244 research outputs found
Elemental Abundance Ratios in Stars of the Outer Galactic Disk. II. Field Red Giants
We summarize a selection process to identify red giants in the direction of
the southern warp of the Galactic disk, employing VI_C photometry and
multi-object spectroscopy. We also present results from follow-up
high-resolution, high-S/N echelle spectroscopy of three field red giants,
finding [Fe/H] values of about -0.5. The field stars, with Galactocentric
distances estimated at 10 to 15 kpc, support the conclusion of Yong, Carney, &
de Almeida (2005) that the Galactic metallicity gradient disappears beyond R_GC
values of 10 to 12 kpc for the older stars and clusters of the outer disk. The
field and cluster stars at such large distances show very similar abundance
patterns, and, in particular, all show enhancements of the "alpha" elements O,
Mg, Si, Ca, and Ti and the r-process element Eu. These results suggest that
Type II supernovae have been significant contributors to star formation in the
outer disk relative to Type Ia supernovae within the past few Gyrs. We also
compare our results with those available for much younger objects. The limited
results for the H II regions and B stars in the outer disk also suggest that
the radial metallicity gradient in the outer disk is shallow or absent. The
much more extensive results for Cepheids confirm these trends, and that the
change in slope of the metallicity gradient may occur at a larger
Galactocentric distance than for the older stars and clusters. However, the
younger stars also show rising alpha element enhancements with increasing R_GC,
at least beyond 12 kpc. These trends are consistent with the idea of a
progressive growth in the size of the Galactic disk with time, and episodic
enrichment by Type II supernovae as part of the disk's growth. [Abridged]Comment: Accepted for publication in A
EOSC Authentication and Authorization Infrastructure (AAI) : Report from the EOSC Executive Board Working Group (WG) Architecture AAI Task Force (TF)
The EOSC Architecture Working Group has assigned the AAI Task Force (AAI TF) the task to establish a common global ecosystem for identity and access control infrastructures for the European Open Science Cloud (EOSC). Since the EOSC is part of an international environment of research and education, the principles established by the EOSC AAI subtask must be globally viable. The EOSC AAI TF has produced a set of deliverables: - EOSC AAI First Principles & Requirements - EOSC AAI Baseline Architecture - EOSC AAI Federation participation guidelines (participation policy and technical framework) - EOSC AAI Best Practise
Storage Resource Manager version 2.2: design, implementation, and testing experience
Storage Services are crucial components of the Worldwide LHC Computing Grid Infrastructure spanning more than 200 sites and serving computing and storage resources to the High Energy Physics LHC communities. Up to tens of Petabytes of data are collected every year by the four LHC experiments at CERN. To process these large data volumes it is important to establish a protocol and a very efficient interface to the various storage solutions adopted by the WLCG sites. In this work we report on the experience acquired during the definition of the Storage Resource Manager v2.2 protocol. In particular, we focus on the study performed to enhance the interface and make it suitable for use by the WLCG communities. At the moment 5 different storage solutions implement the SRM v2.2 interface: BeStMan (LBNL), CASTOR (CERN and RAL), dCache (DESY and FNAL), DPM (CERN), and StoRM (INFN and ICTP). After a detailed inside review of the protocol, various test suites have been written identifying the most effective set of tests: the S2 test suite from CERN and the SRM-Tester test suite from LBNL. Such test suites have helped verifying the consistency and coherence of the proposed protocol and validating existing implementations. We conclude our work describing the results achieved
Abundance Patterns in the Draco, Sextans and Ursa Minor Dwarf Spheroidal Galaxies
The Keck I telescope has been used to obtain HIRES spectra for red giants
belonging to the Draco, Sextans and Ursa Minor dwarf spheroidal (dSph)
galaxies. An analysis of these spectra is presented, along with abundance
ratios for more than 20 elements. The resulting database of element abundances
for 17 stars is the most extensive yet assembled for stars in dSph
environments. Our main findings are summarized as follows: (1) There is
unambiguous evidence for a large internal spread in metallicity in all three
galaxies: our program stars span a range of [Fe/H] = 1.53, 1.40 and 0.73 dex in
Draco, Sextans and Ursa Minor, respectively. (2) The abundance patterns among
the dSph stars are remarkably uniform, suggesting that all three galaxies have
similar nucleosynthetic histories. (3) A comparison of the measured abundance
ratios for our sample of dSph stars with published values for Galactic halo and
disk field stars suggests that the dSph galaxies have 0.02 < [alpha/Fe] < 0.13
dex, whereas the halo field star sample has [alpha/Fe] ~ 0.28 dex over the same
range in metallicity. (4) The most metal-rich dSph stars in our sample have
[Y/Fe] abundances which are significantly lower than those measured for halo
field stars of similar metallicity, while the measured [Ba/Eu] ratios for the
dSph stars suggest that the early chemical evolution of these galaxies was
dominated by the r-process. Taken together, these results suggest that the
Galactic halo is unlikely to have assembled, in its entirety, through the
disruption of dwarf galaxies similar to the low-luminosity dSphs studied here.
(ABRIDGED).Comment: 24 pages, 8 postscript figures (including 6 color figures). Accepted
for publication in the Astrophysical Journa
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