16,906 research outputs found
Critical Collapse in Einstein-Gauss-Bonnet Gravity in Five and Six Dimensions
Einstein-Gauss-Bonnet gravity (EGB) provides a natural higher dimensional and
higher order curvature generalization of Einstein gravity. It contains a new,
presumably microscopic, length scale that should affect short distance
properties of the dynamics, such as Choptuik scaling. We present the results of
a numerical analysis in generalized flat slice co-ordinates of self-gravitating
massless scalar spherical collapse in five and six dimensional EGB gravity near
the threshold of black hole formation. Remarkably, the behaviour is universal
(i.e. independent of initial data) but qualitatively different in five and six
dimensions. In five dimensions there is a minimum horizon radius, suggestive of
a first order transition between black hole and dispersive initial data. In six
dimensions no radius gap is evident. Instead, below the GB scale there is a
change in the critical exponent and echoing period.Comment: 21 pages, 39 figures, a couple of references and two new figures
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Meteorological factors in Earth-satellite propagation
Using the COMSTAR D/4 28.56 GHz beacon as a source, a differential gain experiment was performed by connecting a 5-meter paraboloidal antenna and a 0.6-meter paraboloidal antenna alternately to the same receiver. Substantial differential gain changes were observed during some, but not all, rain events. A site-diversity experiment was implemented which consists of two 28.56 GHz radiometers separated by 9 km. The look-angle corresponds to that of the D/4 beacon, and data were obtained with one radiometer during several weeks of concurrent beacon operation to verify the system calibration. A theoretical study of the effect of scattering from a nonuniform rain distribution along the path is under way to aid in interpreting the results of this experiment. An improved empirical site diversity-gain model was derived from data in the literature relating to 34 diversity experiments. Work on the experiment control and data acquisition system is continuing with a view toward future experiments
Search for the Heliospheric Termination Shock (TS) and Heliosheath (HS)
Voyager 1 continues to measure the very distant Heliospheric Magnetic Field (HMF) beyond 95 AU at ~35 North latitude. The MAG instrument data covers more than a full 22 years solar magnetic cycle. The magnitude of the observed HMF is well described, on average, by Parker's Archimedean spiral structure if due account is made for time variations of the source field strength and solar wind velocity. The V1 magnetic field observations do not provide any evidence for a field increase associated with entry into a subsonic solar wind region, such as the heliosheath is expected to be, nor an exit from this regime. We see no evidence for crossing of the Termination Shock (TS) as has been reported at ~85 AU by the LECP instrument. Merged Interaction Regions are identified by an increased HMF and associated decreases in the flux of >70 MeV/nuc cosmic rays which are then followed by a flux recovery. This CR-B relationship has been identified in V1 data and studied since 1982 when V1 was at 11 AU. The variance of HMF, a direct measure of the energy**1/2 in the HMF fluctuations, shows no significant changes associated with the alleged TS crossings in 2002–2003. Thus, the absence of any HMF increase at the entry into the heliosheath appears not to be due to the onset of mesoscale turbulence as proposed by Fisk. The TS has yet to be directly observed in-situ by the V1 MAG experiment in data through 2003
Ballistic spin transport in exciton gases
Traditional spintronics relies on spin transport by charge carriers, such as
electrons in semiconductor crystals. This brings several complications: the
Pauli principle prevents the carriers from moving with the same speed; Coulomb
repulsion leads to rapid dephasing of electron flows. Spin-optronics is a
valuable alternative to traditional spintronics. In spin-optronic devices the
spin currents are carried by electrically neutral bosonic quasi-particles:
excitons or exciton-polaritons. They can form highly coherent quantum liquids
and carry spins over macroscopic distances. The price to pay is a finite
life-time of the bosonic spin carriers. We present the theory of exciton
ballistic spin transport which may be applied to a range of systems where
bosonic spin transport has been reported, in particular, to indirect excitons
in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit
interaction of electrons and holes on the exciton spin, account for the Zeeman
effect induced by external magnetic fields, long range and short range exchange
splittings of the exciton resonances. We also consider exciton transport in the
non-linear regime and discuss the definitions of exciton spin current,
polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.
Preliminary Results from the Caltech Core-Collapse Project (CCCP)
We present preliminary results from the Caltech Core-Collapse Project (CCCP),
a large observational program focused on the study of core-collapse SNe.
Uniform, high-quality NIR and optical photometry and multi-epoch optical
spectroscopy have been obtained using the 200'' Hale and robotic 60''
telescopes at Palomar, for a sample of 50 nearby core-collapse SNe. The
combination of both well-sampled optical light curves and multi-epoch
spectroscopy will enable spectroscopically and photometrically based subtype
definitions to be disentangled from each other. Multi-epoch spectroscopy is
crucial to identify transition events that evolve among subtypes with time. The
CCCP SN sample includes every core-collapse SN discovered between July 2004 and
September 2005 that was visible from Palomar, found shortly (< 30 days) after
explosion (based on available pre-explosion photometry), and closer than ~120
Mpc. This complete sample allows, for the first time, a study of core-collapse
SNe as a population, rather than as individual events. Here, we present the
full CCCP SN sample and show exemplary data collected. We analyze available
data for the first ~1/3 of the sample and determine the subtypes of 13 SNe II
based on both light curve shapes and spectroscopy. We discuss the relative SN
II subtype fractions in the context of associating SN subtypes with specific
progenitor stars.Comment: To appear in the proceedings of the meeting "The Multicoloured
Landscape of Compact Objects and their Explosive Origins", Cefalu, Italy,
June 2006, to be published by AIP, Eds. L. Burderi et a
The efficacy of cleaning and disinfection on pig farms
Little is known about the effectiveness of the cleaning and disinfection methods in use on commercial pig farms either in Ireland or worldwide. A National Salmonella Control Programme was implemented in Ireland in August 2002 to monitor and control infection with Salmonella spp. in pigs
Depression in Right Hemisphere Disorder
Between 25-79% of stroke survivors suffer depression, which can lead to limited recovery, decreased quality of life, and increased mortality. In adults with right hemisphere disorder (RHD), the cause(s) of depression have been unclear. Our results showed that significantly more adults with RHD were depressed than normal controls, and that adults with RHD were significantly more depressed than normal controls. In both groups, depression was significantly related with loneliness. In adults with RHD, depression was also significantly related with social support. No demographic or lesion-related variables were associated with increased depression in our samples. Suggestions for treatment research are offered
Risk Factors for Depression in Aphasia: Clinical Implications
Between 25-79% of stroke survivors suffer depression, which can limit recovery, decrease quality of life, and increase mortality. In adults with aphasia, the cause(s) of depression, and thus the means by which it can be addressed, have been unclear. Our participants with aphasia did not differ from our normal controls in presence or severity of depression. However, possible causes of depression differed between groups. In both groups, loneliness was a significant factor. In adults with aphasia, other significant factors were time poststroke, severity of language impairment, and desired control over every day events. Suggestions for research and treatment are offered
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