146 research outputs found
Chemical abundances in LMC stellar populations. II. The bar sample
This paper compares the chemical evolution of the Large Magellanic Cloud
(LMC) to that of the Milky Way (MW) and investigates the relation between the
bar and the inner disc of the LMC in the context of the formation of the bar.
We obtained high-resolution and mid signal-to-noise ratio spectra with
FLAMES/GIRAFFE at ESO/VLT and performed a detailed chemical analysis of 106 and
58 LMC field red giant stars (mostly older than 1 Gyr), located in the bar and
the disc of the LMC respectively. We measured elemental abundances for O, Mg,
Si, Ca, Ti, Na, Sc, V, Cr, Co, Ni, Cu, Y, Zr, Ba, La and Eu. We find that the
{\alpha}-element ratios [Mg/Fe] and [O/Fe] are lower in the LMC than in the MW
while the LMC has similar [Si/Fe], [Ca/Fe], and [Ti/Fe] to the MW. As for the
heavy elements, [Ba,La/Eu] exhibit a strong increase with increasing
metallicity starting from [Fe/H]=-0.8 dex, and the LMC has lower [Y+Zr/Ba+La]
ratios than the MW. Cu is almost constant over all metallicities and about 0.5
dex lower in the LMC than in the MW. The LMC bar and inner disc exhibit
differences in their [{\alpha}/Fe] (slightly larger scatter for the bar in the
metallicity range [-1,-0.5]), their Eu (the bar trend is above the disc trend
for [Fe/H] > -0.5 dex), their Y and Zr, their Na and their V (offset between
bar and disc distributions). Our results show that the chemical history of the
LMC experienced a strong contribution from type Ia supernovae as well as a
strong s-process enrichment from metal-poor AGB winds. Massive stars made a
smaller contribution to the chemical enrichment compared to the MW. The
observed differences between the bar and the disc speak in favour of an episode
of enhanced star formation a few Gyr ago, occurring in the central parts of the
LMC and leading to the formation of the bar. This is in agreement with recently
derived star formation histories.Comment: 22 pages, 20 figures; Accepted for publication in A&
Heavy elements Ba, La, Ce, Nd, and Eu in 56 Galactic bulge red giants
Aims. The aim of this work is the study of abundances of the heavy elements
Ba, La, Ce, Nd, and Eu in 56 bulge giants (red giant branch and red clump) with
metallicities ranging from -1.3 dex to 0.5 dex. Methods. We obtained
high-resolution spectra of our giant stars using the FLAMES-UVES spectrograph
on the Very Large Telescope. We inspected four bulge fields along the minor
axis. Results. We measure the chemical evolution of heavy elements, as a
function of metallicity, in the Galactic bulge. Conclusions. The [Ba, La, Ce,
Nd/Fe] vs. [Fe/H] ratios decrease with increasing metallicity, in which aspect
they differ from disc stars. In our metal-poor bulge stars, La and Ba are
enhanced relative to their thick disc counterpart, while in our metal-rich
bulge stars La and Ba are underabundant relative to their disc counterpart.
Therefore, this contrast between bulge and discs trends indicates that bulge
and (solar neighbourhood) thick disc stars could behave differently. An
increase in [La, Nd/Eu] with increasing metallicity, for metal-rich stars with
[Fe/H] > 0 dex, may indicate that the s-process from AGB stars starts to
operate at a metallicity around solar. Finally, [Eu/Fe] follows the
[{\alpha}/Fe] behaviour, as expected, since these elements are produced by SNe
type II.Comment: 12 pages, 10 figures, accepted for publication in A&
Sodium abundances of AGB and RGB stars in Galactic globular clusters II. Analysis and results of NGC 104, NGC 6121, and NGC 6809
Aims. We investigate the Na abundance distribution of asymptotic giant branch (AGB) stars in Galactic globular clusters (GCs) and its possible dependence on GC global properties, especially age and metallicity.
Methods. We analyze high-resolution spectra of a large sample of AGB and red giant branch (RGB) stars in the Galactic GCs NGC 104, NGC 6121, and NGC 6809 obtained with FLAMES/GIRAFFE at ESO/VLT, and determine their Na abundances. This is the first time that the AGB stars in NGC 6809 are targeted. Moreover, to investigate the dependence of AGB Na abundance dispersion on GC parameters, we compare the AGB [Na/H] distributions of a total of nine GCs, with five determined by ourselves with homogeneous method and four from literature, covering a wide range of GC parameters.
Results. NGC 104 and NGC 6809 have comparable AGB and RGB Na abundance distributions revealed by the K−S test, while NGC 6121 shows a lack of very Na-rich AGB stars. By analyzing all nine GCs, we find that the Na abundances and multiple populations of AGB stars form complex picture. In some GCs, AGB stars have similar Na abundances and/or second-population fractions as their RGB counterparts, while some GCs do not have Na-rich second-population AGB stars, and various cases exist between the two extremes. In addition, the fitted relations between fractions of the AGB second population and GC global parameters show that the AGB second-population fraction slightly anticorrelates with GC central concentration, while no robust dependency can be confirmed with other GC parameters.
Conclusions. Current data roughly support the prediction of the fast-rotating massive star (FRMS) scenario. However, considering the weak observational and theoretical trends where scatter and exceptions exist, the fraction of second-population AGB stars can be affected by more than one or two factors, and may even be a result of stochasticity
The role of radial migration in open cluster and field star populations with Gaia dr3
The survival time of a star cluster depends on its total mass, density, and
thus size, as well as on the environment in which it was born and in which
lies. Its dynamical evolution is influenced by various factors such as
gravitational effects of the Galactic bar, spiral structures, and molecular
clouds. Overall, the factors that determine the longevity of a cluster are
complex and not fully understood. This study aims to investigate if open
clusters and field stars respond differently to the perturbations that cause
radial migration. In particular, we aim at understanding the nature of the
oldest surviving clusters. We compared the time evolution of the kinematic
properties of two Gaia DR3 samples: the first sample is composed of 40
open clusters and the second one of 66,000 MSTO field stars. Both
selected samples are composed of stars selected with the same quality
criterion, belonging to the thin disc, in a similar metallicity range, located
in the same Galactocentric region [7.5-9 kpc] and with ages >1 Gyr. We
performed a statistical analysis comparing the properties of the samples of
field stars and of open clusters. A qualitative comparison of kinematic and
orbital properties reveals that clusters younger than 2-3 Gyr are more
resistant to perturbations than field stars and they move along quasi-circular
orbits. Conversely, clusters older than approximately 3 Gyr have more eccentric
and inclined orbits than isolated stars in the same age range. Such orbits lead
them to reach higher elevations on the Galactic plane, maximising their
probability to survive several Gyr longer. A formal statistical analysis
reveals that there are differences among the time evolution of most of the
kinematic and orbital properties of field stars and open clusters. Our results
suggest that oldest survived clusters are usually more massive and move on
orbits with higher eccentricity.Comment: 13 pages, 20 figures, 2 tables. Article accepted for publication in
A&
Ariel stellar characterisation: I -- homogeneous stellar parameters of 187 FGK planet host stars Description and validation of the method
In 2020 the European Space Agency selected Ariel as the next mission to join
the space fleet of observatories to study planets outside our Solar System.
Ariel will be devoted to the characterisation of a thousand planetary
atmospheres, for understanding what exoplanets are made of, how they formed and
how they evolve. To achieve the last two goals all planets need to be studied
within the context of their own host stars, which in turn have to be analysed
with the same technique, in a uniform way. We present the spectro-photometric
method we have developed to infer the atmospheric parameters of the known host
stars in the Tier 1 of the Ariel Reference Sample. Our method is based on an
iterative approach, which combines spectral analysis, the determination of the
surface gravity from {\em Gaia} data, and the determination of stellar masses
from isochrone fitting. We validated our approach with the analysis of a
control sample, composed by members of three open clusters with well-known ages
and metallicities. We measured effective temperature, Teff, surface gravity,
logg, and the metallicity, [Fe/H], of 187 F-G-K stars within the Ariel
Reference Sample. We presented the general properties of the sample, including
their kinematics which allows us to separate them between thin and thick disc
populations. A homogeneous determination of the parameters of the host stars is
fundamental in the study of the stars themselves and their planetary systems.
Our analysis systematically improves agreement with theoretical models and
decreases uncertainties in the mass estimate (from 0.21+/-0.30 to 0.10+/-0.02
M_sun), providing useful data for the Ariel consortium and the astronomical
community at large.Comment: Accepted for publication in A&A, 13 pages, 14 figures, Tables A1 and
A2 in the Appendix will be available at CDS and can be requested by email to:
[email protected]
Determination of stellar parameters for Ariel targets: a comparison analysis between different spectroscopic methods
Ariel has been selected as the next ESA M4 science mission and it is expected to be launched in 2028. During its 4-year mission, Ariel will observe the atmospheres of a large and diversified population of transiting exoplanets. A key factor for the achievement of the scientific goal of Ariel is the selection strategy for the definition of the input target list. A meaningful choice of the targets requires an accurate knowledge of the planet hosting star properties and this is necessary to be obtained well before the launch. In this work, we present the results of a bench-marking analysis between three different spectroscopic techniques used to determine stellar parameters for a selected number of targets belonging to the Ariel reference sample. We aim to consolidate a method that will be used to homogeneously determine the stellar parameters of the complete Ariel reference sample. Homogeneous, accurate and precise derivation of stellar parameters is crucial for characterising exoplanet-host stars and in turn is a key factor for the accuracy of the planet properties
The GALAH survey: Multiple stars and our Galaxy. I. A comprehensive method for deriving properties of FGK binary stars
Binary stellar systems form a large fraction of the Galaxy's stars. They are
useful as laboratories for studying the physical processes taking place within
stars, and must be correctly taken into account when observations of stars are
used to study the structure and evolution of the Galaxy. We present a sample of
12760 well-characterised double-lined spectroscopic binaries that are
appropriate for statistical studies of the binary populations. They were
detected as SB2s using a t-distributed stochastic neighbour embedding (t-SNE)
classification and a cross-correlation analysis of GALAH spectra. This sample
consists mostly of dwarfs, with a significant fraction of evolved stars and
several dozen members of the giant branch. To compute parameters of the primary
and secondary star (, , [Fe/H], ,
, , , and ), we used a
Bayesian approach that includes a parallax prior from Gaia DR2, spectra from
GALAH, and apparent magnitudes from APASS, Gaia DR2, 2MASS, and WISE. The
derived stellar properties and their distributions show trends that are
expected for a population of close binaries (a 10 AU) with mass ratios . The derived metallicity of these binary stars is statistically
lower than that of single dwarf stars from the same magnitude-limited sample.Comment: Accepted for publication in A&
Chemodynamics of a simulated disc galaxy: initial mass functions and Type Ia supernova progenitors
We trace the formation and advection of several elements within a cosmological adaptive mesh refinement simulation of an L� galaxy. We use nine realizations of the same initial conditions with different stellar initial mass functions (IMFs), mass limits for Type II and Type Ia supernovae (SNII, SNIa) and stellar lifetimes to constrain these subgrid phenomena. Our code includes self-gravity, hydrodynamics, star formation, radiative cooling and feedback from multiple sources within a cosmological framework. Under our assumptions of nucleosynthesis we find that SNII with progenitor masses of up to 100 M� are required to match low-metallicity gas oxygen abundances. Tardy SNIa are necessary to reproduce the classical chemical evolution ‘knee’ in [O/Fe]–[Fe/H]: more prompt SNIa delayed time distributions do not reproduce this feature. Within our framework of hydrodynamical mixing of metals and galaxy mergers we find that chemical evolution is sensitive to the shape of the IMF and that there exists a degeneracy with the mass range of SNII. We look at the abundance plane and present the properties of different regions of the plot, noting the distinct chemical properties of satellites and a series of nested discs that have greater velocity dispersions are more α-rich and metal poor with age
Gaia-ESO survey: Lithium abundances in open cluster Red Clump stars
Context. It has recently been suggested that all giant stars with masses below 2 M⊙ suffer an episode of surface lithium enrichment between the tip of the red giant branch (RGB) and the red clump (RC).
Aims. We test if the above result can be confirmed in a sample of RC and RGB stars that are members of open clusters.
Methods. We discuss Li abundances in six open clusters with ages between 1.5 and 4.9 Gyr (turn-off masses between 1.1 and 1.7 M⊙). We compare these observations with the predictions of different models that include rotation-induced mixing, thermohaline instability, mixing induced by the first He flash, and energy losses by neutrino magnetic moment.
Results. In six clusters, we find close to 35% of RC stars have Li abundances that are similar or higher than those of upper RGB stars. This can be a sign of fresh Li production. Because of the extra-mixing episode connected to the luminosity bump, the expectation has been for RC stars to have systematically lower surface Li abundances. However, we cannot confirm that this possible Li production is ubiquitous. For about 65% of RC giants, we can only determine upper limits in abundances that could be hiding very low Li content.
Conclusions. Our results indicate the possibility that Li is being produced in the RC, at levels that would not typically permit the classification of these the stars as Li rich. The determination of their carbon isotopic ratio would help to confirm that the RC giants have suffered extra mixing followed by subsequent Li enrichment. The Li abundances of the RC stars can be qualitatively explained by the models including an additional mixing episode close to the He flash.</jats:p
- …