589 research outputs found
Revisited fluorine abundances in the globular cluster M22 (NGC 6656)
Fluorine is a fairly good tracer of formation histories of multiple stellar
populations in globular clusters as already revealed by several studies. Large
variations in fluorine abundance in red giant stars of the globular cluster M22
have been recently reported by two different groups. Futhermore, one of these
studies claims that the abundance of fluorine is anti-correlated with sodium
abundances in this cluster, leading to strong conclusions on the chemical
history of M22. To validate this important finding, we re-examine the F
abundance determinations of some of the previously studied stars. We have thus
reanalysed some high-resolution VLT/CRIRES spectra of RGB stars found in M22 in
order to re-estimate their fluorine abundance from the spectral synthesis of
the HF line at 2.336microns. Unlike what has been previously estimated, we show
that only upper limits or doubtful fluorine abundances with large uncertainties
in M22 RGB stars can be derived. This is probably caused by an incorrect
identification of continuum fluctuations as the HF signature combined with a
wrong correction of the stellar radial velocity. Such continuum fluctuations
could be the consequences of telluric residuals that are still present in the
analysed spectra. Therefore, no definitive conclusions on the chemical
pollution caused by the M22 first stellar generation can presently be drawn
from the fluorine content of this cluster.Comment: A&A, in pres
Fluorine abundances and the puzzle of globular cluster chemical history
The abundance of fluorine in a few Galactic globular clusters is known to
strongly vary from star-to-star. These unexpected chemical properties are an
additional confirmation of the chemical inhomogeneities already found in
several GC, and probably caused by the first generations of stars formed in
these systems. The aim of this article is to complement our understanding of
the F-behaviour in GC stars and to look for new constraints on the formation
histories of their multiple stellar populations. We have collected near-IR
spectra of 15 RGB stars belonging to GC spanning a wide range of metallicity:
47 Tuc, M4, NGC6397 and M30. F, Na and Fe abundances have been estimated by
spectral synthesis. No anticorrelation between F and Na abundances are found
for the most metal-rich cluster of the sample (47 Tuc). In this GC, RGB stars
indeed exhibit rather small differences in [F/Fe] unlike the larger ones found
for the [Na/Fe] ratios. This reveals a rather inhomogeneous stellar system and
a complex chemical evolution history for 47 Tuc . In M4, one star of our study
confirms the previous Na-F distribution reported by another group in 2005. For
the two very metal-poor GC (NGC6397 and M30), only upper limits of F abundances
have been derived. We show that F abundances could be estimated in such
metal-poor GC with current telescopes and spectrographs only if unexpected
F-rich giants are found and/or exceptional observational conditions are met.
The distribution of the F and Na abundances in GC reveal that their RGB members
seem to belong to two well-separated regions. All the RGB stars analysed so far
in the different GC are indeed found to be either F-rich Na-poor or F-poor
Na-rich. Such well-separated bimodal regimes are consistent with the separate
formation episodes suspected in most galactic GC.Comment: Astronomy & Astrophysics, in pres
Automated derivation of stellar atmospheric parameters and chemical abundances: the MATISSE algorithm
We present an automated procedure for the derivation of atmospheric
parameters (Teff, log g, [M/H]) and individual chemical abundances from stellar
spectra. The MATrix Inversion for Spectral SythEsis (MATISSE) algorithm
determines a basis, B_\theta(\lambda), allowing to derive a particular stellar
parameter \theta by projection of an observed spectrum. The B_\theta(\lambda)
function is determined from an optimal linear combination of theoretical
spectra and it relates, in a quantitative way, the variations in the spectrum
flux with variations in \theta. An application of this method to the GAIA/RVS
spectral range is described, together with its performances for different types
of stars of various metallicities. Blind tests with synthetic spectra of
randomly selected parameters and observed input spectra are also presented. The
method gives rapid, accurate and stable results and it can be efficiently
applied to the study of stellar populations through the analysis of large
spectral data sets, including moderate to low signal to noise spectra
The Origin of Fluorine: Abundances in AGB Carbon Stars Revisited
Revised spectroscopic parameters for the HF molecule and a new CN line list
in the 2.3 mu region have been recently available, allowing a revision of the F
content in AGB stars. AGB carbon stars are the only observationally confirmed
sources of fluorine. Nowadays there is not a consensus on the relevance of AGB
stars in its Galactic chemical evolution. The aim of this article is to better
constrain the contribution of these stars with a more accurate estimate of
their fluorine abundances. Using new spectroscopic tools and LTE spectral
synthesis, we redetermine fluorine abundances from several HF lines in the
K-band in a sample of Galactic and extragalactic AGB carbon stars of spectral
types N, J and SC spanning a wide range of metallicities. On average, the new
derived fluorine abundances are systematically lower by 0.33 dex with respect
to previous determinations. This may derive from a combination of the lower
excitation energies of the HF lines and the larger macroturbulence parameters
used here as well as from the new adopted CN line list. Yet, theoretical
nucleosynthesis models in AGB stars agree with the new fluorine determinations
at solar metallicities. At low metallicities, an agreement between theory and
observations can be found by handling in a different way the
radiative/convective interface at the base of the convective envelope. New
fluorine spectroscopic measurements agree with theoretical models at low and at
solar metallicity. Despite this, complementary sources are needed to explain
its observed abundance in the solar neighbourhood.Comment: 9 pages, 4 figures, accepted in A&
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
Parameter Estimation from an Optimal Projection in a Local Environment
The parameter fit from a model grid is limited by our capability to reduce
the number of models, taking into account the number of parameters and the non
linear variation of the models with the parameters. The Local MultiLinear
Regression (LMLR) algorithms allow one to fit linearly the data in a local
environment. The MATISSE algorithm, developed in the context of the estimation
of stellar parameters from the Gaia RVS spectra, is connected to this class of
estimators. A two-steps procedure was introduced. A raw parameter estimation is
first done in order to localize the parameter environment. The parameters are
then estimated by projection on specific vectors computed for an optimal
estimation. The MATISSE method is compared to the estimation using the
objective analysis. In this framework, the kernel choice plays an important
role. The environment needed for the parameter estimation can result from it.
The determination of a first parameter set can be also avoided for this
analysis. These procedures based on a local projection can be fruitfully
applied to non linear parameter estimation if the number of data sets to be
fitted is greater than the number of models
The AMBRE Project: Stellar Parameterisation of the ESO:UVES archived spectra
The AMBRE Project is a collaboration between the European Southern
Observatory (ESO) and the Observatoire de la Cote d'Azur (OCA) that has been
established in order to carry out the determination of stellar atmospheric
parameters for the archived spectra of four ESO spectrographs.
The analysis of the UVES archived spectra for their stellar parameters has
been completed in the third phase of the AMBRE Project. From the complete
ESO:UVES archive dataset that was received covering the period 2000 to 2010,
51921 spectra for the six standard setups were analysed. The AMBRE analysis
pipeline uses the stellar parameterisation algorithm MATISSE to obtain the
stellar atmospheric parameters. The synthetic grid is currently constrained to
FGKM stars only.
Stellar atmospheric parameters are reported for 12,403 of the 51,921 UVES
archived spectra analysed in AMBRE:UVES. This equates to ~23.9% of the sample
and ~3,708 stars. Effective temperature, surface gravity, metallicity and alpha
element to iron ratio abundances are provided for 10,212 spectra (~19.7%),
while at least effective temperature is provided for the remaining 2,191
spectra. Radial velocities are reported for 36,881 (~71.0%) of the analysed
archive spectra. Typical external errors of sigmaTeff~110dex,
sigmalogg~0.18dex, sigma[M/H]~0.13dex, and sigma[alpha/Fe]~0.05dex with some
reported variation between giants and dwarfs and between setups are reported.
UVES is used to observe an extensive collection of stellar and non-stellar
objects all of which have been included in the archived dataset provided to OCA
by ESO. The AMBRE analysis extracts those objects which lie within the FGKM
parameter space of the AMBRE slow rotating synthetic spectra grid. Thus by
homogeneous blind analysis AMBRE has successfully extracted and parameterised
the targeted FGK stars (23.9% of the analysed sample) from within the ESO:UVES
archive.Comment: 19 pages, 16 figures, 11 table
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
EROS variable stars: discovery of a slow nova in the SMC
We report the discovery of a slow nova found in the core of the Small Magellanic Cloud by the EROS microlensing survey. Nova SMC 1994 is a classical nova with a DQ Her type lightcurve characterized by a deep minimum. Low amplitude variations occuring on time-scales of hours and days are also detected at maximum light. Spectra collected during the nebular phase indicate that Nova SMC 1994 is similar to Galactic novae of the same class. Large helium enhancement in the shell is found and O and N enrichments are suspected. Based on observations collected at the European Southern Observatory, Chile
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