692 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
Rotation of Hot Horizontal Branch Stars in Galactic Globular Clusters
We present high resolution UVES+VLT spectroscopic observations of 61 stars in
the extended blue horizontal branches of the Galactic globular clusters NGC
1904 (M79), NGC 2808, NGC 6093 (M80), and NGC 7078 M15). Our data reveal for
the first time the presence in NGC 1904 of a sizable population of fast (v
sin(i) >= 20 km/s) horizontal branch (HB) rotators, confined to the cool end of
the EHB, similar to that found in M13. We also confirm the fast rotators
already observed in NGC 7078. The cooler stars (T_eff < 11,500 K) in these
three clusters show a range of rotation rates, with a group of stars rotating
at ~ 15 km/s or less, and a fast rotating group at ~ 30 km/s. Apparently, the
fast rotators are relatively more abundant in NGC 1904 and M13, than in NGC
7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the
stars hotter than T_eff ~ 11,500 K have projected rotational velocities vsini<
12 km/s. The connection between photometric gaps in the HB and the change in
the projected rotational velocities is not confirmed by the new data. However,
our data are consistent with a relation between this discontinuity and the HB
jump.Comment: 2 pages, 1 figure, A.S.P. Conf. Ser., in press in Vol. 296, 200
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
Spectroscopic search for binaries among EHB stars in globular clusters
We performed a spectroscopic search for binaries among hot Horizontal Branch
stars in globular clusters. We present final results for a sample of 51 stars
in NGC6752, and preliminary results for the first 15 stars analyzed in M80. The
observed stars are distributed along all the HBs in the range 8000 < Teff <
32000 K, and have been observed during four nights. Radial velocity variations
have been measured with the cross-correlation technique. We carefully analyzed
the statistical and systematic errors associated with the measurements in order
to evaluate the statistical significance of the observed variations. No close
binary system has been detected, neither among cooler stars nor among the
sample of hot EHB stars (18 stars with Teff > 22000 K in NGC6752). The data
corrected for instrumental effects indicate that the radial velocity variations
are always below the 3sigma level of ~15 km/s. These results are in sharp
contrast with those found for field hot subdwarfs, and open new questions about
the formation of EHB stars in globular clusters, and possibly of the field
subdwarfs.Comment: To appear in Baltic Astronomy. Proceedings of the 2nd meeting on Hot
Subdwarf Stars, La Palma, June 2005. 4 pages, 2 figure
Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808
Astronomy and Astrophysics, 452, pp. 493-501, http://dx.doi.org./10.1051/0004-6361:20054593International audienc
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