3,159 research outputs found
The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observations
A high resolution (R = 110.000), very high S/N (>600) spectrum of the
metal-poor turnoff star G271-162 has been obtained in connection with the
commissioning of UVES at VLT/Kueyen. Using both 1D hydrostatic and 3D
hydrodynamical model atmospheres, the lithium isotope ratio has been estimated
from the LiI 670.8 nm line by means of spectral synthesis. The necessary
stellar line broadening (1D: macroturbulence + rotation, 3D: rotation) has been
determined from unblended KI, CaI and FeI lines. The 3D line profiles agree
very well with the observed profiles, including the characteristic line
asymmetries. Both the 1D and 3D analyses reveal a possible detection of 6Li in
G271-162, 6Li/7Li = 0.02 +-0.01 (one sigma). It is discussed if the smaller
amount of 6Li in G271-162 than in the similar halo star HD84937 could be due to
differences in stellar mass and/or metallicity or whether it may reflect an
intrinsic scatter of the Li isotope ratio in the ISM at a given metallicity.Comment: 5 pages with 6 figures. Accepted as a letter in A&
O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is
presented using the [OI] 6300A line, the OI 7774A triplet, and a selection of
weak FeII lines observed on high-resolution spectra acquired with the VLT UVES
spectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 <
[Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] ranging
from -1.0 to -2.7. The abundance analysis was made first using standard model
atmospheres taking into account non-LTE effects on the triplet: the [OI] line
and the triplet give consistent results with [O/Fe] increasing quasi-linearly
with decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. When
hydrodynamical model atmospheres representing stellar granulation in dwarf and
subgiant stars replace standard models, the [O/Fe] from the [OI] and FeII lines
is decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs
[Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] =
-2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] <
-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of the
hydrodynamical models disturbs the broad agreement between the oxygen
abundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve to
erase these differences.Comment: ps file, 18 pages (including 10 figures) - Accepted for publication
in A&
Cosmological Cosmic Rays and the observed Li6 plateau in metal poor halo stars
Very recent observations of the Li6 isotope in halo stars reveal a Li6
plateau about 1000 times above the predicted BBN abundance. We calculate the
evolution of Li6 versus redshift generated from an initial burst of
cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that
the pregalactic production of the Li6 isotope can account for the Li6 plateau
observed in metal poor halo stars without additional over-production of Li7.
The derived relation between the amplitude of the CCR energy spectra and the
redshift of the initial CCR production puts constraints on the physics and
history of the objects, such as pop III stars, responsible for these early
cosmic rays. Consequently, we consider the evolution of Li6 in the Galaxy.
Since Li6 is also produced in Galactic cosmic ray nucleosynthesis, we argue
that halo stars with metallicities between [Fe/H] = -2 and -1, must be somewhat
depleted in Li6.Comment: 8 pages, 6 figures, version accepted for publication in Ap
Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood
Two distinct halo populations were found in the solar neighborhood by a
series of works. They can be clearly separated by [alpha\Fe] and several other
elemental abundance ratios including [Cu/Fe]. Very recently, a non-local
thermodynamic equilibrium (non-LTE) study revealed that relatively large
departures exist between LTE and non-LTE results in copper abundance analysis.
We aim to derive the copper abundances for the stars from the sample of Nissen
et al (2010) with both LTE and non-LTE calculations. Based on our results, we
study the non-LTE effects of copper and investigate whether the high-alpha
population can still be distinguished from the low-alpha population in the
non-LTE [Cu/Fe] results. Our differential abundance ratios are derived from the
high-resolution spectra collected from VLT/UVES and NOT/FIES spectrographs.
Applying the MAFAGS opacity sampling atmospheric models and spectrum synthesis
method, we derive the non-LTE copper abundances based on the new atomic model
with current atomic data obtained from both laboratory and theoretical
calculations. The copper abundances determined from non-LTE calculations are
increased by 0.01 to 0.2 dex depending on the stellar parameters compared with
the LTE results. The non-LTE [Cu/Fe] trend is much flatter than the LTE one in
the metallicity range -1.6<[Fe/H]<-0.8. Taking non-LTE effects into
consideration, the high- and low-alpha stars still show distinguishable copper
abundances, which appear even more clear in a diagram of non-LTE [Cu/Fe] versus
[Fe/H]. The non-LTE effects are strong for copper, especially in metal-poor
stars. Our results confirmed that there are two distinct halo populations in
the solar neighborhood. The dichotomy in copper abundance is a peculiar feature
of each population, suggesting that they formed in different environments and
evolved obeying diverse scenarios.Comment: 9 pages, 7 figures, 2 table
The First Galaxies: Clues from Element Abundances
It has recently become possible to measure directly the abundances of several
chemical elements in a variety of environments at redshifts up to z = 5. In
this review I summarise the latest observations of Lyman break galaxies, damped
Lyman alpha systems and the Lyman alpha forest with a view to uncovering any
clues which these data may offer to the first episodes of star formation. The
picture which is emerging is one where the universe at z = 3 already included
many of the components of today's galaxies--even at these early times we see
evidence for Populations I and II stars, while the `smoking gun' for Population
III objects may be hidden in the chemical composition of the lowest density
regions of the IGM, yet to be deciphered.Comment: 15 pages, LaTex, 8 Postscript Figures. To appear in the Philosophical
Transactions of The Royal Society, Series
Carbon and oxygen in metal-poor halo stars
Carbon and oxygen are key tracers of the Galactic chemical evolution; in
particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor
halo stars could be a signature of nucleosynthesis by massive Population III
stars. We reanalyse carbon, oxygen, and iron abundances in thirty-nine
metal-poor turn-off stars. For the first time, we take into account
three-dimensional (3D) hydrodynamic effects together with departures from local
thermodynamic equilibrium (LTE) when determining both the stellar parameters
and the elemental abundances, by deriving effective temperatures from 3D
non-LTE H profiles, surface gravities from Gaia parallaxes, iron
abundances from 3D LTE Feii equivalent widths, and carbon and oxygen abundances
from 3D non-LTE Ci and Oi equivalent widths. We find that [C/Fe] stays flat
with [Fe/H], whereas [O/Fe] increases linearly up to dex with decreasing
[Fe/H] down to dex. As such [C/O] monotonically decreases towards
decreasing [O/H], in contrast to previous findings, mainly by virtue of less
severe non-LTE effects for Oi at low [Fe/H] with our improved calculations.Comment: 5 pages, 2 figures; published in A&A Letter
Galactic Cosmic Rays from Superbubbles and the Abundances of Lithium, Beryllium, and Boron
In this article we study the galactic evolution of the LiBeB elements within
the framework of a detailed model of the chemical evolution of the Galaxy that
includes galactic cosmic ray nucleosynthesis by particles accelerated in
superbubbles. The chemical composition of the superbubble consists of varying
proportions of ISM and freshly supernova synthesized material. The
observational trends of 6 LiBeB evolution are nicely reproduced by models in
which GCR come from a mixture of 25% of supernova material with 75% of ISM,
except for 6 Li, for which maybe an extra source is required at low
metallicities. To account for 7 Li evolution several additional sources have
been considered (neutrino-induced nucleosynthesis, nova outbursts, C-stars).
The model fulfills the energetic requirements for GCR acceleration.Comment: 25 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
The problem of the Pleiades distance. Constraints from Stromgren photometry of nearby field stars
The discrepancy between the Pleiades cluster distance based on Hipparcos
parallaxes and main sequence fitting is investigated on the basis of Stromgren
photometry of F-type stars. Field stars with the same metallicity as the
Pleiades have been selected from the m1 index and a technique has been
developed to locate the ZAMS of these field stars in color-magnitude diagrams
based on the color/temperature indices b-y, v-y, and beta. Fitting the Pleiades
to these ZAMS relations results in a distance modulus of 5.61+/-0.03 mag in
contrast to the Hipparcos modulus of 5.36+/-0.06 mag. Hence, we cannot confirm
the recent claim by Grenon (1999) that the distance problem is solved by
adopting a low metallicity of the Pleiades ([Fe/H]=-0.11) as determined from
Geneva photometry. The metallicity sensitivity of the ZAMS determined by the
field stars is investigated, and by combining this sensitivity in all three
color/temperature indices b-y, v-y, and beta we get a independent test of the
Pleiades distance modulus which support our value of 5.61 mag. Furthermore, the
field star sample used for the comparison is tested against theoretical
isochrones of different ages to show that evolutionary effects in the field
star sample are not biasing our distance modulus estimate significantly.
Possible explanations of the Pleiades distance problem are discussed and it is
suggested that the discrepancy in the derived moduli may be linked to a
non-spherical shape of the cluster.Comment: 11 pages, 5 figures, accepted for publication in A&
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