4,557 research outputs found
The Spectator Electromagnetic Effect on Charged Pion Spectra in Peripheral Ultrarelativistic Heavy-Ion Collisions
We estimate the electromagnetic effect of the spectator charge on the
momentum spectra of and produced in peripheral Pb+Pb collisions
at SPS energies. We find that the effect is large and results in strongly
varying structures in the dependence of the ratio,
especially at low transverse momenta where a deep valley in the above ratio is
predicted at 0.15 -- 0.20.
It appears that the effect depends on initial conditions. Thus, it provides
new information on the space and time evolution of the non-perturbative pion
creation process.Comment: 20 pages and 8 figure
Search for synchrotron emission from secondary leptons in dense cold starless cores
We report radio continuum observations with the Australia Telescope Compact
Array of two molecular clouds. The impetus for these observations is a search
for synchrotron radiation by cosmic ray secondary electrons/positrons in a
region of enhanced density and possibly high magnetic field. We present
modelling which shows that there should be an appreciable flux of synchrotron
above the more diffuse, galactic synchrotron background.
The starless core G333.125-0.562 and infrared source IRAS 15596-5301 were
observed at 1384 and 2368 MHz. For G333.125-0.562, we find no significant
levels of radio emission from this source at either frequency, nor any
appreciable polarisation: we place an upper limit on the radio continuum flux
from this source of 0.5 mJy per beam at both 1384 and 2368 MHz. Due to the
higher than expected flux density limits, we also obtained archival ATCA data
at 8640 MHz for this cloud and place an upper limit on the flux density of 50
micro-Jy per beam. Assuming the cosmic ray spectrum is similar to that near the
Sun, and given the cloud's molecular density and mass, we place an upper limit
on the magnetic field of 500 micro-G. IRAS 15596-5301, with an RMS of 50
micro-Jy per beam at 1384 MHz, shows an HII region consistent with optically
thin free-free emission already detected at 4800 MHz. We use the same
prescription as G333 to constrain the magnetic field from this cloud to be less
than 500 micro-G. We find that these values are not inconsistent with the view
that magnetic field values scale with the average density of the molecular
cloud.Comment: 6 pages, 5 pdf figures, accepted for publication in PAS
The inner regions of protoplanetary disks
To understand how planetary systems form in the dusty disks around
pre-main-sequence stars a detailed knowledge of the structure and evolution of
these disks is required. While this is reasonably well understood for the
regions of the disk beyond about 1 AU, the structure of these disks inward of 1
AU remains a puzzle. This is partly because it is very difficult to spatially
resolve these regions with current telescopes. But it is also because the
physics of this region, where the disk becomes so hot that the dust starts to
evaporate, is poorly understood. With infrared interferometry it has become
possible in recent years to directly spatially resolve the inner AU of
protoplanetary disks, albeit in a somewhat limited way. These observations have
partly confirmed current models of these regions, but also posed new questions
and puzzles. Moreover, it has turned out that the numerical modeling of these
regions is extremely challenging. In this review we give a rough overview of
the history and recent developments in this exciting field of astrophysics.Comment: 45 pages with 14 Figures. to appear in Annual Review of Astronomy and
Astrophysics (2010, Vol. 48
Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization
The spectra of the first galaxies and quasars in the Universe should be
strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by
neutral hydrogen (HI) in the intervening intergalactic medium. However, the
Lyman-alpha line photons emitted by these sources are not eliminated but rather
scatter until they redshift out of resonance and escape due to the Hubble
expansion of the surrounding intergalactic HI. We calculate the resulting
brightness distribution and the spectral shape of the diffuse Lyman-alpha line
emission around high redshift sources, before the intergalactic medium was
reionized. Typically, the Lyman-alpha photons emitted by a source at z=10
scatter over a characteristic angular radius of order 15 arcseconds around the
source and compose a line which is broadened and redshifted by about a thousand
km/s relative to the source. The scattered photons are highly polarized.
Detection of the diffuse Lyman-alpha halos around high redshift sources would
provide a unique tool for probing the neutral intergalactic medium before the
epoch of reionization. On sufficiently large scales where the Hubble flow is
smooth and the gas is neutral, the Lyman-alpha brightness distribution can be
used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3
corrected; new section added on the detectability of Lyman alpha halos;
conclusions update
A survey of backward proton and pion production in p+C interactions at beam momenta from 1 to 400 GeV/c
New data on proton and pion production in p+C interactions from the CERN PS
and SPS accelerators are used in conjunction with other available data sets to
perform a comprehensive survey of backward hadronic cross sections. This survey
covers the complete backward hemisphere in the range of lab angles from 10 to
180 degrees, from 0.2 to 1.4 GeV/c in lab momentum and from 1 to 400 GeV/c in
projectile momentum. Using the constraints of continuity and smoothness of the
angular, momentum and energy dependences a consistent description of the
inclusive cross sections is established which allows the control of the
internal consistency of the nineteen available data sets.Comment: 52 pages 47 figure
Locally continuously perfect groups of homeomorphisms
The notion of a locally continuously perfect group is introduced and studied.
This notion generalizes locally smoothly perfect groups introduced by Haller
and Teichmann. Next, we prove that the path connected identity component of the
group of all homeomorphisms of a manifold is locally continuously perfect. The
case of equivariant homeomorphism group and other examples are also considered.Comment: 14 page
Fundamental analysis of the failure of polymer-based fiber reinforced composites
A mathematical model is described which will permit predictions of the strength of fiber reinforced composites containing known flaws to be made from the basic properties of their constituents. The approach was to embed a local heterogeneous region (LHR) surrounding the crack tip into an anisotropic elastic continuum. The model should (1) permit an explicit analysis of the micromechanical processes involved in the fracture process, and (2) remain simple enough to be useful in practical computations. Computations for arbitrary flaw size and orientation under arbitrary applied load combinations were performed from unidirectional composites with linear elastic-brittle constituent behavior. The mechanical properties were nominally those of graphite epoxy. With the rupture properties arbitrarily varied to test the capability of the model to reflect real fracture modes in fiber composites, it was shown that fiber breakage, matrix crazing, crack bridging, matrix-fiber debonding, and axial splitting can all occur during a period of (gradually) increasing load prior to catastrophic fracture. The computations reveal qualitatively the sequential nature of the stable crack process that precedes fracture
Distributed Computing in the Asynchronous LOCAL model
The LOCAL model is among the main models for studying locality in the
framework of distributed network computing. This model is however subject to
pertinent criticisms, including the facts that all nodes wake up
simultaneously, perform in lock steps, and are failure-free. We show that
relaxing these hypotheses to some extent does not hurt local computing. In
particular, we show that, for any construction task associated to a locally
checkable labeling (LCL), if is solvable in rounds in the LOCAL model,
then remains solvable in rounds in the asynchronous LOCAL model.
This improves the result by Casta\~neda et al. [SSS 2016], which was restricted
to 3-coloring the rings. More generally, the main contribution of this paper is
to show that, perhaps surprisingly, asynchrony and failures in the computations
do not restrict the power of the LOCAL model, as long as the communications
remain synchronous and failure-free
Device-spectroscopy of magnetic field effects in a polyfluorene organic light-emitting diode
We perform charge-induced absorption and electroluminescence spectroscopy in
a polyfluorene organic magnetoresistive device. Our experiments allow us to
measure the singlet exciton, triplet exciton and polaron densities in a live
device under an applied magnetic field, and to distinguish between three
different models that were proposed to explain organic magnetoresistance. These
models are based on different spin-dependent interactions, namely exciton
formation, triplet exciton-polaron quenching and bipolaron formation. We show
that the singlet exciton, triplet exciton and polaron densities and
conductivity all increase with increasing magnetic field. Our data are
inconsistent with the exciton formation and triplet-exciton polaron quenching
models.Comment: 4 pages, two figure
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