477 research outputs found
On-line determination of stellar atmospheric parameters Teff, log g, [Fe/H] from ELODIE echelle spectra. II - The library of F5 to K7 stars
A library of 211 echelle spectra taken with ELODIE at the Observatoire de
Haute-Provence is presented. It provides a set of spectroscopic standards
covering the full range of gravities and metallicities in the effective
temperature interval [4000 K, 6300 K]. The spectra are straightened, wavelength
calibrated, cleaned of cosmic ray hits, bad pixels and telluric lines. They
cover the spectral range [440 nm, 680 nm] with an instrumental resolution of
42000. For each star, basic data were compiled from the Hipparcos catalogue and
the Hipparcos Input Catalogue. Radial velocities with a precision better than
100 m/s are given. Atmospheric parameters (Teff, log g, [Fe/H]) from the
literature are discussed. Because of scattered determinations in the
bibliography, even for the most well-known stars, these parameters were
adjusted by an iterative process which takes account of common or different
spectral features between the standards, using our homogeneous set of spectra.
Revised values of (Teff, log g, [Fe/H]) are proposed. They are still consistent
with the literature, and also lead to the self-consistency of the library, in
the sense that similar spectra have similar atmospheric parameters. This
adjustment was performed by using step by step a method based on the least
square comparison of carefully prepared spectra, which was originally developed
for the on-line estimation of the atmospheric parameters of faint field stars
(companion paper in the main journal). The spectra and corresponding data will
only be available in electronic form at the CDS (ftp cdsarc.u-strasbg.fr or
http://cdsweb.u-strasbg.fr/Abstract.html).Comment: 7 pages, 7 figures, accepted for publication in A&A Supplement Serie
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
Nitrogen Overabundance: Globular Cluster and Halo Formation
Halo globular clusters pose four succinct issues that must be solved in any
scenario of their formation: single-age, single metallicity stellar
populations; a lower limit ([Fe/H] ~ -2.3) to their average metallicity;
comprising only 1% of the stellar halo mass, and being among the oldest stars
in our Galaxy. New spectra are presented of Galactic stars and integrated
spectra of Galactic globular clusters which extend to 3250 angstroms. These
spectra show show that the most metal-poor and among the best-studied Galactic
globular clusters show strong NH3360 absorption, even though their spectral
energy distributions in the near-UV are dominated by blue horizontal branch,
AF-type stars. These strong NH features must be coming from the main sequence
stars in these clusters. These new data are combined with existing data on the
wide range of carbon and nitrogen abundance in very metal-poor ([Fe/H] < -3.5)
halo giant and dwarf stars, together with recent models of zero-metal star
formation, to make a strawman scenario for globular cluster formation that can
reproduce three of the above four issues, and well as related two of the three
issues pertaining to nitrogen overabundance. This strawman proposal makes
observational and theoretical predictions that are testable, needing specific
help from the modelers to understand all of the elemental constraints on
globular cluster and halo formation.Comment: to be published in ApJL, 2 figures, one tabl
The chemical evolution of Barium and Europium in the Milky Way
We compute the evolution of the abundances of barium and europium in the
Milky Way and we compare our results with the observed abundances from the
recent UVES Large Program "First Stars". We use a chemical evolution model
which already reproduces the majority of observational constraints. We confirm
that barium is a neutron capture element mainly produced in the low mass AGB
stars during the thermal-pulsing phase by the 13C neutron source, in a slow
neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H]
as well as the Ba solar abundance, we suggest that Ba should be also produced
as an r-process element by massive stars in the range 10-30 solar masses. On
the other hand, europium should be only an r-process element produced in the
same range of masses (10-30 solar masses), at variance with previous
suggestions indicating a smaller mass range for the Eu producers. As it is well
known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low
metallicities, although smaller in the newest data. With our model we estimate
for both elements (Ba and Eu) the ranges for the r-process yields from massive
stars which better reproduce the trend of the data. We find that with the same
yields which are able to explain the observed trends, the large spread in the
[Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an
inhomogeneous models for the chemical evolution of our Galaxy. We therefore
derive the amount by which the yields should be modified to fully account for
the observed spread. We then discuss several possibilities to explain the size
of the spread. We finally suggest that the production ratio of [Ba/Eu] could be
almost constant in the massive stars.Comment: 14 pages, 17 figures, accepted for pubblication in A&
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