25,509 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
Quasi-periodic Oscillations in the X-ray Light Curves from Relativistic Tori
We use a relativistic ray-tracing code to analyze the X-ray emission from a
pressure-supported oscillating relativistic torus around a black hole. We show
that a strong correlation exists between the {\it intrinsic} frequencies of the
torus normal modes and the {\it extrinsic} frequencies seen in the observed
light curve power spectrum. This correlation demonstrates the feasibility of
the oscillating-torus model to explain the multiple peaks seen in black hole
high-frequency quasi-periodic oscillations. Using an optically thin,
monochromatic emission model, we also determine how a relativistically
broadened emission line and the amplitude of the X-ray modulations are
dependent on the observer's inclination angle and on the torus oscillation
amplitudes. Observations of these features can provide important information
about the torus as well as the black hole.Comment: 4 pages, 3 figures, submitted to ApJ
A Methodology to Engineer and Validate Dynamic Multi-level Multi-agent Based Simulations
This article proposes a methodology to model and simulate complex systems,
based on IRM4MLS, a generic agent-based meta-model able to deal with
multi-level systems. This methodology permits the engineering of dynamic
multi-level agent-based models, to represent complex systems over several
scales and domains of interest. Its goal is to simulate a phenomenon using
dynamically the lightest representation to save computer resources without loss
of information. This methodology is based on two mechanisms: (1) the activation
or deactivation of agents representing different domain parts of the same
phenomenon and (2) the aggregation or disaggregation of agents representing the
same phenomenon at different scales.Comment: Presented at 3th International Workshop on Multi-Agent Based
Simulation, Valencia, Spain, 5th June 201
Inert states of spin-S systems
We present a simple but efficient geometrical method for determining the
inert states of spin-S systems. It can be used if the system is described by a
spin vector of a spin-S particle and its energy is invariant in spin rotations
and phase changes. Our method is applicable to an arbitrary S and it is based
on the representation of a pure spin state of a spin-S particle in terms of 2S
points on the surface of a sphere. We use this method to find candidates for
some of the ground states of spinor Bose-Einstein condensates.Comment: 4 pages, 2 figures, minor changes, references added, typos correcte
Electrical transport and low-temperature scanning tunneling microscopy of microsoldered graphene
Using the recently developed technique of microsoldering, we perform a
systematic transport study of the influence of PMMA on graphene flakes
revealing a doping effect of up to 3.8x10^12 1/cm^2, but a negligible influence
on mobility and gate voltage induced hysteresis. Moreover, we show that the
microsoldered graphene is free of contamination and exhibits a very similar
intrinsic rippling as has been found for lithographically contacted flakes.
Finally, we demonstrate a current induced closing of the previously found
phonon gap appearing in scanning tunneling spectroscopy experiments, strongly
non-linear features at higher bias probably caused by vibrations of the flake
and a B-field induced double peak attributed to the 0.Landau level of graphene.Comment: 8 pages, 3 figure
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