297 research outputs found
Abundances in Sagittarius: present state and perspectives for the use of VLT
Sagittarius, the nearest external galaxy, will be amenable to detailed
abundance studies with VLT and other 8m class telescopes. Such data, in
conjuction with the similar data of our own galaxy, will allow a deeper
understanding of chemical evolution. Our study conducted with NTT shows the
presence of metal-rich stars with radial velocities compatible with Sagittarius
membership and of stars as metal-poor as [Fe/H]=-1.5. In this talk I shall
address the way in which VLT instruments will allow to clarify this intricate
situation.Comment: Invited talk at the ESO VLT Opening Symposium, Antofagasta, March 1-4
199
Low metallicity stars in our Galaxy
The advent of m class telescopes has allowed the detailed spectroscopic study
of sizeable numbers of extremely metal-poor Galactic stars which are the
witnesses of the formation of the early Galaxy. Their chemical composition
displays some distinctive trends which should provide a strong constraint on
the physical nature of the first generation(s) of stars and on their
nucleosynthetic output. I will review recent results in the field following the
periodic table, from lithium to uranium and shortly comment on the intriguing
classes of Carbon Enhanced Metal Poor (CEMP) stars, for many of which there is
no analogue among solar metallicity stars. In spite of these exciting results,
the number of known stars of metallicity below [Fe/H]=-3.3 remains quite small
and it would be desirable to discover more, both to clearly understand the
metal-weak tail of Halo metallicity distribution and to clarify the abundance
trends at the lowest metallicities. Most of these extremely rare objects have
been discovered by the wide field objective prism surveys, HK survey and
Hamburg-ESO survey. In the near future the Sloan Digital Sky Survey and its
continuation SEGUE are expected to boost significantly the numbers of known
extremely metal poor stars. We are living exciting times but an even more
exciting future lies ahead !Comment: To appear in the proceedings of the CRAL-Conference Series I
"Chemodynamics: from first stars to local galaxies", Lyon 10-14 July 2006,
France, Eds. Emsellem, Wozniak, Massacrier, Gonzalez, Devriendt, Champavert,
EAS Publications Serie
The primordial lithium abundance
Lithium abundances in a selected sample of halo stars have been revised by
using the new accurate IRFM effective temperatures by Alonso, Arribas &
Martinez-Roger (1996a). From 41 plateau stars (Teff > 5700 and [Fe/H] <= -1.5)
we found no evidence for intrinsic dispersion, a tiny trend with Teff and no
trend with [Fe/H]. The trend with the Teff is fully consistent with the
standard Li isochrones of Deliyannis, Demarque & Kawaler (1990) implying a
primordial value for Li of A(Li) = 2.238 +/- 0.012 {1 sigma} +/- 0.05{sys}. The
present results argue against any kind of depletion predicted by diffusion,
rotational mixing or stellar winds. Therefore the Li observed in Pop II stars
provides a direct and reliable estimate of the baryonic density that can rival
other baryonic indicators such as the deuterium in high redshift systems. The
present upwards revision of primordial Li in the framework of SBBN gives at 1
sigma two solutions for the baryonic density: Omega_{B}h^2 = 0.0062
{+0.0018,-0.0011} or Omega_{B}h^2 = 0.0146 {+0.0029,-0.0033} .Comment: Tex, uses MN.tex, 18 .ps figures; accepted MNRA
Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models
Context. Copper is an element whose interesting evolution with metallicity is
not fully understood. Observations of copper abundances rely on a very limited
number of lines, the strongest are the Cu I lines of Mult. 1 at 324.7 nm and
327.3 nm which can be measured even at extremely low metallicities. Aims. We
investigate the quality of these lines as abundance indicators. Method. We
measure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752
and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3D
hydrodynamical model atmospheres computed with the CO5BOLD code. These
abundances are compared to the Cu abundances measured in giant stars of the
same clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm. Results.
The abundances derived from the lines of Mult. 1 in TO stars differ from the
abundances of giants of the same clusters. This is true both using CO5BOLD
models and using traditional 1D model atmospheres. The LTE 3D corrections for
TO stars are large, while they are small for giant stars. Conclusions. The Cu I
resonance lines of Mult. 1 are not reliable abundance indicators. It is likely
that departures from LTE should be taken into account to properly describe
these lines, although it is not clear if these alone can account for the
observations. An investigation of these departures is indeed encouraged for
both dwarfs and giants. Our recommendation to those interested in the study of
the evolution of copper abundances is to rely on the measurements in giants,
based on the lines of Mult. 2. We caution, however, that NLTE studies may imply
a revision in all the Cu abundances, both in dwarfs and giants.Comment: to be published on A\&
On the origin of HE0107-5240, the most iron deficient star presently known
We show that the "puzzling" chemical composition observed in the extremely
metal poor star HE0107-5240 may be naturally explained by the concurrent
pollution of at least two supernovae. In the simplest possible model a
supernova of quite low mass (~15 Msun), underwent a "normal" explosion and
ejected ~0.06 Msun of 56Ni while a second one was massive enough (~35 Msun) to
experience a strong fall back that locked in a compact remnant all the
carbon-oxygen core. In a more general scenario, the pristine gas clouds were
polluted by one or more supernovae of relatively low mass (less than ~25 Msun).
The successive explosion of a quite massive star experiencing an extended fall
back would have largely raised the abundances of the light elements in its
close neighborhood.Comment: 10 pages; 3 figures; accepted for publication in the The
Astrophysical Journal Letter
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