644 research outputs found
Pre-Existing Superbubbles as the Sites of Gamma-Ray Bursts
According to recent models, gamma-ray bursts apparently explode in a wide
variety of ambient densities ranging from ~ 10^{-3} to 30 cm^{-3}. The lowest
density environments seem, at first sight, to be incompatible with bursts in or
near molecular clouds or with dense stellar winds and hence with the
association of gamma-ray bursts with massive stars. We argue that low ambient
density regions naturally exist in areas of active star formation as the
interiors of superbubbles. The evolution of the interior bubble density as a
function of time for different assumptions about the evaporative or
hydrodynamical mass loading of the bubble interior is discussed. We present a
number of reasons why there should exist a large range of inferred afterglow
ambient densities whether gamma-ray bursts arise in massive stars or some
version of compact star coalescence. We predict that many gamma-ray bursts will
be identified with X-ray bright regions of galaxies, corresponding to
superbubbles, rather than with blue localized regions of star formation.
Massive star progenitors are expected to have their own circumstellar winds.
The lack of evidence for individual stellar winds associated with the
progenitor stars for the cases with afterglows in especially low density
environments may imply low wind densities and hence low mass loss rates
combined with high velocities. If gamma-ray bursts are associated with massive
stars, this combination might be expected for compact progenitors with
atmospheres dominated by carbon, oxygen or heavier elements, that is,
progenitors resembling Type Ic supernovae.Comment: 14 pages, no figures, submitted to The Astrophysical Journa
Sources of UHECRs in view of the TUS and JEM-EUSO experiments
The origin of ultra-high-energy cosmic rays (UHECRs) is one of the most
intriguing problems of modern cosmic ray physics. We briefly review the main
astrophysical models of their origin and the forthcoming orbital experiments
TUS and JEM-EUSO, and discuss how the new data can help one solve the
long-standing puzzle.Comment: 4 pages; prepared for ECRS-2012 (http://ecrs2012.sinp.msu.ru/); v2: a
reference adde
Motivations and barriers to prosthesis users participation in physical activity, exercise and sport : a review of the literature
The UK will host the Paralympics in 2012 and the Commonwealth Games in 2014 showcasing the talents of elite athletes and aiming to inspire the population to become involved. However, low levels of physical activity (PA) are prevalent: only 40% of men and 28% of women meet the minimum UK recommendations. The limb absent population is no exception. To determine if people with limb amputations are participating in physical activity and sport; whether post-amputation activity levels match pre-amputation levels; and if there are motivations and barriers to participation. Study design: Literature review Five reviewers systematically search of peer reviewed and gray literature in seven bibliographic databases and the Cochrane Library. Results: Following rigorous elimination, 12 articles were finally included in the review and critically appraised. Four themes were identified: components, rehabilitation outcomes, body image and motivations and barriers to participation. People with limb absence are not participating in PA conducive to health benefits, and only a minority participate in exercise and sports. Participation following amputation does not mirror that of pre-amputation levels, and more barriers than motivations exist to adopting and maintaining a physically active lifestyle. This literature review aims to inform those involved in rehabilitation and ongoing care of those with limb absence about what motivates or precludes their participation in physical activity, exercise and sport. Such knowledge could be applied to improving health and well being in this population
Absence of correlation between built-in electric dipole moment and quantum Stark effect in InAs/GaAs self-assembled quantum dots
We report significant deviations from the usual quadratic dependence of the
ground state interband transition energy on applied electric fields in
InAs/GaAs self-assembled quantum dots. In particular, we show that conventional
second-order perturbation theory fails to correctly describe the Stark shift
for electric field below kV/cm in high dots. Eight-band calculations demonstrate this effect is predominantly due to
the three-dimensional strain field distribution which for various dot shapes
and stoichiometric compositions drastically affects the hole ground state. Our
conclusions are supported by two independent experiments.Comment: 4 pages, 4 figure
Nonlinear Radiation Pressure and Stochasticity in Ultraintense Laser Fields
The radiation force on a single electron in an ultraintense plane wave () is calculated and shown to be proportional to in the
high- limit for arbitrary waveform and polarization. The cyclotron motion of
an electron in a constant magnetic field and an ultraintense plane wave is
numerically found to be quasiperiodic even in the high- limit if the
magnetic field is not too strong, as suggested by previous analytical work. A
strong magnetic field causes highly chaotic electron motion and the boundary of
the highly chaotic region of parameter space is determined numerically.
Applications to experiments and astrophysics are briefly discussed.Comment: 5 pages, 4 figures; uses RevTex, epsf macros. Corrected, expanded
versio
Freezing by Monte Carlo Phase-Switch
We describe a Monte Carlo procedure which allows sampling of the disjoint
configuration spaces associated with crystalline and fluid phases, within a
single simulation. The method utilises biased sampling techniques to enhance
the probabilities of gateway states (in each phase) which are such that a
global switch (to the other phase) can be implemented. Equilibrium
freezing-point parameters can be determined directly; statistical uncertainties
prescribed transparently; and finite-size effects quantified systematically.
The method is potentially quite general; we apply it to the freezing of hard
spheres.Comment: 5 pages, 2 figure
Predator-Prey Interactions between Droplets Driven by Nonreciprocal Oil Exchange
Chemotactic interactions are ubiquitous in nature and can lead to
nonreciprocal and complex emergent behavior in multibody systems. Here we show
how chemotactic signaling between microscale oil droplets of different
chemistries in micellar surfactant solutions can result in predator-prey-like
chasing interactions. The interactions and dynamic self-organization result
from the net directional, micelle-mediated transport of oil between emulsion
droplets of differing composition and are powered by the free energy of mixing.
The nonreciprocal behavior occurs in a wide variety of oil and surfactant
conditions, and we systematically elucidate chemical design rules for tuning
the interactions between droplets by varying oil and surfactant chemical
structure and concentration. Through integration of experiment and simulation,
we also investigate the active behavior and dynamic reorganization of
multi-droplet clusters. Our findings demonstrate how chemically-minimal systems
can be designed with controllable, non-reciprocal chemotactic interactions to
generate emergent self-organization and collective behaviors reminiscent of
biological systems
Does Turbulent Pressure Behave as a Logatrope?
We present numerical simulations of an isothermal turbulent gas undergoing
gravitational collapse, aimed at testing for ``logatropic'' behavior of the
form , where is the ``turbulent pressure'' and
is the density. To this end, we monitor the evolution of the turbulent velocity
dispersion as the density increases during the collapse. A logatropic
behavior would require that , a result which,
however, is not verified in the simulations. Instead, the velocity dispersion
increases with density, implying a polytropic behavior of . This behavior
is found both in purely hydrodynamic as well as hydromagnetic runs. For purely
hydrodynamic and rapidly-collapsing magnetic cases, the velocity dispersion
increases roughly as , implying ,
where is the turbulent pressure. For slowly-collapsing magnetic cases the
behavior is close to , which implies . We thus suggest that the logatropic ``equation of state'' may
represent only the statistically most probable state of an ensemble of clouds
in equilibrium between self-gravity and kinetic support, but does not
adequately represent the behavior of the ``turbulent pressure'' within a cloud
undergoing a dynamic compression due to gravitational collapse. Finally, we
discuss the importance of the underlying physical model for the clouds (in
equilibrium vs. dynamic) on the results obtained.Comment: Accepted in ApJ. 10 pages, 3 postscript figure
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