5,226 research outputs found
Hyperbola-generator for location of aperiodic events
Plotting device, when used in conjunction with three or more detectors and local receiver and recorder, can quickly pinpoint location of any aperiodic event. Operation requires minimal training and is readily adapted to the field. Mechanical error in device prototype is less than or equal to 3 percent
A Special Case Of A Conjecture By Widom With Implications To Fermionic Entanglement Entropy
We prove a special case of a conjecture in asymptotic analysis by Harold
Widom. More precisely, we establish the leading and next-to-leading term of a
semi-classical expansion of the trace of the square of certain integral
operators on the Hilbert space . As already observed by Gioev and
Klich, this implies that the bi-partite entanglement entropy of the free Fermi
gas in its ground state grows at least as fast as the surface area of the
spatially bounded part times a logarithmic enhancement.Comment: 20 pages, 3 figures, improvement of the presentation, some references
added or updated, proof of Theorem 12 (formerly Lemma 11) adde
Young and intermediate-age massive star clusters
An overview of our current understanding of the formation and evolution of
star clusters is given, with main emphasis on high-mass clusters. Clusters form
deeply embedded within dense clouds of molecular gas. Left-over gas is cleared
within a few million years and, depending on the efficiency of star formation,
the clusters may disperse almost immediately or remain gravitationally bound.
Current evidence suggests that a few percent of star formation occurs in
clusters that remain bound, although it is not yet clear if this fraction is
truly universal. Internal two-body relaxation and external shocks will lead to
further, gradual dissolution on timescales of up to a few hundred million years
for low-mass open clusters in the Milky Way, while the most massive clusters (>
10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe.
The low-mass end of the initial cluster mass function is well approximated by a
power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that
quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5
Msun. In starburst galaxies and old globular cluster systems, this limit
appears to be higher, at least several x 10^6 Msun. The difference is likely
related to the higher gas densities and pressures in starburst galaxies, which
allow denser, more massive giant molecular clouds to form. Low-mass clusters
may thus trace star formation quite universally, while the more long-lived,
massive clusters appear to form preferentially in the context of violent star
formation.Comment: 21 pages, 3 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
PDFLaTeX, requires rspublic.cls style fil
Boundary conditions and defect lines in the Abelian sandpile model
We add a defect line of dissipation, or crack, to the Abelian sandpile model.
We find that the defect line renormalizes to separate the two-dimensional plane
into two half planes with open boundary conditions. We also show that varying
the amount of dissipation at a boundary of the Abelian sandpile model does not
affect the universality class of the boundary condition. We demonstrate that a
universal coefficient associated with height probabilities near the defect can
be used to classify boundary conditions.Comment: 8 pages, 1 figure; suggestions from referees incorporated; to be
published in Phys. Rev.
Distance-redshift from an optical metric that includes absorption
We show that it is possible to equate the intensity reduction of a light wave
caused by weak absorption with a geometrical reduction in intensity caused by a
"transverse" conformal transformation of the spacetime metric in which the wave
travels. We are consequently able to modify Gordon's optical metric to account
for electromagnetic properties of ponderable material whose properties include
both refraction and absorption. Unlike refraction alone however, including
absorption requires a modification of the optical metric that depends on the
eikonal of the wave itself. We derive the distance-redshift relation from the
modified optical metric for Friedman-Lema\^itre-Robertson-Walker spacetimes
whose cosmic fluid has associated refraction and absorption coefficients. We
then fit the current supernovae data and provide an alternate explanation
(other than dark energy) of the apparent acceleration of the universe.Comment: 2 figure
Chromosome mapping: radiation hybrid data and stochastic spin models
This work approaches human chromosome mapping by developing algorithms for
ordering markers associated with radiation hybrid data. Motivated by recent
work of Boehnke et al. [1], we formulate the ordering problem by developing
stochastic spin models to search for minimum-break marker configurations. As a
particular application, the methods developed are applied to 14 human
chromosome-21 markers tested by Cox et al. [2]. The methods generate
configurations consistent with the best found by others. Additionally, we find
that the set of low-lying configurations is described by a Markov-like ordering
probability distribution. The distribution displays cluster correlations
reflecting closely linked loci.Comment: 26 Pages, uuencoded LaTex, Submitted to Phys. Rev. E,
[email protected], [email protected]
Nonlinear theory of resonant slow waves in anisotropic and dispersive plasmas
The solar corona is a typical example of a plasma with strongly anisotropic transport processes. The main dissipative mechanisms in the solar corona acting on slow magnetoacoustic waves are the anisotropic thermal conductivity and viscosity [Ballai et al., Phys. Plasmas 5, 252 (1998)] developed the nonlinear theory of driven slow resonant waves in such a regime. In the present paper the nonlinear behavior of driven magnetohydrodynamic waves in the slow dissipative layer in plasmas with strongly anisotropic viscosity and thermal conductivity is expanded by considering dispersive effects due to Hall currents. The nonlinear governing equation describing the dynamics of nonlinear resonant slow waves is supplemented by a term which describes nonlinear dispersion and is of the same order of magnitude as nonlinearity and dissipation. The connection formulas are found to be similar to their nondispersive counterparts
Absorption Line Studies in the Halo
Significant progress has been made over the last few years to explore the
gaseous halo of the Milky Way by way of absorption spectroscopy. I review
recent results on absorption line studies in the halo using various
instruments, such as the Far Ultraviolet Spectroscopic Explorer, the Space
Telescope Imaging Spectrograph, and others. The new studies imply that the
infall of low-metallicity gas, the interaction with the Magellanic Clouds, and
the Galactic Fountain are responsible for the phenomenon of the intermediate-
and high-velocity clouds in the halo. New measurements of highly-ionized gas in
the vicinity of the Milky Way indicate that these clouds are embedded in a
corona of hot gas that extends deep into the intergalactic space.Comment: 7 pages, 1 figure; Invited review at the conference "How does the
Galaxy work ?", Granada/Spain, June 200
The final fate of spherical inhomogeneous dust collapse II: Initial data and causal structure of singularity
Further to results in [9], pointing out the role of initial density and
velocity distributions towards determining the final outcome of spherical dust
collapse, the causal structure of singularity is examined here in terms of
evolution of the apparent horizon. We also bring out several related features
which throw some useful light towards understanding the nature of this
singularity, including the behaviour of geodesic families coming out and some
aspects related to the stability of singularity.Comment: Latex file, uses epsf.sty, 15 pages and 3 eps figures. Paragraph on
role of smooth functions rewritten. Four references added. To appear in
Classical & Quantum Gravit
Feedback Heating by Cosmic Rays in Clusters of Galaxies
Recent observations show that the cooling flows in the central regions of
galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles
are a leading candidate for suppressing cooling, usually via some form of
mechanical heating. At the same time, observed X-ray cavities and synchrotron
emission point toward a significant non-thermal particle population. Previous
studies have focused on the dynamical effects of cosmic-ray pressure support,
but none have built successful models in which cosmic-ray heating is
significant. Here we investigate a new model of AGN heating, in which the
intracluster medium is efficiently heated by cosmic-rays, which are injected
into the ICM through diffusion or the shredding of the bubbles by
Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal
conduction as well. Using numerical simulations, we show that the cooling
catastrophe is efficiently suppressed. The cluster quickly relaxes to a
quasi-equilibrium state with a highly reduced accretion rate and temperature
and density profiles which match observations. Unlike the conduction-only case,
no fine-tuning of the Spitzer conduction suppression factor f is needed. The
cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within
observational bounds. Cosmic ray heating is a very attractive alternative to
mechanical heating, and may become particularly compelling if GLAST detects the
gamma-ray signature of cosmic-rays in clusters.Comment: Revised version accepted for publication in MNRAS. Significantly
expanded discussion and new simulations exploring parameter space/model
robustness; conclusions unchange
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