52 research outputs found
LiBeB Production by Nuclei and Neutrinos
The production of LiBeB isotopes by nuclear and neutrino spallation are
compared in the framework of galactic evolutionary models. As motivated by
-ray observations of Orion, different possible sources of low-energy C
and O nuclei are considered, such as supernovae of various masses and WC stars.
We confirm that the low energy nuclei (LEN), injected in molecular clouds by
stellar winds and type II supernovae originating from the most massive
progenitors, can very naturally reproduce the observed Be and B evolution in
the early galaxy (halo phase). Assuming the global importance of the LEN
component, we compute upper and lower bounds to the neutrino process
contribution corresponding to limiting cases of LEN particle spectra. A
consistent solution is found with a spectrum of the kind proposed by Ramaty
\etal (1995a,b), e.g. flat up to MeV/n and decreasing abruptly above.
This solution fulfills the challenge of explaining at the same time the general
Be and B evolution, and their solar system abundances without overproducing
\li7 at very low metallicities, and the meteoritic \b11/\b10 ratio. In this
case, neutrino spallation is constrained to play a limited role in the genesis
of the solar system \b11. Galactic cosmic rays (GCR) become operative late in
the evolution of the disk ([Fe/H]-1), but their contribution to the solar
abundances of \be9, \b10 and \b11 is not dominant (35\%, 30\% and 20\%
respectively). Thus, with this LEN spectrum, GCR are {\it not}\ the main source
of \be9 and B in the Galaxy. The most favorable case for neutrinos, (adopting
the same kind of spectrum) has MeV/n. Even in this case, the neutrino
yields of Woosley and Weaver (1995) must to be reduced by a factor of 5 to
avoid \b11 overproduction. Furthermore, this solution leads to a high B/BeComment: 19 pages, 5 postscript figures, uses plain LaTeX, also available at
http://www.nd.edu/~bfields/vcfo.htm
Cosmologie
Chaque homme, sur cette Terre, converti au réductionnisme qui lui a martelé les oreilles, ahane que les objets sont faits de particules élémentaires microscopiques, dont la configuration d’ensemble constitue l’univers. Cette vision est incorrecte, car elle fait l’économie de toute magie quantique. Les particules peuvent être en superposition interférentielle, alors que les objets sensibles ne peuvent l’être. La connexion entre objets macroscopiques définis en fonction de notre expérience et les objets microscopiques définis dans le cadre de la théorie physique est plus subtile et plus conforme à la notion de reliance. « Quand je parle de complexité, je me réfère au sens latin élémentaire du mot complexus, “ce qui est tissé ensemble”. Les constituants sont différents, mais il faut voir comme dans une tapisserie la figure d’ensemble » (Morin, 1995)
Constraints on the injection energy of positrons in the Galactic centre region
Recent observations of the 511 keV positron-electron annihilation line in the
Galactic centre region by the INTEGRAL/SPI spectrometer have stirred up new
speculations about the origin of the large corresponding positron injection
rate. Beyond astrophysical candidates, new mechanisms have been put forward. We
focus on the annihilation of light dark matter particles and review the various
gamma-ray radiation components related to such a source of mono-energetic
positrons in addition to the 511 keV line itself. We study the influence of the
degree of ionisation of the bulge on this radiation, and its possible effects
on the observational constraints on the mass of the hypothetical light dark
matter particle or the injection energy of a mono-energetic source of positrons
in general.Comment: 4 pages, 7 figures, 1 table. Accepted for publication in the
proceedings of the 6th INTEGRAL Workshop on the Obscured Universe (ESA
SP-622). 2-8 July 2006, Moscow, Russi
Evolution of Lithium-Beryllium-Boron and Oxygen in the early Galaxy
Oxygen is a much better evolutionary index than iron to describe the history
of Lithium-Beryllium-Boron (LiBeB) since it is the main producer of these light
elements at least in the early Galaxy. The O-Fe relation is crucial to the
determination of the exact physical process responsible for the LiBeB
production. At low metallicity, if [O/Fe] vs [Fe/H] is flat, then the
production mode is independent of the interstellar metallicity, BeB is
proportional to oxygen, i.e. is of primary nature. If not, the production mode
is function of the progressive enrichment in O of the interstellar medium, BeB
varies rather as the square of O, i.e. is of secondary nature. In the first
case, fast nuclei enriched into He, C and O injected by supernovae and
accelerated in surrounding superbubbles would explain the primary trend.
In the second case, the main spallative agent would be the standard galactic
cosmic rays. Calculated nucleosynthetic yields of massive stars, estimates of
the energy cost of production of beryllium nuclei, and above all recent
observations reported in this meeting seem to favor the primary mechanism, at
least in the early Galaxy.Comment: invited review, IAU Symposium, JD8, Manchester, August 2000, New
Astronomy Review, in press 6 pages 1 figur
Contact resistances in trigate and FinFET devices in a Non-Equilibrium Green's Functions approach
We compute the contact resistances in trigate and FinFET devices
with widths and heights in the 4 to 24 nm range using a Non-Equilibrium Green's
Functions approach. Electron-phonon, surface roughness and Coulomb scattering
are taken into account. We show that represents a significant part
of the total resistance of devices with sub-30 nm gate lengths. The analysis of
the quasi-Fermi level profile reveals that the spacers between the heavily
doped source/drain and the gate are major contributors to the contact
resistance. The conductance is indeed limited by the poor electrostatic control
over the carrier density under the spacers. We then disentangle the ballistic
and diffusive components of , and analyze the impact of different
design parameters (cross section and doping profile in the contacts) on the
electrical performances of the devices. The contact resistance and variability
rapidly increase when the cross sectional area of the channel goes below
nm. We also highlight the role of the charges trapped at the
interface between silicon and the spacer material.Comment: 16 pages, 15 figure
Nucleosynthesis and Gamma Ray-Line Astronomy
The most energetic part of the electromagnetic spectrum bears the purest
clues to the synthesis of atomic nuclei in the universe. The decay of
radioactive species, synthesized in stellar environments and ejected into the
interstellar medium, gives rise to specific gamma ray lines. The observations
gathered up to now show evidence for radioactivities throughout the galactic
disk, in young supernova remnants (Cas A, Vela), and in nearby extragalactic
supernovae (SN 1987A, SN 1991T and SN1998bu), in the form of specific gamma ray
lines resulting, respectively, from the radioactive decay of 26Al, 44Ti and
56Co. The various astrophysical sites of thermal nucleosynthesis of the
radioactive nuclei were discussed: AGB and Wolf-Rayet stars, novae, and type Ia
and type II supernovae. Nuclear excitations by fast particles also produce
gamma ray lines which have been observed in great detail from solar flares, and
more hypothetically from active star forming regions where massive supernovae
and WR stars abound. This non thermal process and its nucleosynthetic
consequences was reviewed. The 511 keV line arising from e+ + e- annihilation
also provides important information on explosive nucleosynthesis, as well as on
the nature of the interstellar medium where the positrons annihilate. INTEGRAL,
the main mission devoted to high resolution nuclear spectroscopy, should lead
to important progress in this field.Comment: 4 page
Lithium-Beryllium-Boron : Origin and Evolution
The origin and evolution of Lithium-Beryllium-Boron is a crossing point
between different astrophysical fields : optical and gamma spectroscopy, non
thermal nucleosynthesis, Big Bang and stellar nucleosynthesis and finally
galactic evolution. We describe the production and the evolution of
Lithium-Beryllium-Boron from Big Bang up to now through the interaction of the
Standard Galactic Cosmic Rays with the interstellar medium, supernova neutrino
spallation and a low energy component related to supernova explosions in
galactic superbubbles.Comment: 28 pages, 7 figures, to be published in a special memorial volume of
Physics Reports in honor of David Schram
Soft gamma-ray background and light Dark Matter annihilation
The bulk of the extragalactic background between 10 keV and 10 GeV is likely
to be explained by the emission of Seyfert galaxies, type Ia supernovae, and
blazars. However, as revealed by the INTEGRAL satellite, the bulge of our
galaxy is an intense source of a 511 keV gamma-ray line, indicating the
production of a large number of positrons that annihilate. The origin of the
latter is debated, and they could be produced, in particular, by the (S- or
P-wave) annihilation of light Dark Matter particles into e+e-. In any case, the
cumulated effect of similar sources at all redshifts could lead to a new
background of hard X-ray and soft gamma-ray photons. On the basis of the
hierarchical model of galaxy formation, we compute analytically the SNIa
contribution to the background, and add it to Seyfert and blazars emission
models. We find that any extra contribution to this unresolved background at
511 keV should be lower than about 4 keV/cm^2/s/sr. We also estimate
analytically the extragalactic background due to Dark Matter annihilation,
increasing the accuracy of the earlier computations. Indeed, we take into
account the large positron escape fraction from low mass dark matter halos,
unable to confine a dense and magnetized interstellar medium. Our new
background estimate turns out to be one order of magnitude lower, so that the
hypothesis of a light Dark Matter candidate remains compatible with the
observed extragalactic background for a wider range of particle masses and
cross-sections.Comment: 10 pages, 8 figures, 1 table, accepted for publication in Physical
Review D, improved with 4 new figures, 1 new table and 1 new par
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