3,381 research outputs found
Hypernovae and light dark matter as possible Galactic positron sources
The electron-positron annihilation source in the Galactic center region has
recently been observed with INTEGRAL/SPI, which shows that this 511 keV source
is strong and its extension is consistent with the Galactic bulge geometry. The
positron production rate, estimated to more than 10 per second, is very
high and raises a challenging question about the nature of the Galactic
positron source. Commonly considered astrophysical positron injectors, namely
type Ia supernovae are rare events and fall short to explain the observed
positron production rate. In this paper, we study the possibility of Galactic
positron production by hypernovae events, exemplified by the recently observed
SN2003dh/GRB030329, an asymmetric explosion of a Wolf-Rayet star associated
with a gamma-ray burst. In these kinds of events, the ejected material becomes
quickly transparent to positrons, which spread out in the interstellar medium.
Non radioactive processes, such as decays of heavy dark matter particles
(neutralinos) predicted by most extensions of the standard model of particle
physics, could also produce positrons as byproducts. However they are expected
to be accompanied by a large flux of high-energy gamma-rays, which were not
observed by EGRET and ground based Tcherenkov experiments. In this context we
explore the possibility of direct positron production by annihilation of light
dark matter particles.Comment: 8 pages, 0 figures, 35th COSPAR, accepted in July 2005 by Elsevier
Science for publication in "Advances in Space Research
Psychometric Properties of Questionnaires on Functional Health Status in Oropharyngeal Dysphagia: A Systematic Literature Review
Introduction. Questionnaires on Functional Health Status (FHS) are part of the assessment of oropharyngeal dysphagia. Objective. To conduct a systematic review of the literature on the psychometric properties of English-language FHS questionnaires in adults with oropharyngeal dysphagia. Methods. A systematic search was performed using the electronic databases Pubmed and Embase. The psychometric properties of the questionnaires were determined based on the COSMIN taxonomy of measurement properties and definitions for health-related patient-reported outcomes and the COSMIN checklist using preset psychometric criteria. Results. Three questionnaires were included: the Eating Assessment Tool (EAT-10), the Swallowing Outcome after Laryngectomy (SOAL), and the Self-report Symptom Inventory. The Sydney Swallow Questionnaire (SSQ) proved to be identical to the Modified Self-report Symptom Inventory. All FHS questionnaires obtained poor overall methodological quality scores for most measurement properties. Conclusions. The retrieved FHS questionnaires need psychometric reevaluation; if the overall methodological quality shows satisfactory improvement on most measurement properties, the use of the questionnaires in daily clinic and research can be justified. However, in case of insufficient validity and/or reliability scores, new FHS questionnaires need to be developed using and reporting on preestablished psychometric criteria as recommended in literature
Volatile inventories in clathrate hydrates formed in the primordial nebula
Examination of ambient thermodynamic conditions suggest that clathrate
hydrates could exist in the martian permafrost, on the surface and in the
interior of Titan, as well as in other icy satellites. Clathrate hydrates
probably formed in a significant fraction of planetesimals in the solar system.
Thus, these crystalline solids may have been accreted in comets, in the forming
giant planets and in their surrounding satellite systems. In this work, we use
a statistical thermodynamic model to investigate the composition of clathrate
hydrates that may have formed in the primordial nebula. In our approach, we
consider the formation sequence of the different ices occurring during the
cooling of the nebula, a reasonable idealization of the process by which
volatiles are trapped in planetesimals. We then determine the fractional
occupancies of guests in each clathrate hydrate formed at given temperature.
The major ingredient of our model is the description of the guest-clathrate
hydrate interaction by a spherically averaged Kihara potential with a nominal
set of parameters, most of which being fitted on experimental equilibrium data.
Our model allows us to find that Kr, Ar and N can be efficiently encaged in
clathrate hydrates formed at temperatures higher than 48.5 K in the
primitive nebula, instead of forming pure condensates below 30 K. However, we
find at the same time that the determination of the relative abundances of
guest species incorporated in these clathrate hydrates strongly depends on the
choice of the parameters of the Kihara potential and also on the adopted size
of cages. Indeed, testing different potential parameters, we have noted that
even minor dispersions between the different existing sets can lead to
non-negligible variations in the determination of the volatiles trapped in
clathrate hydrates formed in the primordial nebula.Comment: Accepted for publication in Faraday Discussion
Integral and Light Dark Matter
The nature of Dark Matter remains one of the outstanding questions of modern
astrophysics. The success of the Cold Dark Matter cosmological model argues
strongly in favor of a major component of the dark matter being in the form of
elementary particles, not yet discovered. Based on earlier theoretical
considerations, a possible link between the recent SPI/INTEGRAL measurement of
an intense and extended emission of 511 keV photons (positron annihilation)
from the central Galaxy, and this mysterious component of the Universe, has
been established advocating the existence of a light dark matter particle at
variance with the neutralino, in general considered as very heavy. We show that
it can explain the 511 keV emission mapped with SPI/INTEGRAL without
overproducing undesirable signals like high energy gamma-rays arising from
decays, and radio synchrotron photons emitted by high energy
positrons circulating in magnetic fields. Combining the annihilation line
constraint with the cosmological one (i.e. that the relic LDM energy density
reaches about 23% of the density of the Universe), one can restrict the main
properties of the light dark matter particle. Its mass should lie between 1 and
100 MeV, and the required annihilation cross section, velocity dependent,
should be significantly larger than for weak interactions, and may be induced
by the virtual production of a new light neutral spin 1 boson . On
astrophysical grounds, the best target to validate the LDM proposal seems to be
the observation by SPI/INTEGRAL and future gamma ray telescopes of the
annihilation line from the Sagittarius dwarf galaxy and the Palomar-13 globular
cluster, thought to be dominated by dark matter.Comment: 7 pages, 0 figures. To appear in the Proceedings of the 5th INTEGRAL
Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich, German
An estimate of the chemical composition of Titan's lakes
Hundreds of radar-dark patches interpreted as lakes have been discovered in
the north and south polar regions of Titan. We have estimated the composition
of these lakes by using the direct abundance measurements from the Gas
Chromatograph Mass Spectrometer (GCMS) aboard the Huygens probe and recent
photochemical models based on the vertical temperature profile derived by the
Huygens Atmospheric Structure Instrument (HASI). Thermodynamic equilibrium is
assumed between the atmosphere and the lakes, which are also considered as
nonideal solutions. We find that the main constituents of the lakes are ethane
(C2H6) (~76-79%), propane (C3H8) (~7-8%), methane (CH4) (~5-10%), hydrogen
cyanide (HCN) (~2-3%), butene (C4H8) (~1%), butane (C4H10) (~1%) and acetylene
(C2H2) (~1%). The calculated composition of lakes is then substantially
different from what has been expected from models elaborated prior to the
exploration of Titan by the Cassini-Huygens spacecraft.Comment: 5 pages, 2 figures, accepted in ApJ
Structure prediction based on ab initio simulated annealing for boron nitride
Possible crystalline modifications of chemical compounds at low temperatures
correspond to local minima of the energy landscape. Determining these minima
via simulated annealing is one method for the prediction of crystal structures,
where the number of atoms per unit cell is the only information used. It is
demonstrated that this method can be applied to covalent systems, at the
example of boron nitride, using ab initio energies in all stages of the
optimization, i.e. both during the global search and the subsequent local
optimization. Ten low lying structure candidates are presented, including both
layered structures and 3d-network structures such as the wurtzite and zinc
blende types, as well as a structure corresponding to the beta-BeO type
SVOM pointing strategy: how to optimize the redshift measurements?
The Sino-French SVOM mission (Space-based multi-band astronomical Variable
Objects Monitor) has been designed to detect all known types of gamma-ray
bursts (GRBs) and to provide fast and reliable GRB positions. In this study we
present the SVOM pointing strategy which should ensure the largest number of
localized bursts allowing a redshift measurement. The redshift measurement can
only be performed by large telescopes located on Earth. The best scientific
return will be achieved if we are able to combine constraints from both space
segment (platform and payload) and ground telescopes (visibility).Comment: Proceedings of Gamma-Ray Bursts 2007 conference, Santa Fe, USA, 5-9
November 2007. Published in AIP conf. proc. 1000, 585-588 (2008
A continuous non-linear shadowing model of columnar growth
We propose the first continuous model with long range screening (shadowing)
that described columnar growth in one space dimension, as observed in plasma
sputter deposition. It is based on a new continuous partial derivative equation
with non-linear diffusion and where the shadowing effects apply on all the
different processes.Comment: Fast Track Communicatio
A-STAR: The All-Sky Transient Astrophysics Reporter
The small mission A-STAR (All-Sky Transient Astrophysics Reporter) aims to
locate the X-ray counterparts to ALIGO and other gravitational wave detector
sources, to study the poorly-understood low luminosity gamma-ray bursts, and to
find a wide variety of transient high-energy source types, A-STAR will survey
the entire available sky twice per 24 hours. The payload consists of a coded
mask instrument, Owl, operating in the novel low energy band 4-150 keV, and a
sensitive wide-field focussing soft X-ray instrument, Lobster, working over
0.15-5 keV. A-STAR will trigger on ~100 GRBs/yr, rapidly distributing their
locations.Comment: Accepted for the European Astronomical Society Publications Series:
Proceedings of the Fall 2012 Gamma-Ray Burst Symposium held in Marbella,
Spain, 8-12 Oct 201
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