental
abundances, rotational velocity, and also, e.g., activity and accretion
indicators in young stars and membership probability in star clusters for more
than 114,000 stars. The spectral analysis is coordinated by a number of Working
Groups (WGs) within the Survey, which specialise in the various stellar
samples. Common targets are analysed across WGs to allow for comparisons (and
calibrations) amongst instrumental setups and spectral types. Here we describe
the procedures employed to ensure all Survey results are placed on a common
scale to arrive at a single set of recommended results for all Survey
collaborators to use. We also present some general quality and consistency
checks performed over all Survey results.Comment: A&A accepted, minor revision, 36 pages, 38 figure
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