113 research outputs found
Finding the Most Metal-poor Stars of the Galactic Halo with the Hamburg/ESO Objective-Prism Survey
I review the status of the search for extremely metal-poor halo stars with
the Hamburg/ESO objective-prism survey (HES). 2194 candidate metal-poor
turn-off stars and 6133 giants in the magnitude range 14 < B < 17.5 have been
selected from 329 (out of 380) HES fields, covering an effective area of 6400
square degrees in the southern extragalactic sky. Moderate-resolution follow-up
observations for 3200 candidates have been obtained so far, and ~200 new stars
with [Fe/H] <- 3.0 have been found, which trebles the total number of such
extremely low-metallicity stars identified by all previous surveys.
We use VLT-UT2/UVES, Keck/HIRES, Subaru/HDS, TNG/SARG, and Magellan/MIKE for
high-resolution spectroscopy of HES metal-poor stars. I provide an overview of
the scientific aims of these programs, and highlight several recent results.Comment: 16 pages, 6 figure
The Hamburg/ESO R-process Enhanced Star survey (HERES) IX. Constraining pure r-process Ba/Eu abundance ratio from observations of r-II stars
The oldest stars born before the onset of the main s-process are expected to
reveal a pure r-process Ba/Eu abundance ratio. We revised barium and europium
abundances of selected very metal-poor (VMP) and strongly r-process enhanced
(r-II) stars to evaluate an empirical r-process Ba/Eu ratio. Our calculations
were based on non-local thermodynamic equilibrium (NLTE) line formation for Ba
II and Eu II in the classical 1D MARCS model atmospheres. Homogeneous stellar
abundances were determined from the Ba II subordinate and resonance lines by
applying a common Ba isotope mixture. We used high-quality VLT/UVES spectra and
observational material from the literature. For most investigated stars, NLTE
leads to a lower Ba, but a higher Eu abundance. The resulting elemental ratio
of the NLTE abundances amounts, on average, log(Ba/Eu) = 0.78+-0.06. This is a
new constraint to pure r-process production of Ba and Eu. The obtained Ba/Eu
abundance ratio of the r-II stars supports the corresponding Solar System
r-process ratio as predicted by recent Galactic chemical evolution calculations
of Bisterzo, Travaglio, Gallino, Wiescher, and Kappeler. We present the NLTE
abundance corrections for lines of Ba II and Eu II in the grid of VMP model
atmospheres.Comment: 12 pages, 8 tables, accepted for publication in A&
Stellar abundances and presolar grains trace the nucleosynthetic origin of molybdenum and ruthenium
This work presents a large consistent study of molybdenum (Mo) and ruthenium
(Ru) abundances in the Milky Way. These two elements are important
nucleosynthetic diagnostics. In our sample of 71 Galactic metal-poor field
stars, we detect Ru and/or Mo in 51 of these (59 including upper limits). The
sample consists of high-resolution, high signal-to-noise spectra covering both
dwarfs and giants from [Fe/H]=-0.63 down to -3.16. Thus we provide information
on the behaviour of Mo I and Ru I at higher and lower metallicity than is
currently known. We find a wide spread in the Mo and Ru abundances, which is
typical of heavy elements. This indicates that several formation processes, in
addition to high entropy winds, can be responsible for the formation of Mo and
Ru. The formation processes are traced by comparing Mo and Ru to elements (Sr,
Zr, Pd, Ag, Ba, and Eu) with known formation processes. We find contributions
from different formation channels, namely p-, slow (s-), and rapid (r-)
neutron-capture processes. Molybdenum is a highly convolved element that
receives contributions from several processes, whereas Ru is mainly formed by
the weak r-process as is silver. We also compare our absolute elemental stellar
abundances to relative isotopic abundances of presolar grains extracted from
meteorites. Their isotopic abundances can be directly linked to the formation
process (e.g. r-only isotopes) providing a unique comparison between
observationally derived abundances and the nuclear formation process. The
comparison to abundances in presolar grains shows that the r-/s-process ratios
from the presolar grains match the total elemental chemical composition derived
from metal-poor halo stars with [Fe/H]~ -1.5 to -1.1 dex. This indicates that
both grains and stars around and above [Fe/H]=-1.5 are equally (well) mixed and
therefore do not support a heterogeneous presolar nebula... Abridged.Comment: 18 pages, 12 figures, accepted by A&
HE 1327-2326, an unevolved star with [Fe/H]<-5.0. II. New 3D-1D corrected abundances from a VLT/UVES spectrum
We present a new abundance analysis of HE 1327-2326, the currently most
iron-poor star, based on observational data obtained with VLT/UVES. We correct
the 1D LTE abundances for 3D effects to provide an abundance pattern that
supersedes previous works, and should be used to observationally test current
models of the chemical yields of the first-generation SNe. Apart from
confirming the 1D LTE abundances found in previous studies before accounting
for 3D effects, we make use of a novel technique to apply the 3D-1D corrections
for CNO which are a function of excitation potential and line strength for the
molecular lines that comprise the observable CH, NH, and OH features. We find
that the fit to the NH band at 3360 A is greatly improved due to the
application of the 3D-1D corrections. This may indicate that 3D effects are
actually observable in this star. We also report the first detection of several
weak Ni lines. The cosmologically important element Li is still not detected;
the new Li upper limit is extremely low, A(Li)<0.62, and in stark contrast with
results not only from WMAP but also from other metal-poor stars. We also
discuss how the new corrected abundance pattern of HE 1327-2326 is being
reproduced by individual and integrated yields of SNe.Comment: 43 pages, incl. 17 figures, accepted for publication in Ap
The Hamburg/ESO R-process Enhanced Star survey (HERES)VII. Thorium abundances in metal-poor stars
We report thorium abundances for 77 metal-poor stars in the metallicity range
of -3.5<[Fe/H]}<-1.0, based on "snapshot" spectra obtained with VLT-UT2/UVES
during the HERES Survey. We were able to determine the thorium abundances with
better than 1-sigma confidence for 17 stars, while for 60 stars we derived
upper limits. For five stars common with previous studies, our results were in
good agreement with the literature results. The thorium abundances span a wide
range of about 4.0 dex, and scatter exists in the distribution of log(Th/Eu)
ratios for lower metallicity stars, supporting previous studies suggesting the
r-process is not universal. We derived ages from the log(Th/Eu) ratios for 12
stars, resulting in large scattered ages, and two stars with significant
enhancement of Th relative to Eu are found, indicating the "actinide boost"
does not seem to be a rare phenomenon and thus highlighting the risk in using
log(Th/Eu) to derive stellar ages.Comment: 12 pages, 8 figure
The Oxygen Abundance of HE 1327-2326
From a newly obtained VLT/UVES spectrum we have determined the oxygen
abundance of HE 1327-2326, the most iron-poor star known to date. UV-OH lines
yield a 1D LTE abundance of [O/Fe]_OH = 3.7 (subgiant case) and [O/Fe]_OH = 3.4
(dwarf case). Using a correction of -1.0 dex to account for 3D effects on OH
line formation, the abundances are lowered to [O/Fe] = 2.8 and [O/Fe] = 2.5,
respectively, which we adopt. Without 3D corrections, the UV-OH based abundance
would be in disagreement with the upper limits derived from the OI triplet
lines: [O/Fe]_trip < 2.8 (subgiant) and [O/Fe]_trip < 3.0 (dwarf). We also
correct the previously determined carbon and nitrogen abundances for 3D
effects. Knowledge of the O abundance of HE 1327-2326 has implications for the
interpretation of its abundance pattern. A large O abundance is in accordance
with HE 1327-2326 being an early Population II star which formed from material
chemically enriched by a first generation supernova. Our derived abundances,
however, do not exclude other possibilities such as a Population III scenario.Comment: 13 pages, accepted for publication in ApJ
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