8,247 research outputs found
Extra dimensions, orthopositronium decay, and stellar cooling
In a class of extra dimensional models with a warped metric and a single
brane the photon can be localized on the brane by gravity only. An intriguing
feature of these models is the possibility of the photon escaping into the
extra dimensions. The search for this effect has motivated the present round of
precision orthopositronium decay experiments. We point out that in this
framework a photon in plasma should be metastable. We consider the
astrophysical consequences of this observation, in particular, what it implies
for the plasmon decay rate in globular cluster stars and for the core-collapse
supernova cooling rate. The resulting bounds on the model parameter exceed the
possible reach of orthopositronium experiments by many orders of magnitude.Comment: 13 pages, no figure
Coulomb tunneling for fusion reactions in dense matter: Path integral Monte Carlo versus mean field
We compare Path Integral Monte Carlo calculations by Militzer and Pollock
(Phys. Rev. B 71, 134303, 2005) of Coulomb tunneling in nuclear reactions in
dense matter to semiclassical calculations assuming WKB Coulomb barrier
penetration through the radial mean-field potential. We find a very good
agreement of two approaches at temperatures higher than ~1/5 of the ion plasma
temperature. We obtain a simple parameterization of the mean field potential
and of the respective reaction rates. We analyze Gamow-peak energies of
reacting ions in various reaction regimes and discuss theoretical uncertainties
of nuclear reaction rates taking carbon burning in dense stellar matter as an
example.Comment: 13 pages, 7 figures, to appear in Phys. Rev.
The Australian Incident Monitoring Study in Intensive Care: AIMS-ICU. The development and evaluation of an incident reporting system in intensive care
Publisher's copy made available with the permission of the publisher © 1996 Australian Society of AnaesthetistsIntensive care units are complex, dynamic patient management environments. Incidents and accidents can be caused by human error, by problems inherent in complex systems, or by a combination of these. Study objectives were to develop and evaluate an incident reporting system. A report form was designed eliciting a description of the incident, contextual information and contributing factors. Staff group sessions using open-ended questions, observations in the workplace and a review of earlier narratives were used to develop the report form. Three intensive care units participated in a two-month evaluation study. Feedback questionnaires were used to assess staff attitudes and understanding, project design and organization. These demonstrated a positive attitude and good understanding by more than 90% participants. Errors in communication, technique, problem recognition and charting were the predisposing factors most commonly chosen in the 128 incidents reported. It was concluded that incident monitoring may be a suitable technique for improving patient safety in intensive care.U. Beckman, L.F. West, G.J. Groombridge, I. Baldwin, G.K. Hart, D.G. Clayton, R.K. Webb, W.B. Runcima
Gamma ray constraints on the galactic supernova rate
Monte Carlo simulations of the expected gamma-ray signatures of galactic supernovae of all types are performed in order to estimate the significance of the lack of a gamma-ray signal due to supernovae occurring during the last millenium. Using recent estimates of nuclear yields, we determine galactic supernova rates consistent with the historic supernova record and the gamma-ray limits. Another objective of these calculations of galactic supernova histories is their application to surveys of diffuse galactic gamma-ray line emission
Chaos driven fusion enhancement factor at astrophysical energies
We perform molecular dynamics simulations to assess the screening effects by
bound target electrons in low energy nuclear reactions in laboratories. Quantum
effects corresponding to the Pauli and Heisenberg principle are enforced by
constraints. We show that the enhancement of the average cross section and of
its variance is due to the perturbations induced by the electrons.This gives a
correlation between the maximum amplitudes of the inter-nuclear oscillational
motion and the enhancement factor. It suggests that the chaotic behavior of the
electronic motion affects the magnitude of the enhancement factor.Comment: 4 pages, 3 figure
Starburst-like Dust Extinction in the Small Magellanic Cloud
The recent discovery that the UV dust extinction in starburst galaxies is similar to that found in the Small Magellanic Cloud (SMC) motivated us to re-investigate the ultraviolet (UV) extinction found in the SMC. We have been able to improve significantly on previous studies by carefully choosing pairs of well matched reddened and unreddened stars. In addition, we benefited from the improved S/N of the NEWSIPS IUE data and the larger sample of SMC stars now available. Searching the IUE Final Archive, we found only four suitable early-type stars that were significantly reddened and had well matched comparison stars. The extinction for three of these stars is remarkably similar. The curves are roughly linear with 1/lambda and have no measurable 2175 A bump. The fourth star has an extinction curve with a significant 2175 A bump and weaker far-UV extinction. The dust along all four sightlines is thought to be local to the SMC. There is no significant Galactic foreground component. The first three stars lie in the SMC Bar and the line-of-sight for each of them passes through regions of recent star formation. The fourth star belongs to the SMC Wing and its line-of-sight passes though a much more quiescent region. Thus, the behavior of the dust extinction in the SMC supports a dependence of dust properties on star formation activity. However, other environmental factors (such as galactic metallicity) must also be important. Dust in the 30 Dor region of the LMC, where much more active star formation is present, does not share the extreme extinction properties seen in SMC dust
Nuclear Gamma Rays from 7Li in the Galactic Cosmic Radiation
The observation of a y-ray line feature from the direction of the galactic center by Johnson, Harnden, and Haymes is interpreted as the 478-keV nuclear de-excitation of low-energy 7Li cosmic rays as they inelastically scatter from the interstellar gas. The prediction of an associated line at 432 keV is proposed as a definitive test of this idea
Gamma ray constraints on the Galactic supernova rate
We perform Monte Carlo simulations of the expected gamma ray signatures of Galactic supernovae of all types to estimate the significance of the lack of a gamma ray signal due to supernovae occurring during the last millenium. Using recent estimates of the nuclear yields, we determine mean Galactic supernova rates consistent with the historic supernova record and the gamma ray limits. Another objective of these calculations of Galactic supernova histories is their application to surveys of diffuse Galactic gamma ray line emission
Evolution of Phase-Space Density in Dark Matter Halos
The evolution of the phase-space density profile in dark matter (DM) halos is
investigated by means of constrained simulations, designed to control the
merging history of a given DM halo. Halos evolve through a series of quiescent
phases of a slow accretion intermitted by violent events of major mergers. In
the quiescent phases the density of the halo closely follows the NFW profile
and the phase-space density profile, Q(r), is given by the Taylor & Navarro
power law, r^{-beta}, where beta ~ 1.9 and stays remarkably stable over the
Hubble time. Expressing the phase-space density by the NFW parameters, Q(r)=Qs
(r/Rs)^{-beta}, the evolution of Q is determined by Qs. We have found that the
effective mass surface density within Rs, Sigma_s = rhos Rs, remains constant
throughout the evolution of a given DM halo along the main branch of its
merging tree. This invariance entails that Qs ~ Rs^{-5/2} and Q(r) ~
Sigma_s^{-1/2} Rs^{-5/2} (r/ Rs)^{-beta}. It follows that the phase-space
density remains constant, in the sense of Qs=const., in the quiescent phases
and it decreases as Rs^{-5/2} in the violent ones. The physical origin of the
NFW density profile and the phase-space density power law is still unknown.
Yet, the numerical experiments show that halos recover these relations after
the violent phases. The major mergers drive Rs to increase and Qs to decrease
discontinuously while keeping Qs Rs^{5/2} = const. The virial equilibrium in
the quiescent phases implies that a DM halos evolves along a sequence of NFW
profiles with constant energy per unit volume (i.e., pressure) within Rs.Comment: 7 pages, 5 figures, accepted by the Astrophysical Journal. Revised, 2
figures adde
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