353 research outputs found
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
SD electronics: simulations on the dynamic range
The surface detector electronics of the Pierre Auger Observatory is characterized by a large dynamic range due to the variation of the signal intensity of the Cherenkov tanks as a function of the distance from the core. In this paper, we present results of simulations and discuss the impact of the dynamic range on the shower reconstruction
Design of the photomultiplier bases for the surface detectors of the Pierre Auger Observatory
The design of the photomultiplier bases for the surface detectors of the Pierre Auger Observatory is presented. The bleeder is purely resistive. The base comprises two outputs: one from the anode and another one from the last dynode followed by an amplifier. The charge ratio between the anode and the amplified dynode is around 30. The design ensures a low consumption (less than 100 mu A at 2 kV), a stability of the gain and of the base line during the whole period of measurement (20 mu s per event) and for the whole dynamic range (max. 1 to 3x10^4 in amplitude). First measurement with a prototype base on the Hamamatsu R5912 photomultiplier tube are presented
Heart of endosymbioses : transcriptomics reveals a conserved genetic program among arbuscular mycorrhizal, actinorhizal and legume-rhizobial symbioses
To improve their nutrition, most plants associate with soil microorganisms, particularly fungi, to form mycorrhizae. A few lineages, including actinorhizal plants and legumes are also able to interact with nitrogen-fixing bacteria hosted intracellularly inside root nodules. Fossil and molecular data suggest that the molecular mechanisms involved in these root nodule symbioses (RNS) have been partially recycled from more ancient and widespread arbuscular mycorrhizal (AM) symbiosis. We used a comparative transcriptomics approach to identify genes involved in establishing these 3 endosymbioses and their functioning. We analysed global changes in gene expression in AM in the actinorhizal tree C. glauca. A comparison with genes induced in AM in Medicago truncatula and Oryza sativa revealed a common set of genes induced in AM. A comparison with genes induced in nitrogen-fixing nodules of C. glauca and M. truncatula also made it possible to define a common set of genes induced in these three endosymbioses. The existence of this core set of genes is in accordance with the proposed recycling of ancient AM genes for new functions related to nodulation in legumes and actinorhizal plants
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
Transport of Cosmic Rays in Chaotic Magnetic Fields
The transport of charged particles in disorganised magnetic fields is an
important issue which concerns the propagation of cosmic rays of all energies
in a variety of astrophysical environments, such as the interplanetary,
interstellar and even extra-galactic media, as well as the efficiency of Fermi
acceleration processes. We have performed detailed numerical experiments using
Monte-Carlo simulations of particle propagation in stochastic magnetic fields
in order to measure the parallel and transverse spatial diffusion coefficients
and the pitch angle scattering time as a function of rigidity and strength of
the turbulent magnetic component. We confirm the extrapolation to high
turbulence levels of the scaling predicted by the quasi-linear approximation
for the scattering frequency and parallel diffusion coefficient at low
rigidity. We show that the widely used Bohm diffusion coefficient does not
provide a satisfactory approximation to diffusion even in the extreme case
where the mean field vanishes. We find that diffusion also takes place for
particles with Larmor radii larger than the coherence length of the turbulence.
We argue that transverse diffusion is much more effective than predicted by the
quasi-linear approximation, and appears compatible with chaotic magnetic
diffusion of the field lines. We provide numerical estimates of the Kolmogorov
length and magnetic line diffusion coefficient as a function of the level of
turbulence. Finally we comment on applications of our results to astrophysical
turbulence and the acceleration of high energy cosmic rays in supernovae
remnants, in super-bubbles, and in jets and hot spots of powerful
radio-galaxies.Comment: To be published in Physical Review D, 20 pages 9 figure
Light Element Evolution and Cosmic Ray Energetics
Using cosmic-ray energetics as a discriminator, we investigate evolutionary
models of LiBeB. We employ a Monte Carlo code which incorporates the delayed
mixing into the ISM both of the synthesized Fe, due to its incorporation into
high velocity dust grains, and of the cosmic-ray produced LiBeB, due to the
transport of the cosmic rays. We normalize the LiBeB production to the integral
energy imparted to cosmic rays per supernova. Models in which the cosmic rays
are accelerated mainly out of the average ISM significantly under predict the
measured Be abundance of the early Galaxy, the increase in [O/Fe] with
decreasing [Fe/H] notwithstanding. We suggest that this increase could be due
to the delayed mixing of the Fe. But, if the cosmic-ray metals are accelerated
out of supernova ejecta enriched superbubbles, the measured Be abundances are
consistent with a cosmic-ray acceleration efficiency that is in very good
agreement with the current epoch data. We also find that neither the above
cosmic-ray origin models nor a model employing low energy cosmic rays
originating from the supernovae of only very massive progenitors can account
for the Li data at values of [Fe/H] below 2.Comment: latex 19 pages, 2 tables, 10 eps figures, uses aastex.cls natbib.sty
Submitted to the Astrophysical Journa
Testing Spallation Processes With Beryllium and Boron
The nucleosynthesis of Be and B by spallation processes provides unique
insight into the origin of cosmic rays. Namely, different spallation schemes
predict sharply different trends for the growth of LiBeB abundances with
respect to oxygen. ``Primary'' mechanisms predict BeB O, and are well
motivated by the data if O/Fe is constant at low metallicity. In contrast,
``secondary'' mechanisms predict BeB O and are consistent with
the data if O/Fe increases towards low metallicity as some recent data suggest.
Clearly, any primary mechanism, if operative, will dominate early in the
history of the Galaxy. In this paper, we fit the BeB data to a two-component
scheme which includes both primary and secondary trends. In this way, the data
can be used to probe the period in which primary mechanisms are effective. We
analyze the data using consistent stellar atmospheric parameters based on
Balmer line data and the continuum infrared flux. Results depend sensitively on
Pop II O abundances and, unfortunately, on the choice of stellar parameters.
When using recent results which show O/Fe increasing toward lower metallicity,
a two-component Be-O fits indicates that primary and secondary components
contribute equally at [O/H] = -1.8 for Balmer line data; and
[O/H] = -1.4 to -1.8 for IRFM. We apply these constraints to recent
models for LiBeB origin. The Balmer line data does not show any evidence for
primary production. On the other hand, the IRFM data does indicate a preference
for a two-component model, such as a combination of standard GCR and
metal-enriched particles accelerated in superbubbles. These conclusions rely on
a detailed understanding of the abundance data including systematic effects
which may alter the derived O-Fe and BeB-Fe relations.Comment: 40 pages including 11 ps figures. Written in AASTe
An Ultra-High-Resolution Survey of the Interstellar ^7Li-to-^6Li Isotope Ratio in the Solar Neighborhood
In an effort to probe the extent of variations in the interstellar ^7Li/^6Li
ratio seen previously, ultra-high-resolution (R ~ 360,000), high
signal-to-noise spectra of stars in the Perseus OB2 and Scorpius OB2
Associations were obtained. These measurements confirm our earlier findings of
an interstellar ^7Li/^6Li ratio of about 2 toward o Per, the value predicted
from models of Galactic cosmic ray spallation reactions. Observations of other
nearby stars yield limits consistent with the isotopic ratio ~ 12 seen in
carbonaceous chondrite meteorites. If this ratio originally represented the gas
toward o Per, then to decrease the original isotope ratio to its current value
an order of magnitude increase in the Li abundance is expected, but is not
seen. The elemental K/Li ratio is not unusual, although Li and K are formed via
different nucleosynthetic pathways. Several proposals to account for the low
^7Li/^6Li ratio were considered, but none seems satisfactory.
Analysis of the Li and K abundances from our survey highlighted two sight
lines where depletion effects are prevalent. There is evidence for enhanced
depletion toward X Per, since both abundances are lower by a factor of 4 when
compared to other sight lines. Moreover, a smaller Li/H abundance is observed
toward 20 Aql, but the K/H abundance is normal, suggesting enhanced Li
depletion (relative to K) in this direction. Our results suggest that the
^7Li/^6Li ratio has not changed significantly during the last 4.5 billion years
and that a ratio ~ 12 represents most gas in the solar neighborhood. In
addition, there appears to be a constant stellar contribution of ^7Li,
indicating that one or two processes dominate its production in the Galaxy.Comment: 54 pages, accepted for publication in the Astrophysical Journa
Production of Lithium, Beryllium, and Boron by Hypernovae
We investigate a possible nucleosynthetic signature of highly energetic
explosions of C-O cores ("hypernovae," HNe) which might be associated with
gamma-ray bursts (GRBs). We note that the direct impact of C- and O-enriched
hypernova ejecta on the ambient hydrogen and helium leads to spallation
reactions which can produce large amounts of the light nuclides lithium,
beryllium, and boron (LiBeB). Using analytic velocity spectra of the hypernova
ejecta, we calculate the LiBeB yields of different exploding C-O cores
associated with observed hypernovae. The deduced yields are times
higher than those produced by similar (direct) means in normal Type II
supernovae, and are higher than the commonly used ones arising from shock wave
acceleration induced by Type II supernova (SN) explosions. To avoid
overproduction of these elements in our Galaxy, hypernovae should be rare
events, with \la 10^{-3} hypernovae per supernova, assuming a constant HN/SN
ratio over time. This rate is in good agreement with that of long duration GRBs
if we assume that the gamma-ray emission is focussed with a beaming factor
\Omega/4\pi \la 10^{-2}. This encouraging result supports the possible HN-GRB
association. Thus, Galactic LiBeB abundance measurements offer a promising way
to probe the HN rate history and the possible HN-GRB correlation. On the other
hand, if hypernovae are associated to very massive pregalactic stars
(Population III) they would produce a LiBeB pre-enrichment in proto-galactic
gas, which could show up as a plateau in the lowest metallicities of the Be-Fe
relation in halo stars.Comment: 12 pages, AASTeX, no figure
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