250 research outputs found
Catalogue of [Fe/H] determinations for FGK stars : 2001 edition
The catalogue presented here is a compilation of published atmospheric
parameters (Teff, logg, [Fe/H]) obtained from high resolution, high
signal-to-noise spectroscopic observations. This new edition has changed
compared to the five previous versions. It is now restricted to intermediate
and low mass stars (F, G and K stars). It contains 6354 determinations of
(Teff, logg, [Fe/H]) for 3356 stars, including 909 stars in 79 stellar systems.
The literature is complete between January 1980 and December 2000 and includes
378 references. The catalogue is made up of two tables, one for field stars and
one for stars in galactic associations, open and globular clusters and external
galaxies. The catalogue is distributed through the CDS database. Access to the
catalogue with cross-identification to other sets of data is also possible with
VizieR (Ochsenbein et al. 2000).Comment: 5 pages, 4 figures, published in A&A 373, 159. Data can be fetched
directly from: ftp://cdsarc.u-strasbg.fr/cats/III/22
The Ages of Stars
The age of an individual star cannot be measured, only estimated through
mostly model-dependent or empirical methods, and no single method works well
for a broad range of stellar types or for a full range in age. This review
presents a summary of the available techniques for age-dating stars and
ensembles of stars, their realms of applicability, and their strengths and
weaknesses. My emphasis is on low-mass stars because they are present from all
epochs of star formation in the Galaxy and because they present both special
opportunities and problems. The ages of open clusters are important for
understanding the limitations of stellar models and for calibrating empirical
age indicators. For individual stars, a hierarchy of quality for the available
age-dating methods is described. Although our present ability to determine the
ages of even the nearest stars is mediocre, the next few years hold great
promise as asteroseismology probes beyond stellar surfaces and starts to
provide precise interior properties of stars and as models continue to improve
when stressed by better observations.Comment: To appear in the 2010 volume of Annual Reviews of Astronomy and
Astrophysics
On the metallicity of the Milky Way thin disc and photometric abundance scales
The mean metallicity of the Milky Way thin disc in the solar neighbourhood is
still a matter of debate, and has recently been subject to upward revision
(Haywood, 2001). Our star sample was drawn from a set of solar neighbourhood
dwarfs with photometric metallicities. In a recent study, Reid (2002) suggests
that our metallicity calibration, based on Geneva photometry, is biased. We
show here that the effect detected by Reid is not a consequence of our adopted
metallicity scale, and we confirm that our findings are robust. On the
contrary, the application to Stromgren photometry of the Schuster & Nissen
metallicity scale is problematic. Systematic discrepancies of about 0.1 to 0.3
dex affect the photometric metallicity determination of metal rich stars, on
the colour interval 0.22< b-y <0.59, i.e including F and G stars. For F stars,
it is shown that this is a consequence of a mismatch between the standard
sequence m_1(b-y) of the Hyades used by Schuster & Nissen to calibrate their
metallicity scale, and the system of Olsen (1993, 1994ab). It means that
although Schuster & Nissen calibration and Olsen photometry are intrinsically
correct, there are mutually incompatible for metal rich, F-type stars. For G
stars, the discrepancy is most probably the continuation of the same problem,
albeit worthen by the lack of spectroscopic calibrating stars. A corrected
calibration is proposed which renders the calibration of Schuster & Nissen
applicable to the catalogues of Olsen. We also give a simpler calibration
referenced to the Hyades sequence, valid over the same color and metallicity
ranges.Comment: 11 pages, 11 figures, accepted in MNRA
The PASTEL catalogue of stellar parameters
The PASTEL catalogue is an update of the [Fe/H] catalogue, published in 1997
and 2001. It is a bibliographical compilation of stellar atmospheric parameters
providing (Teff,logg,[Fe/H]) determinations obtained from the analysis of high
resolution, high signal-to-noise spectra, carried out with model atmospheres.
PASTEL also provides determinations of the one parameter Teff based on various
methods. It is aimed in the future to provide also homogenized atmospheric
parameters and elemental abundances, radial and rotational velocities. A web
interface has been created to query the catalogue on elaborated criteria.
PASTEL is also distributed through the CDS database and VizieR. To make it as
complete as possible, the main journals have been surveyed, as well as the CDS
database, to find relevant publications. The catalogue is regularly updated
with new determinations found in the literature. As of Febuary 2010, PASTEL
includes 30151 determinations of either Teff or (Teff,logg,[Fe/H]) for 16649
different stars corresponding to 865 bibliographical references. Nearly 6000
stars have a determination of the three parameters (Teff,logg,[Fe/H]) with a
high quality spectroscopic metallicity.Comment: 5 pages, accepted for publication in A&A. The PASTEL catalogue can be
queried at http://pastel.obs.u-bordeaux1.fr/ or
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/paste
The Na I D resonance lines in main sequence late-type stars
We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) in late-type
dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between
0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained
medium resolution echelle spectra using the 2.15-m telescope at the argentinian
observatory CASLEO. The observations have been performed periodically since
1999. The spectra were calibrated in wavelength and in flux. A definition of
the pseudo-continuum level is found for all our observations. We also define a
continuum level for calibration purposes. The equivalent width of the D lines
is computed in detail for all our spectra and related to the colour index (B-V)
of the stars. When possible, we perform a careful comparison with previous
studies. Finally, we construct a spectral index (R_D') as the ratio between the
flux in the D lines, and the bolometric flux. We find that, once corrected for
the photospheric contribution, this index can be used as a chromospheric
activity indicator in stars with a high level of activity. Additionally, we
find that combining some of our results, we obtain a method to calibrate in
flux stars of unknown colour.Comment: 12 pages, including 14 figures and 4 tables. Accepted for publication
in MNRA
Impact of granulation effects on the use of Balmer lines as temperature indicators
Balmer lines serve as important indicators of stellar effective temperatures
in late-type stellar spectra. One of their modelling uncertainties is the
influence of convective flows on their shape. We aim to characterize the
influence of convection on the wings of Balmer lines. We perform a differential
comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical
model atmospheres and 1D hydrostatic standard ones. The model parameters are
appropriate for F,G,K dwarf and subgiant stars of metallicity ranging from
solar to 1/1000 solar. The shape of the Balmer lines predicted by 3D models can
never be exactly reproduced by a 1D model, irrespective of its effective
temperature. We introduce the concept of a 3D temperature correction, as the
effective temperature difference between a 3D model and a 1D model which
provides the closest match to the 3D profile. The temperature correction is
different for the different members of the Balmer series and depends on the
adopted mixing-length parameter in the 1D model. Among the investigated models,
the 3D correction ranges from -300K to +300K. Horizontal temperature
fluctuations tend to reduce the 3D correction. Accurate effective temperatures
cannot be derived from the wings of Balmer lines, unless the effects of
convection are properly accounted for. The 3D models offer a physically well
justified way of doing so. The use of 1D models treating convection with the
mixing-length theory do not appear to be suitable for this purpose. In
particular, there are indications that it is not possible to determine a single
value of the mixing-length parameter which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters.Comment: 6 pages, 3 figures, accepted for publication in A&
The Top Ten solar analogs in the ELODIE library
Several solar analogs have been identified in the library of high resolution
stellar spectra taken with the echelle spectrograph ELODIE. A purely
differential method has been used, based on the chi2 comparison of a large
number of G dwarf spectra to 8 spectra of the Sun, taken on the Moon and Ceres.
HD 146233 keeps its status of closest ever solar twin (Porto de Mello & da
Silva 1997). Some other spectroscopic analogs have never been studied before,
while the two planet-host stars HD095128 and HD186427 are also part of the
selection. The fundamental parameters found in the literature for these stars
show a surprising dispersion, partly due to the uncertainties which affect
them.
We discuss the advantages and drawbacks of photometric and spectroscopic
methods to search for solar analogs and conclude that they have to be used
jointly to find real solar twins.Comment: 12 pages, accepted in A&
A catalogue of [Fe/H] determinations : 1996 edition
The Catalogue of [Fe/H] determinations has been updated and now includes 5946
determinations for 3247 stars (with 751 stars in 84 associations, clusters or
galaxies). The 700 bibliographical references are complete up to December 1995
and refer only to [Fe/H] determinations obtained from high-resolution
spectroscopic observations. The Catalogue also gives the spectral type, the
object type, the visual magnitude and colour, as well as determinations of the
effective temperature and gravity for each star, when available. In this paper,
we comment on the Catalogue, the parameters listed in it, its stellar
content,and the precautions that should be observed when using it. The full
Catalogue is only available in electronic form at the CDS or upon request
([email protected]).Comment: 7 pages (Latex, A&A style), 4 PostScript figures, to be published in
A&A Supplement Series in jul
The extended star formation history of omega Centauri
For the first time, the abundances of a large sample of subgiant and turn-off
region stars in omega Centauri have been measured, the data base being medium
resolution spectroscopy from FORS2 at the VLT. Absolute iron abundances were
derived for about 400 member stars from newly defined line indices with an
accuracy of +/-0.15 dex. The abundances range between -2.2<[Fe/H]<-0.7 dex,
resembling the large metallicity spread found for red giant branch stars. The
combination of the spectroscopic results with the location of the stars in the
colour magnitude diagram has been used to estimate ages for the individual
stars. Whereas most of the metal-poor stars are consistent with a single old
stellar population, stars with abundances higher than [Fe/H]=-1.3 dex are
younger. The total age spread in omega Cent is about 3 Gyr. The monotonically
increasing age-metallicity relation seems to level off above [Fe/H]=-1.0 dex.
Whether the star formation in omega Cen occured continuously or rather
episodically has to be shown by combining more accurate abundances with highest
quality photometry.Comment: 4 pages, 3 figures, accepted for publication in Astronomy &
Astrophysics (Letters
Solar twins in M67
The discovery of true solar analogues is fundamental for a better
understanding of the Sun and of the solar system. The open cluster M67 offers a
unique opportunity to search for solar analogues because its chemical
composition and age are very similar to those of the Sun. We analyze FLAMES
spectra of a large number of M67 main sequence stars to identify solar
analogues in this cluster.We first determine cluster members which are likely
not binaries, by combining proper motions and radial velocity measurements. We
concentrate our analysis on the determination of stellar effective temperature,
using analyses of line-depth ratios and H wings, making a direct
comparison with the solar spectrum obtained with the same instrument. We also
compute the lithium abundance for all the stars.Ten stars have both the
temperature derived by line-depth ratios and H wings within 100 K from
the Sun. From these stars we derive, assuming a cluster reddening
, the solar colour and a cluster
distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun
and candidates to be true solar twins. These stars have also a low Li content,
comparable to the photospheric abundance of the Sun, likely indicating a
similar mixing evolution. We find several candidates for the best solar
analogues ever. These stars are amenable to further spectroscopic
investigations and planet search. The solar colours are determined with rather
high accuracy with an independent method, as well as the cluster distance
modulus.Comment: 13pages and 8 figures, accepted for publication in A&
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