28 research outputs found
A snapshot of the inner dusty regions of a RCrB-type variable
R Coronae Borealis variable stars are suspected to sporadically eject
optically thick dust clouds causing, when one of them lies on the
line-of-sight, a huge brightness decline in visible light. Mid-infrared
interferometric observations of RYSgr allowed us to explore the circumstellar
regions very close to the central star (~20-40 mas) in order to look for the
signature of any heterogeneities. Using the VLTI/MIDI instrument, five
dispersed visibility curves were recorded with different projected baselines
oriented towards two roughly perpendicular directions. The large spatial
frequencies visibility curves exhibit a sinusoidal shape whereas, at shorter
spatial frequencies visibility curves follow a Gaussian decrease. These
observations are well interpreted with a geometrical model consisting in a
central star surrounded by an extended circumstellar envelope in which one
bright cloud is embedded. Within this simple geometrical scheme, the inner
110AU dusty environment of RYSgr is dominated at the time of observations by a
single dusty cloud which, at 10mic represents ~10% of the total flux of the
whole system. The cloud is located at about 100stellar radii from the centre
toward the East-North-East direction (or the symmetric direction with respect
to centre) within a circumstellar envelope which FWHM is about 120stellar
radii. This first detection of a cloud so close to the central star, supports
the classical scenario of the RCrB brightness variations in the optical
spectral domain
The AMBRE Project: Parameterisation of FGK-type stars from the ESO:HARPS archived spectra
The AMBRE project is a collaboration between the European Southern
Observatory (ESO) and the Observatoire de la Cote d'Azur (OCA). It has been
established to determine the stellar atmospheric parameters (effective
temperature, surface gravity, global metallicities and abundance of
alpha-elements over iron) of the archived spectra of four ESO spectrographs.
The analysis of the ESO:HARPS archived spectra is presented. The sample being
analysed (AMBRE:HARPS) covers the period from 2003 to 2010 and is comprised of
126688 scientific spectra corresponding to 17218 different stars. For the
analysis of the spectral sample, the automated pipeline developed for the
analysis of the AMBRE:FEROS archived spectra has been adapted to the
characteristics of the HARPS spectra. Within the pipeline, the stellar
parameters are determined by the MATISSE algorithm, developed at OCA for the
analysis of large samples of stellar spectra in the framework of galactic
archaeology. In the present application, MATISSE uses the AMBRE grid of
synthetic spectra, which covers FGKM-type stars for a range of gravities and
metallicities. We first determined the radial velocity and its associated error
for the ~15% of the AMBRE:HARPS spectra, for which this velocity had not been
derived by the ESO:HARPS reduction pipeline. The stellar atmospheric parameters
and the associated chemical index [alpha/Fe] with their associated errors have
then been estimated for all the spectra of the AMBRE:HARPS archived sample.
Based on quality criteria, we accepted and delivered the parameterisation of
~71% of the total sample to ESO. These spectra correspond to ~10706 stars; each
are observed between one and several hundred times. This automatic
parameterisation of the AMBRE:HARPS spectra shows that the large majority of
these stars are cool main-sequence dwarfs with metallicities greater than -0.5
dex
Characterisation of Galactic carbon stars and related stars from Gaia-EDR3
We extend here a previous investigation on the characteristics of Galactic
carbon stars using more accurate EDR3 astrometry measurements. Based on a much
larger statistics, we confirm that N- and SC-type carbon stars share a very
similar luminosity function, while the luminosities of J-type stars (Mbol) are
fainter by half a magnitude on average. R-hot type carbon stars have
luminosities throughout the RGB, which favours the hypothesis of an external
origin for their carbon enhancement. Moreover, the kinematic properties of a
significant fraction of the R-hot stars are compatible with the thick-disc
population, in contrast with that of N- and SC-type stars, which would belong
mostly to the thin disk. We also derive the luminosity function of a large
number of Galactic extrinsic and intrinsic (O-rich) S stars and show that the
luminosities of the latter are typically higher than the predicted onset of the
third dredge-up during the AGB for solar metallicity. This result is consistent
with these stars being genuine thermally pulsing AGB stars. On the other hand,
using the so-called Gaia-2MASS diagram, we show that the overwhelming majority
of the carbon stars identified in the LAMOST survey as AGB stars are probably
R-hot and/or CH-type stars. Finally, we report the identification of 2660 new
carbon stars candidates that we identified through their 2MASS photometry,
their Gaia astrometry, and their location in the Gaia-2MASS diagram.Comment: 14 pages, 16 figures, accepted for publication in A&
Fluorine Abundances in the Globular Cluster M 4
We present chemical abundances for the elements carbon, sodium, and fluorine
in 15 red giants of the globular cluster M 4, as well as six red giants of the
globular cluster Centauri. The chemical abundances were calculated in
LTE via spectral synthesis. The spectra analyzed are high-resolution spectra
obtained in the near-infrared region around 2.3m with the Phoenix
spectrograph on the 8.1m Gemini South Telescope, the IGRINS spectrograph on the
McDonald Observatory 2.7m Telescope, and the CRIRES spectrograph on the ESO
8.2m Very Large Telescope. The results indicate a significant reduction in the
fluorine abundances when compared to previous values from the literature for M
4 and Centauri, due to a downward revision in the excitation
potentials of the HF(1-0) R9 line used in the analysis. The fluorine abundances
obtained for the M 4 red giants are found to be anti-correlated with those of
Na, following the typical pattern of abundance variations seen in globular
clusters between distinct stellar populations. In M 4, as the Na abundance
increases by +0.4 dex, the F abundance decreases by -0.2 dex. A
comparison with abundance predictions from two sets of stellar evolution models
finds that the models predict somewhat less F depletion (-0.1 dex) for
the same increase of +0.4 dex in Na
On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios
Using a semi-analytical model of the evolution of the Milky Way, we show how
secular evolution can create distinct overdensities in the phase space of
various properties (e.g. age vs metallicity or abundance ratios vs age)
corresponding to the thin and thick discs. In particular, we show how key
properties of the Solar vicinity can be obtained by secular evolution, with no
need for external or special events, like galaxy mergers or paucity in star
formation. This concerns the long established double-branch behaviour of
[alpha/Fe] vs metallicity and the recently found non-monotonic evolution of the
stellar abundance gradient, evaluated at the birth radii of stars. We extend
the discussion to other abundance ratios and we suggest a classification
scheme, based on the nature of the corresponding yields (primary vs secondary
or odd elements) and on the lifetimes of their sources (short-lived vs
long-lived ones). The latter property is critical in determining the single- or
double- branch behavior of an elementary abundance ratio in the Solar
neighborhood. We underline the high diagnostic potential of this finding, which
can help to separate clearly elements with sources evolving on different
timescales and help determining the site of e.g. the r-process(es). We define
the "abundance distance" between the thin and thick disc sequences as an
important element for such a separation. We also show how the inside-out
evolution of the Milky Way disc leads rather to a single-branch behavior in
other disc regions.Comment: 20 pages, 16 figures, to appear in MNRA
The cerium content of the Milky Way as revealed by Gaia DR3 GSP-Spec abundances
[Abstract]: The recent Gaia third data release contains a homogeneous analysis of millions of high-quality Radial Velocity Spectrometer (RVS) stellar spectra by the GSP-Spec module. This led to the estimation of millions of individual chemical abundances and allows us to chemically map the Milky Way. The published GSP-Spec abundances include three heavy elements produced by neutron-captures in stellar interiors: Ce, Zr, and Nd. Aims. We study the Galactic content in cerium based on these Gaia/RVS data and discuss the chemical evolution of this element. Methods. We used a sample of about 30 000 local thermal equilibrium Ce abundances, selected after applying different combinations of GSP-Spec flags. Based on the Gaia DR3 astrometric data and radial velocities, we explore the cerium content in the Milky Way and, in particular, in its halo and disc components. Results. The high quality of the Ce GSP-Spec abundances is quantified through literature comparisons. We found a rather flat [Ce/Fe] versus [M/H] trend. We also found a flat radial gradient in the disc derived from field stars and, independently, from about 50 open clusters. This agrees with previous studies. The [Ce/Fe] vertical gradient was also estimated. We also report an increasing [Ce/Ca] versus [Ca/H] in the disc, illustrating the late contribution of asymptotic giant branch stars with respect to supernovae of type II. Our cerium abundances in the disc, including the young massive population, are well reproduced by a new three-infall chemical evolution model. In the halo population, the M 4 globular cluster is found to be enriched in cerium. Moreover, 11 stars with cerium abundances belonging to the Thamnos, Helmi Stream, and Gaia-Sausage-Enceladus accreted systems were identified from chemo-dynamical diagnostics. We found that the Helmi Stream might be slightly underabundant in cerium compared to the two other systems. Conclusions. This work illustrates the high quality of the GSP-Spec chemical abundances, which significantly contribute to unveiling the heavy-element evolution history of the Milky Way.We thank the referee for their valuable comments. ES received funding from the European Unionâs Horizon 2020 research and innovation program under SPACE-H2020 grant agreement number 101004214 (EXPLORE project). ARB also acknowledges support from this Horizon program. PAP and EP thanks the Centre National dâEtudes Spatiales (CNES) for funding support. VG acknowledges support from the European Research Council Consolidator Grant funding scheme (project ASTEROCHRONOMETRY, G.A. n. 772293, http://www.asterochronometry.eu ). Special thanks to Niels Nieuwmunster and Botebar for grateful comments on figures. This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement
Estimating stellar birth radii and the time evolution of Milky Wayâs ISM metallicity gradient
We present a semi-empirical, largelymodel-independent approach for estimatingGalactic birth radii, rbirth, for Milky Way disc stars. The technique relies on the justifiable assumption that a negative radial metallicity gradient in the interstellar medium (ISM) existed for most of the disc lifetime. Stars are projected back to their birth positions according to the observationally derived age and [Fe/H] with no kinematical information required. Applying our approach to the AMBRE:HARPS and HARPSâGTO local samples, we show that we can constrain the ISM metallicity evolution with Galactic radius and cosmic time, [Fe/H]ISM(r, t), by requiring a physically meaningful rbirth distribution. We find that the data are consistent with an ISM radial metallicity gradient that flattens with time from ~â 0.15 dex kpcâ1 at the beginning of disc formation, to its measured present-day value (â0.07 dex kpcâ1). We present several chemokinematical relations in terms of mono-rbirth populations. One remarkable result is that the kinematically hottest stars would have been born locally or in the outer disc, consistent with thick disc formation from the nested flares of mono-age populations and predictions from cosmological simulations. This phenomenon can be also seen in the observed ageâvelocity dispersion relation, in that its upper boundary is dominated by stars born at larger radii. We also find that the flatness of the local ageâmetallicity relation (AMR) is the result of the superposition of the AMRs of mono-rbirth populations, each with a well-defined negative slope. The solar birth radius is estimated to be 7.3 ± 0.6 kpc, for a current Galactocentric radius of 8 kpc
The sixth data release of the Radial Velocity Experiment (RAVE). I. Survey description, spectra and radial velocities
The Radial Velocity Experiment (RAVE) is a magnitude-limited (9<I<12)
spectroscopic survey of Galactic stars randomly selected in the southern
hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet
region (8410-8795A). The 6th and final data release (DR6 or FDR) is based on
518387 observations of 451783 unique stars. RAVE observations were taken
between 12 April 2003 and 4 April 2013. Here we present the genesis, setup and
data reduction of RAVE as well as wavelength-calibrated and flux-normalized
spectra and error spectra for all observations in RAVE DR6. Furthermore, we
present derived spectral classification and radial velocities for the RAVE
targets, complemented by cross matches with Gaia DR2 and other relevant
catalogs. A comparison between internal error estimates, variances derived from
stars with more than one observing epoch and a comparison with radial
velocities of Gaia DR2 reveals consistently that 68% of the objects have a
velocity accuracy better than 1.4 km/s, while 95% of the objects have radial
velocities better than 4.0 km/s. Stellar atmospheric parameters, abundances and
distances are presented in subsequent publication. The data can be accessed via
the RAVE Web (http://rave-survey.org) or the Vizier database.Comment: 32 pages, 11 figures, accepted for publication to A