1,423 research outputs found
On the chemical evolution of the Milky Way
I discuss three different topics concerning the chemical evolution of the
Milky Way (MW). 1) The metallicity distribution of the MW halo; it is shown
that this distribution can be analytically derived in the framework of the
hierarchical merging scenario for galaxy formation, assuming that the component
sub-haloes had chemical properties similar to those of the progenitors of
satellite galaxies of the MW. 2) The age-metallicity relationship (AMR) in the
solar neighborhood; I argue for caution in deriving from data with important
uncertainties (such as the age uncertainties in the Geneva-Kopenhaguen survey)
a relationship between average metallicity and age: derived relationships are
shown to be systematically flatter than the true ones and should not be
directly compared to models. 3) The radial mixing of stars in the disk, which
may have important effects on various observables (scatter in AMR, extension of
the tails of the metallicity distribution, flatenning of disk abundance
profiles). Recent SPH + N-body simulations find considerable radial mixing, but
only comparison to observations will ultimately determine the extent of that
mixing.Comment: 12 pages, 13 figures, Invited talk at IAU Symposium No. 254 "The
Galaxy Disk in Cosmological Context", Eds. J. Andersen et al., in pres
Kinematic segregation of nearby disk stars from the Hipparcos database
To better understand our Galaxy, we investigate the pertinency of describing
the sys tem of nearby disk stars in terms of a two-components Schwarzschild
velocity distributio n.Using the proper motion and parallax information of
Hipparcos database, we determine t he parameters characterizing the local
stellar velocity field of a sample of 22000 disk stars. The sample we use is
essentially the same as the one described by the criteria ad opted to study the
LSR and the stream motion of the nearby stellar populationComment: 19 page
[alpha/Fe] in the thin and the thick disk towards an automatic parametrization of stellar spectra
We test an automatic procedure to measure [Fe/H] and [alpha/Fe] on high
resolution spectra. The test sample is the intersection of the ELODIE library
and a catalogue of 830 stars having well determined abundances.Comment: Three-dimensional Universe with Gaia, 4-7 October 2004, Observatoire
de Paris-Meudon, France (ESA SP-576), eds M. Perryman & C. Turo
The incorrect rotation curve of the Milky Way
In the fundamental quest of the rotation curve of the Milky Way, the
tangent-point (TP) method has long been the simplest way to infer velocities
for the inner, low latitude regions of the Galactic disk from observations of
the gas component. We test the validity of the method on realistic gas
distribution and kinematics of the Milky Way, using a numerical simulation of
the Galaxy. We show that the resulting velocity profile strongly deviates from
the true rotation curve of the simulation, as it overstimates it in the central
regions, and underestimates it around the bar corotation. Also, its shape
strongly depends on the orientation of the stellar bar. The discrepancies are
caused by highly non-uniform azimuthal velocities, and the systematic selection
by the TP method of high-velocity gas along the bar and spiral arms, or
low-velocity gas in less dense regions. The velocity profile is in good
agreement with the rotation curve only beyond corotation, far from massive
asymmetric structures. Therefore the observed velocity profile of the Milky Way
inferred by the TP method is expected to be very close to the true Galactic
rotation curve for 4.5<R<8 kpc. Another consequence is that the Galactic
velocity profile for R<4-4.5 kpc is very likely flawed by the non-uniform
azimuthal velocities, and does not represent the true Galactic rotation curve,
but instead local motions. The real shape of the innermost rotation curve is
probably shallower than previously thought. Using a wrong rotation curve has a
dramatic impact on the modelling of the mass distribution, in particular for
the bulge component of which derived enclosed mass within the central kpc and
scale radius are, respectively, twice and half of the actual values. We thus
strongly argue against using terminal velocities or the velocity curve from the
TP method for modelling the mass distribution of the Milky Way. (abridged)Comment: Accepted for publication in Astronomy & Astrophysics, 8 pages, 10
figures, revised version after A&A language editin
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
Vertical distribution of Galactic disk stars : II. The surface mass density in the Galactic plane
High resolution spectra data of red clump stars towards the NGP have been
obtained with the high resolution spectrograph Elodie at OHP for Tycho-2
selected stars. Combined with Hipparcos local analogues, we determine both the
gravitational force law perpendicaular to the Galactic plane, and the total
surface mass density and thickness of the Galactic disk. The surface mass
density of the Galactic disk within 800 pc derived from this analysis is
Sigma(|z|<800pc)=76 Msol.pc-2 and, removing the dark halo contribution, the
total disk mass density is Sigma0=67 Msol.pc-2 at solar radius. The thickness
of the total disk mass distribution is dynamicaly measured for the first time
and is found to be 390pc in relative agreement with the old stellar disk scale
height. All dynamical evidences concerning the structure of the disk (its local
volume density -i.e. the Oort limit-, its surface density and its thickness)
are compatible with our knowledge of the corresponding stellar disk properties.Comment: 11 pages, 9 figures, Accepted for publication in A&
On the metallicity of open clusters. III. Homogenised sample
Open clusters are known as excellent tools for various topics in Galactic
research. For example, they allow accurately tracing the chemical structure of
the Galactic disc. However, the metallicity is known only for a rather low
percentage of the open cluster population, and these values are based on a
variety of methods and data. Therefore, a large and homogeneous sample is
highly desirable. In the third part of our series we compile a large sample of
homogenised open cluster metallicities using a wide variety of different
sources. These data and a sample of Cepheids are used to investigate the radial
metallicity gradient, age effects, and to test current models. We used
photometric and spectroscopic data to derive cluster metallicities. The
different sources were checked and tested for possible offsets and
correlations. In total, metallicities for 172 open cluster were derived. We
used the spectroscopic data of 100 objects for a study of the radial
metallicity distribution and the age-metallicity relation. We found a possible
increase of metallicity with age, which, if confirmed, would provide
observational evidence for radial migration. Although a statistical
significance is given, more studies are certainly needed to exclude selection
effects, for example. The comparison of open clusters and Cepheids with recent
Galactic models agrees well in general. However, the models do not reproduce
the flat gradient of the open clusters in the outer disc. Thus, the effect of
radial migration is either underestimated in the models, or an additional
mechanism is at work. [abridged]Comment: Accepted for publication in Astronomy and Astrophysics: 18 pages, 10
figures, 4 table
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