430 research outputs found
On the nature of Lithium-rich giant stars: constraints from Beryllium abundances
We have derived beryllium abundances for 7 Li-rich giant (A(Li) > 1.5) stars
and 10 other Li-normal giants, with the aim of investigating the origin of the
Lithium in the Li-rich giants. In particular, we test the predictions of the
engulfment scenario proposed by Siess & Livio (1999), where the engulfment of a
brown dwarf or one or more giant planets would lead to a simultaneous
enrichment of 7Li and 9Be. We show that regardless their nature, none of the
stars studied in this paper were found to have detectable beryllium. Using
simple dilution arguments we show that the engulfment of an external object as
the sole source of Li enrichment is ruled out by the Li and Be abundance data.
The present results favor the idea that Li has been produced in the interior of
the stars by a Cameron-Fowler process and brought up to the surface by an extra
mixing mechanism.Comment: Accepted in A&
The AMBRE Project: searching for the closest solar siblings
Finding solar siblings, that is, stars that formed in the same cluster as the
Sun, will yield information about the conditions at the Sun's birthplace. We
search for solar sibling candidates in AMBRE, the very large spectra database
of solar vicinity stars. Since the ages and chemical abundances of solar
siblings are very similar to those of the Sun, we carried out a chemistry- and
age-based search for solar sibling candidates. We used high-resolution spectra
to derive precise stellar parameters and chemical abundances of the stars. We
used these spectroscopic parameters together with Gaia DR2 astrometric data to
derive stellar isochronal ages. Gaia data were also used to study the
kinematics of the sibling candidates. From the about 17000 stars that are
characterized within the AMBRE project, we first selected 55 stars whose
metallicities are closest to the solar value (-0.1 < [Fe/H] < 0.1 dex). For
these stars we derived precise chemical abundances of several iron-peak, alpha-
and neutron-capture elements, based on which we selected 12 solar sibling
candidates with average abundances and metallicities between -0.03 to 0.03 dex.
Our further selection left us with 4 candidates with stellar ages that are
compatible with the solar age within observational uncertainties. For the 2 of
the hottest candidates, we derived the carbon isotopic ratios, which are
compatible with the solar value. HD186302 is the most precisely characterized
and probably the most probable candidate of our 4 best candidates. Very precise
chemical characterization and age estimation is necessary to identify solar
siblings. We propose that in addition to typical chemical tagging, the study of
isotopic ratios can give further important information about the relation of
sibling candidates with the Sun. Ideally, asteroseismic age determinations of
the candidates could solve the problem of imprecise isochronal ages.Comment: Accepted for publication in A&
Properties of carbon stars in the Solar neighbourhood based on Gaia DR2 astrometry
We find that the combined LF of N- and SC-type stars are consistent with a
Gaussian distribution peaking at M_bol~ -5.2 mag. The resulting LF however
shows two tails at lower and higher luminosities more extended than those
previously found, indicating that AGB carbon stars with Solar metallicity may
reach M_bol~-6.0 mag. We find that J-type stars are about half a magnitude
fainter on average than N- and SC-type stars, while R-hot stars are half a
magnitude brighter than previously found. The Galactic spatial distribution and
velocity components of the N-, SC- and J-type stars are very similar, while
about 30 % of the R-hot stars in the sample are located at distances larger
than ~ 500 pc from the Galactic Plane, and show a significant drift with
respect to the local standard of rest. The LF derived for N- and SC-type in the
Solar neighbourhood fully agrees with the expected luminosity of stars of 1.5-3
M_o on the AGB. On a theoretical basis, the existence of an extended low
luminosity tail would require a contribution of extrinsic low mass carbon
stars, while the high luminosity one would imply that stars with mass up to ~5
Mo may become carbon star on the AGB. J-type stars not only differ
significantly in their chemical composition with respect to the N- and SC-types
but also in their LF, which reinforces the idea that these carbon stars belong
to a dvifferent type whose origin is still unknown. The derived luminosities of
R-hot stars make these stars unlikely to be in the red-clump as previously
claimed. On the other hand, the derived spatial distribution and kinematic
properties, together with their metallicity, indicate that most of the N-, SC-
and J-type stars belong to the thin disc population, while a significant
fraction of R-hot stars show characteristics compatible with the thick disc.Comment: Accepted in Astronomy & Astrophysic
The Gaia-ESO Survey: Separating disk chemical substructures with cluster models
(Abridged) Recent spectroscopic surveys have begun to explore the Galactic
disk system outside the solar neighborhood on the basis of large data samples.
In this way, they provide valuable information for testing spatial and temporal
variations of disk structure kinematics and chemical evolution. We used a
Gaussian mixture model algorithm, as a rigurous mathematical approach, to
separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample from the
Gaia-ESO survey internal data release 2. We find that the sample is separated
into five groups associated with major Galactic components; the metal-rich end
of the halo, the thick disk, and three subgroups for the thin disk sequence.
This is confirmed with a sample of red clump stars from the Apache Point
Observatory Galactic Evolution Experiment (APOGEE) survey. The two
metal-intermediate and metal-rich groups of the thin disk decomposition
([Fe/H]>-0.25 dex) highlight a change in the slope at solar metallicity. This
holds true at different radial regions. The distribution of Galactocentric
radial distances of the metal-poor part of the thin disk ([Fe/H]<-0.25 dex) is
shifted to larger distances than those of the more metal-rich parts. Moreover,
the metal-poor part of the thin disk presents indications of a scale height
intermediate between those of the thick and the rest of the thin disk, and it
displays higher azimuthal velocities than the latter. These stars might have
formed and evolved in parallel and/or dissociated from the inside-out formation
taking place in the internal thin disk. Their enhancement levels might be due
to their origin from gas pre-enriched by outflows from the thick disk or the
inner halo. The smooth trends of their properties (their spatial distribution
with respect to the plane, in particular) with [Fe/H] and [Mg/Fe] suggested by
the data indicates a quiet dynamical evolution, with no relevant merger events
Beyond the two-infall model I. Indications for a recent gas infall with Gaia DR3 chemical abundances
The recent Gaia Data Release 3 (DR3) represents an unparalleled revolution in
Galactic Archaeology, providing us with numerous radial velocities chemical
abundances for millions of stars, with all-sky coverage. We present a new
chemical evolution model for the Galactic disc components (high- and low-
sequence stars) designed to reproduce the new abundance ratios
provided by the GSP-spec module for the Gaia DR3 and also constrained by the
detailed star formation histories for both the thick and thin disc stars
inferred from previous Gaia releases. Sophisticated modeling based on previous
Gaia releases has found evidence for narrow episodes of enhanced SF inferred in
recent time. Additionally, Gaia DR3 highlighted the presence of young (massive)
low- disc stars which show evidence of a recent chemical impoverishment
in several elements. Hence, in this study, we compare Gaia DR3 chemical
abundances with the predictions of a three-infall chemical evolution model for
the high- and low- components. The proposed three-infall chemical
evolution model nicely reproduces the main features of the abundance ratio
[X/Fe] versus [M/H] (X=Mg, Si, Ca, Ti, ) of Gaia DR3 stars in different
age bins for the considered elements. Moreover, the most recent gas
infall - which started 2.7 Gyr ago - allows us to predict well the Gaia
DR3 young population which has experienced a recent chemical impoverishment.Comment: Accepted for publication in Astronomy and Astrophysics (A&A), 17
pages, 16 figure
The Gaia-ESO Survey: Tracing interstellar extinction
Large spectroscopic surveys have enabled in the recent years the computation
of three-dimensional interstellar extinction maps thanks to accurate stellar
atmospheric parameters and line-of-sight distances. Such maps are complementary
to 3D maps extracted from photometry, allowing a more thorough study of the
dust properties. Our goal is to use the high-resolution spectroscopic survey
Gaia-ESO in order to obtain with a good distance resolution the interstellar
extinction and its dependency as a function of the environment and the
Galactocentric position. We use the stellar atmospheric parameters of more than
5000 stars, obtained from the Gaia-ESO survey second internal data release, and
combine them with optical (SDSS) and near-infrared (VISTA) photometry as well
as different sets of theoretical stellar isochrones, in order to calculate
line-of-sight extinction and distances. The extinction coefficients are then
compared with the literature to discuss their dependancy on the stellar
parameters and position in the Galaxy. Within the errors of our method, our
work does not show that there is any dependence of the interstellar extinction
coefficient on the atmospheric parameters of the stars. We do not find any
evidence of the variation of E(J-H)/E(J-K) with the angle from the Galactic
centre nor with Galactocentric distance. This suggests that we are dealing with
a uniform extinction law in the SDSS ugriz bands and the near-IR JHKs bands.
Therefore, extinction maps using mean colour-excesses and assuming a constant
extinction coefficient can be used without introducing any systematic errors.Comment: 13 pages, 14 figures, 1 Appendix accepted for publication in
Astronomy&Astrophysic
s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-element
content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass
s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are
derived using the spectral synthesis technique from high-resolution spectra.
The N-stars analyzed are of nearly solar metallicity and show moderate
s-element enhancements, similar to those found in S stars, but smaller than
those found in the only previous similar study (Utsumi 1985), and also smaller
than those found in supergiant post-AGB stars. This is in agreement with the
present understanding of the envelope s-element enrichment in giant stars,
which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the
AGB phase. We compare the observational data with recent -process
nucleosynthesis models for different metallicities and stellar masses. Good
agreement is obtained between low mass AGB star models (M < 3 M_o) and
s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction
is the main source of neutrons for the s-process; a moderate spread, however,
must exist in the abundance of 13C that is burnt in different stars. By
combining information deriving from the detection of Tc, the infrared colours
and the theoretical relations between stellar mass, metallicity and the final
C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this
work are intrinsic, thermally-pulsing AGB stars; their abundances are the
consequence of the operation of third dredge-up and are not to be ascribed to
mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap
A Search for Very Active Stars in the Galaxy
We report the first results of a systematic search near the plane of the
Galaxy for the so called very active stars (VAS), which are characterized by a
hard X-ray spectrum and activity in the radio domain. Candidates with hard
X-ray binary-like spectra have been selected from the Bright ROSAT Source
Catalogue in the Zone of Avoidance () and were tentatively
identified in GB6/PMM/NVSS radio surveys. Most of them were observed with the
ATCA and VLA. Precise radio coordinates have led to unambiguous optical
identification for 60 candidates, and a sub-sample of five of themhas been
observed with the VLT. Also some discovery and confirmatory spectra were
obtained with the AAT (4-m) and BTA (6-m). Spectroscopy with moderate
dispersion, made with the FORS1 spectrograph of the VLT has revealed two
stellar objects (one of them, VASC J1628-41, is definitivelya binary VAS), one
new AGN and two featureless spectrum sources. One of these objects, VASC
J1353-66, shows a marginal evidence of proper motion, which, if confirmed,
would imply the discovery of a new type of galactic source.Comment: to appear in A&A, 7 figure
The Gaia-ESO survey : Processing FLAMES-UVES spectra
Date of Acceptance: 19/03/2014The Gaia-ESO Survey is a large public spectroscopic survey that aims to derive radial velocities and fundamental parameters of about 105 Milky Way stars in the field and in clusters. Observations are carried out with the multi-object optical spectrograph FLAMES, using simultaneously the medium-resolution (R ~ 20 000) GIRAFFE spectrograph and the high-resolution (R ~ 47 000) UVES spectrograph. In this paper we describe the methods and the software used for the data reduction, the derivation of the radial velocities, and the quality control of the FLAMES-UVES spectra. Data reduction has been performed using a workflow specifically developed for this project. This workflow runs the ESO public pipeline optimizing the data reduction for the Gaia-ESO Survey, automatically performs sky subtraction, barycentric correction and normalisation, and calculates radial velocities and a first guess of the rotational velocities. The quality control is performed using the output parameters from the ESO pipeline, by a visual inspection of the spectra and by the analysis of the signal-to-noise ratio of the spectra. Using the observations of the first 18 months, specifically targets observed multiple times at different epochs, stars observed with both GIRAFFE and UVES, and observations of radial velocity standards, we estimated the precision and the accuracy of the radial velocities. The statistical error on the radial velocities is σ ~ 0.4 km s-1 and is mainly due to uncertainties in the zero point of the wavelength calibration. However, we found a systematic bias with respect to the GIRAFFE spectra (~0.9 km s-1) and to the radial velocities of the standard stars (~0.5 km s-1) retrieved from the literature. This bias will be corrected in the future data releases, when a common zero point for all the set-ups and instruments used for the survey is be established.Peer reviewe
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