453 research outputs found
Pregalactic LiBeB Production by Supernova Cosmic Rays
I calculate the evolution of Be and B abundances produced by cosmic rays
generated by massive stars in the pregalactic phase of the universe. The inputs
for calculation, i.e. the star formation rate and the nuclear abundances of
cosmic rays, which I assume to be the same as those of the ISM, are taken from
the results of a detailed cosmic chemical evolution model with its parameters
best fitted from several items of observational information including an early
reionization of the IGM by . I found that when the Li plateau
abundance observed in metal-poor halo stars originated in the pregalactic
cosmological cosmic ray nucleosynthesis, Be and B simultaneously produced with
Li amount to the lowest levels ever detected in metal-poor halo stars. It
is desirable to observe Be and B abundances in metal-poor halo stars with
[Fe/H] in order to elucidate the possibility of early LiBeB
production by pregalactic supernova cosmic ray nucleosynthesis.Comment: 13 pages, 8 figures, ApJ accepte
The metallicity dependence of the Cepheid PL-relation
A sample of 37 Galactic, 10 LMC and 6 SMC cepheids is compiled for which
individual metallicity estimates exist and BVIK photometry in almost all cases.
The Galactic cepheids all have an individual distance estimate available. For
the MC objects different sources of photometry are combined to obtain improved
periods and mean magnitudes. A multi-parameter Period-Luminosity relation is
fitted to the data which also solves for the distance to the LMC and SMC. When
all three galaxies are considered, without metallicity effect, a significant
quadratic term in log P is found, as previously observed and also predicted in
some theoretical calculations. For the present sample it is empirically
determined that for log P < 1.65 linear PL-relations may be adopted, but this
restricts the sample to only 4 LMC and 1 SMC cepheid. Considering the Galactic
sample a metallicity effect is found in the zero point in the VIWK PL-relation
(-0.6 \pm 0.4 or -0.8 \pm 0.3 mag/dex depending on the in- or exclusion of one
object), in the sense that metal-rich cepheids are brighter. The small
significance is mostly due to the fact that the Galactic sample spans a narrow
metallicity range. The error is to a significant part due to the error in the
metallicity determinations and not to the error in the fit. Including the 5 MC
cepheids broadens the observed metallicity range and a metallity effect of
about -0.27 \pm 0.08 mag/dex in the zero point is found in VIWK, in agreement
with some previous empirical estimates, but now derived using direct
metallicity determinations for the cepheids themselves.Comment: Accepted Astronomy and Astrophysics, February 20, 200
Cosmic Ray production of Beryllium and Boron at high redshift
Recently, new observations of Li6 in Pop II stars of the galactic halo have
shown a surprisingly high abundance of this isotope, about a thousand times
higher than its predicted primordial value. In previous papers, a cosmological
model for the cosmic ray-induced production of this isotope in the IGM has been
developed to explain the observed abundance at low metallicity. In this paper,
given this constraint on the Li6, we calculate the non-thermal evolution with
redshift of D, Be, and B in the IGM. In addition to cosmological cosmic ray
interactions in the IGM, we include additional processes driven by SN
explosions: neutrino spallation and a low energy component in the structures
ejected by outflows to the IGM. We take into account CNO CRs impinging on the
intergalactic gas. Although subdominant in the galactic disk, this process is
shown to produce the bulk of Be and B in the IGM, due to the differential metal
enrichment between structures (where CRs originate) and the IGM. We also
consider the resulting extragalactic gamma-ray background which we find to be
well below existing data. The computation is performed in the framework of
hierarchical structure formation considering several star formation histories
including Pop III stars. We find that D production is negligible and that a
potentially detectable Be and B plateau is produced by these processes at the
time of the formation of the Galaxy (z ~ 3).Comment: 9 pages, 7 figure
Beryllium in Ultra-Lithium-Deficient Halo Stars - The Blue Straggler Connection
There are nine metal-deficient stars that have Li abundances well below the
Li plateau that is defined by over 100 unevolved stars with temperatures above
5800 K and values of [Fe/H] 1.0. Abundances of Be have been determined
for most of these ultra-Li-deficient stars in order to investigate the cause of
the Li deficiencies. High-resolution and high signal-to-noise spectra have been
obtained in the Be II spectral region near 3130 \AA for six ultra-Li-deficient
stars with the Keck I telescope and its new uv-sensitive CCD on the upgraded
HIRES. The spectrum synthesis technique has been used to determine Be
abundances. All six stars are found to have Be deficiencies also. Two have
measurable - but reduced - Be and four have only upper limits on Be. These
results are consistent with the idea that these Li- and Be-deficient stars are
analogous to blue stragglers. The stars have undergone mass transfer events (or
mergers) which destroy or dilute both Li and Be. The findings cannot be matched
by the models that predict that the deficiencies are due to extra-mixing in a
subset of halo stars that were initially rapid rotators, with the possible
exception of one star, G 139-8. Because the ultra-Li-deficient stars are also
Be-deficient, they appear to be genuine outliers in population of halo stars
used to determine the value of primordial Li; they no longer have the Li in
their atmospheres that was produced in the Big Bang.Comment: 17 pages of text, 12 figures, 3 tables Submitted to Ap
Observational signatures of lithium depletion in the metal-poor globular cluster NGC6397
The "stellar" solution to the cosmological lithium problem proposes that
surface depletion of lithium in low-mass, metal-poor stars can reconcile the
lower abundances found for Galactic halo stars with the primordial prediction.
Globular clusters are ideal environments for studies of the surface evolution
of lithium, with large number statistics possible to obtain for main sequence
stars as well as giants. We discuss the Li abundances measured for >450 stars
in the globular cluster NGC6397, focusing on the evidence for lithium depletion
and especially highlighting how the inferred abundances and interpretations are
affected by early cluster self-enrichment and systematic uncertainties in the
effective temperature determination.Comment: 6 pages, 2 figures, conference proceedings for IAU symposium 26
New results on catalyzed BBN with a long-lived negatively-charged massive particle
It has been proposed that the apparent discrepancies between the inferred
primordial abundances of 6Li and 7Li and the predictions of big bang
nucleosynthesis (BBN) can be resolved by the existence of a negatively-charged
massive unstable supersymmetric particle (X-) during the BBN epoch. Here, we
present new BBN calculations with an X- particle utilizing an improved nuclear
reaction network including captures of nuclei by the particle, nuclear
reactions and beta-decays of normal nuclei and nuclei bound to the X- particles
(X-nuclei), and new reaction rates derived from recent rigorous quantum
many-body dynamical calculations. We find that this is still a viable model to
explain the observed 6Li and 7Li abundances. However, contrary to previous
results, neutral X-nuclei cannot significantly affect the BBN light-element
abundances. We also show that with the new rates the production of heavier
nuclei is suppressed and there is no signature on abundances of nuclei heavier
than Be in the X--particle catalyzed BBN model as has been previously proposed.
We also consider the version of this model whereby the X- particle decays into
the present cold dark matter. We analyze the this paradigm in light of the
recent constraints on the dark-matter mass deduced from the possible detected
events in the CDMS-II experiment. We conclude that based upon the inferred
range for the dark-matter mass, only X- decay via the weak interaction can
achieve the desired 7Li destruction while also reproducing the observed 6Li
abundance.Comment: 6 pages, 2 figure
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&
Chemical abundances in LMC stellar populations. II. The bar sample
This paper compares the chemical evolution of the Large Magellanic Cloud
(LMC) to that of the Milky Way (MW) and investigates the relation between the
bar and the inner disc of the LMC in the context of the formation of the bar.
We obtained high-resolution and mid signal-to-noise ratio spectra with
FLAMES/GIRAFFE at ESO/VLT and performed a detailed chemical analysis of 106 and
58 LMC field red giant stars (mostly older than 1 Gyr), located in the bar and
the disc of the LMC respectively. We measured elemental abundances for O, Mg,
Si, Ca, Ti, Na, Sc, V, Cr, Co, Ni, Cu, Y, Zr, Ba, La and Eu. We find that the
{\alpha}-element ratios [Mg/Fe] and [O/Fe] are lower in the LMC than in the MW
while the LMC has similar [Si/Fe], [Ca/Fe], and [Ti/Fe] to the MW. As for the
heavy elements, [Ba,La/Eu] exhibit a strong increase with increasing
metallicity starting from [Fe/H]=-0.8 dex, and the LMC has lower [Y+Zr/Ba+La]
ratios than the MW. Cu is almost constant over all metallicities and about 0.5
dex lower in the LMC than in the MW. The LMC bar and inner disc exhibit
differences in their [{\alpha}/Fe] (slightly larger scatter for the bar in the
metallicity range [-1,-0.5]), their Eu (the bar trend is above the disc trend
for [Fe/H] > -0.5 dex), their Y and Zr, their Na and their V (offset between
bar and disc distributions). Our results show that the chemical history of the
LMC experienced a strong contribution from type Ia supernovae as well as a
strong s-process enrichment from metal-poor AGB winds. Massive stars made a
smaller contribution to the chemical enrichment compared to the MW. The
observed differences between the bar and the disc speak in favour of an episode
of enhanced star formation a few Gyr ago, occurring in the central parts of the
LMC and leading to the formation of the bar. This is in agreement with recently
derived star formation histories.Comment: 22 pages, 20 figures; Accepted for publication in A&
Light Element Production in the Circumstellar Matter of Energetic Type Ic Supernovae
We investigate energetic type Ic supernovae as production sites for Li6 and
Be in the early stages of the Milky Way. Recent observations have revealed that
some very metal-poor stars with [Fe/H]<-2.5 possess unexpectedly high
abundances of Li6. Some also exbihit enhanced abundances of Be as well as N.
From a theoretical point of view, recent studies of the evolution of
metal-poor massive stars show that rotation-induced mixing can enrich the outer
H and He layers with C, N, and O (CNO) elements, particularly N, and at the
same time cause intense mass loss of these layers. Here we consider energetic
supernova explosions occurring after the progeniter star has lost all but a
small fraction of the He layer. The fastest portion of the supernova ejecta can
interact directly with the circumstellar matter (CSM), both composed of He and
CNO, and induce light element production through spallation and He-He fusion
reactions. The CSM should be sufficiently thick to energetic particles so that
the interactions terminate within its innermost regions. We calculate the
resulting Li6/O and Be9/O ratios in the ejecta+CSM material out of which the
very metal-poor stars may form. We find that they are consistent with the
observed values if the mass of the He layer remaining on the pre-explosion core
is 0.01-0.1 solar mass, and the mass fraction of N mixed in the He layer is
about 0.01. Further observations of Li6, Be and N at low metallicity should
provide critical tests of this production scenario.Comment: 12 pages, 2 figures, revised with referee suggestions, final version
accepted in ApJ Letter
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