127 research outputs found
On the correlation of elemental abundances with kinematics among galactic disk stars
We have performed the detailed analysis of 174 high-resolution spectra of FGK
dwarfs obtained with the ELODIE echelle spectrograph at the Observatoire de
Haute-Provence. Abundances of Fe, Si and Ni have been determined from
equivalent widths under LTE approximation, whereas abundances of Mg have been
determined under NLTE approximation using equivalent widths of 4 lines and
profiles of 5 lines. Spatial velocities with an accuracy better than 1 km/s, as
well as orbits, have been computed for all stars. They have been used to define
2 subsamples kinematically representative of the thin disk and the thick disk
in order to highlight their respective properties. A transition occurs at
[Fe/H]=-0.3. Stars more metal-rich than this value have a flat distribution
with Zmax<1 kpc and sigma_W<20 km/s, and a narrow distribution of [alpha/Fe].
There exist stars in this metallicity regime which cannot belong to the thin
disk because of their excentric orbits, neither to the thick disk because of
their low scale height. Several thin disk stars are identified down to
[Fe/H]=-0.80. Their Mg enrichment is lower than thick disk stars with the same
metallicity. We confirm from a larger sample the results of Feltzing et al
(2003) and Bensby et al (2003) showing a decrease of [alpha/Fe] with [Fe/H] in
the thick disk interpreted as the signature of the SNIa which have
progressively enriched the ISM with iron. However our data suggest that the
star formation in the thick disk stopped when the enrichment was [Fe/H]=-0.30,
[Mg/Fe]=+0.20, [Si/Fe]=+0.17. A vertical gradient in [alpha/Fe] may exist in
the thick disk but should be confirmed with a larger sample. Finally we have
identified 2 new candidates of the HR1614 moving group.Comment: Accepted in A&A, 16 pages, 14 figure
Barium and Yttrium abundance in intermediate-age and old open clusters
Barium is a neutron capture element, that, in open clusters, is frequently
over-abundant with respect to the Iron. A clear explanation for this is still
missing. Additionally, its gradient across the Galactic disk is poorly
constrained. We measure the abundance of yttrium and barium using the synthetic
spectrum method from UVES high-resolution spectra of eight distant open
clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75,
NGC 6192, NGC 6404, and NGC 6583. The barium abundance was estimated using NLTE
approximation. We confirm that Barium is indeed over-abundant in most clusters,
especially young clusters. Finally, we investigated the trend of yttrium and
barium abundances as a function of distance in the Galaxy and ages. Several
scenarios for the barium over-abundance are then discussed.Comment: 9 pages, 10 figure
On the subject of the Ba overabundance in the open clusters stars
For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttrium and barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one young cluster (Ruprecht 7) demonstrate significant barium overabundance( 3c0.55 dex) that can not be due to the determination error. We have considered the Ba abundance determination errors due to LTE approach, saturation of the lines, synthetic and observed barium line fitting, and the causes of the Ba overabundance associated with the Galactic disc enrichment or the origin of open clusters. Possible explanation for this overabundance can be the origin of n-capture elements enrichment of the clusters (galactic or extragalactic) or additional sources of the Ba production
Superflare G and K Stars and the Lithium abundance
We analyzed here the connection of superflares and the lithium abundance in G
and K stars based on Li abundance determinations conducted with the echelle
spectra of a full set of 280 stars obtained with the ELODIE spectrograph. For
high-active stars we show a definite correlation between and the
chromosphere activity. We show that sets of stars with high Li abundance and
having superflares possess common properties. It relates, firstly, to stars
with activity saturation. We consider the X-ray data for G, K, and M stars
separately, and show that transition from a saturation mode to solar-type
activity takes place at values of rotation periods 1.1, 3.3, and 7.2 days for
G2, K4 and M3 spectral types, respectively. We discuss bimodal distribution of
a number of G and K main-sequence stars versus an axial rotation and location
of superflare stars with respect to other Kepler stars. We conclude that
superflare G and K stars are mainly fast rotating young objects, but some of
them belong to stars with solar-type activity. At the same time, we found a
group of G stars with high Li content , but being
slower rotators with rotation periods > 10 days, which are characterized by low
chromospheric activity. This agrees with a large spread in Li abundances in
superflare stars. A mechanism leading to this effect is discussed.Comment: 6 pages, 8 figures. The 19th Cambridge Workshop on Cool Stars,
Stellar Systems, and the Su
Abundances of Cu and Zn in metal-poor stars: clues for Galaxy evolution
We present new observations of copper and zinc abundances in 90 metal-poor
stars, belonging to the metallicity range -3< [Fe/H] < -0.5. The present study
is based on high resolution spectroscopic measurements collected at the Haute
Provence Observatoire (R= 42000, S/N > 100). The trend of Cu and Zn abundances
as a function of the metallicity [Fe/H] is discussed and compared to that of
other heavy elements beyond iron. We also estimate spatial velocities and
galactic orbital parameters for our target stars in order to disentangle the
population of disk stars from that of halo stars using kinematic criteria. In
the absence of a firm a priori knowledge of the nucleosynthesis mechanisms
controlling Cu and Zn production, and of the relative stellar sites, we derive
constraints on these last from the trend of the observed ratios [Cu/Fe] and
[Zn/Fe] throughout the history of the Galaxy, as well as from a few well
established properties of basic nucleosynthesis processes in stars. We thus
confirm that the production of Cu and Zn requires a number of different sources
(neutron captures in massive stars, s-processing in low and intermediate mass
stars, explosive nucleosynthesis in various supernova types). We also attempt a
ranking of the relative roles played by different production mechanisms, and
verify these hints through a simple estimate of the galactic enrichment in Cu
and Zn. In agreement with suggestions presented earlier, we find evidence that
Type Ia Supernovae must play a relevant role, especially for the production of
Cu.Comment: Accepted for A&A, 27 pages, 14 figure
Spectroscopy of high proper motion stars in the ground--based UV
Based on high quality spectral data (spectral resolution R>60000) within the
wavelength range of 3550-5000 AA we determined main parameters (effective
temperature, surface gravity, microturbulent velocity, and chemical element
abundances including heavy metals from Sr to Dy) for 14 metal-deficient G-K
stars with large proper motions. The stars we studied have a wide range of
metallicity: [Fe/H]=-0.3 \div -2.9. Abundances of Mg, Al, Sr and Ba were
calculated with non-LTE line-formation effects accounted for. Abundances both
of the radioactive element Th and r-process element Eu were determined using
synthetic spectrum calculations. We selected stars that belong to different
galactic populations according to the kinematical criterion and parameters
determined by us. We found that the studied stars with large proper motions
refer to different components of the Galaxy: thin, thick disks and halo. The
chemical composition of the star BD+80 245 located far from the galactic plane
agrees with its belonging to the accreted halo. For the giant HD115444 we
obtained [Fe/H]=-2.91, underabundance of Mn, overabundance of heavy metals from
Ba to Dy, and, especially high excess of the r-process element Europium:
[Eu/Fe]=+1.26. Contrary to its chemical composition typical for halo stars,
HD115444 belongs to the disc population according to its kinematic parameters.Comment: 16 pages, 4 figures, 5 tables, "UV Universe-2010 (2nd NUVA Symposium)
conference
New insights on Ba overabundance in open clusters. Evidence for the intermediate neutron-capture process at play?
Recently, an increasing number of studies were devoted to measure the abundances of neutron-capture elements heavier than iron in stars belonging to Galactic Open Clusters (OCs). OCs span a sizeable range in metallicity (-0.6 <= [Fe/H] <= +0.4), and they show abundances of light elements similar to disc stars of the same age. A different pattern is observed for heavy elements. A large scatter is observed for Ba, with most OCs showing [Ba/Fe] and [Ba/La] overabundant with respect to the Sun. The origin of this overabundance is not clearly understood. With the goal of providing new observational insights, we determined radial velocities, atmospheric parameters and chemical composition of 27 giant stars members of five OCs: Cr 110, Cr 261, NGC 2477, NGC 2506 and NGC 5822. We used high-resolution spectra obtained with the UVES spectrograph at European Southern Observatory Paranal. We perform a detailed spectroscopic analysis of these stars to measure the abundance of up to 22 elements per star. We study the dependence of element abundance on metallicity and age with unprecedented detail, complementing our analysis with data culled from the literature. We confirm the trend of Ba overabundance in OCs, and show its large dispersion for clusters younger than ˜4 Gyr. Finally, the implications of our results for stellar nucleosynthesis are discussed. We show in this work that the Ba enrichment compared to other neutron-capture elements in OCs cannot be explained by the contributions from the slow neutron-capture process and the rapid neutron-capture process. Instead, we argue that this anomalous signature can be explained by assuming an additional contribution by the intermediate neutron-capture process
The Galactic thick and thin disks: differences in evolution
Recent observations demonstrate that the thin and thick disks of the Galaxy
have different chemical abundance trends and evolution timescales. The relative
abundances of -elements in the thick Galactic disk are increased
relative to the thin disk. Our goal is to investigate the cause of such
differences in thick and thin disk abundances. We investigate the chemical
evolution of the Galactic disk in the framework of the open two-zone model with
gas inflow. The Galactic abundance trends for -elements (Mg, Si, O) and
Fe are predicted for the thin and thick Galactic disks. The star formation
histories of the thin and thick disks must have been different and the gas
infall must have been more intense during the thick disk evolution that the
thin disk evolution.Comment: 9 pages, 10 figures, A&A accepte
Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects
(Abridged) Aims: We study the effects related to departures from non-local
thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red
giant stars in Galactic globular cluster NGC 6752, to assess their influence on
the formation of Ba II lines. Methods: One-dimensional (1D) local thermodynamic
equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D
ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were
obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE
abundances using 3D-1D abundance corrections, i.e., the differences between the
abundances obtained from the same spectral line using the 3D hydrodynamical
(CO5BOLD) and classical 1D (LHD) stellar model atmospheres. Results: The mean
1D barium-to-iron abundance ratios derived for 20 giants are _{1D
NLTE} = 0.05 \pm0.06 (stat.) \pm0.08 (sys.). The 3D-1D abundance correction
obtained for 8 giants is small (~+0.05 dex), thus leads to only minor
adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for
the 20 giants, _{3D+NLTE} = 0.10 \pm0.06(stat.) \pm0.10(sys.). The
intrinsic abundance spread between the individual cluster stars is small and
can be explained in terms of uncertainties in the abundance determinations.
Conclusions: Deviations from LTE play an important role in the formation of
barium lines in the atmospheres of red giants studied here. The role of 3D
hydrodynamical effects should not be dismissed either, even if the obtained
3D-1D abundance corrections are small. This result is a consequence of subtle
fine-tuning of individual contributions from horizontal temperature
fluctuations and differences between the average temperature profiles in the 3D
and 1D model atmospheres: owing to the comparable size and opposite sign, their
contributions nearly cancel each other.Comment: Minor typos corrected. Accepted for publication in A&A (9 pages, 3
figures, 6 tables
New Lithium Measurements in Metal-Poor Stars
We provide *lambda*6708 Li 1 measurements in 37 metal-poor stars, most of
which are poorly-studied or have no previous measurements, from high-resolution
and high-S/N spectroscopy obtained with the McDonald Observatory 2.1m and 2.7m
telescopes. The typical line strength and abundance uncertainties, confirmed by
the thinness of the Spite plateau manifested by our data and by comparison with
previous measurements, are <=4 mAng and <=0.07-0.10 dex respectively. Two rare
moderately metal-poor solar-Teff dwarfs, HIP 36491 and 40613, with
significantly depleted but still detectable Li are identified; future light
element determinations in the more heavily depeleted HIP 40613 may provide
constraints on the Li depletion mechanism acting in this star. We note two
moderately metal-poor and slightly evolved stars, HIP 105888 and G265-39, that
appear to be analogs of the low-Li moderately metal-poor subgiant HD 201889.
Preliminary abundance analysis of G 265-39 finds no abnormalities that suggest
the low Li content is associated with AGB mass-transfer or deep mixing and
p-capture. We also detect line doubling in HIP 4754, heretofore classified as
SB1.Comment: Accepted for publication in PASP, volume 912 (Feb 2012) 15 pages, 3
figures, 2 table
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