1,664 research outputs found
A Theoretical Analysis of Thermal Radiation from Neutron Stars
As soon as it was realized that the direct URCA process is allowed by many modern nuclear equation of state, an analysis of its effect on the cooling of neutron stars was undertaken. A primary study showed that the occurrence of the direct URCA process makes the surface temperature of a neutron star suddenly drop by almost an order of magnitude when the cold wave from the core reaches the surface when the star is a few years old. The results of this study are published in Page and Applegate. As a work in progress, we are presently extending the above work. Improved expressions for the effect of nucleon pairing on the neutrino emissivity and specific heat are now available, and we have incorporated them in a recalculation of rate of the direct URCA process
Production of Beryllium and Boron by Spallation in Supernova Ejecta
The abundances of beryllium and boron have been measured in halo stars of
metallicities as low as [Fe/H] =-3. The observations show that the ratios Be/Fe
and B/Fe are independent of metallicity and approximately equal to their solar
values over the entire range of observed metallicity. These observations are in
contradiction with the predictions of simple models of beryllium and boron
production by spallation in the interstellar medium of a well mixed galaxy. We
propose that beryllium and boron are produced by spallation in the ejecta of
type II supernovae. In our picture, protons and alpha particles are accelerated
early in the supernova event and irradiate the heavy elements in the ejecta
long before the ejecta mixes with the interstellar medium. We follow the
propagation of the accelerated particles with a Monte-Carlo code and find that
the energy per spallation reaction is about 5 GeV for a variety of initial
particle spectra and ejecta compositions. Reproducing the observed Be/Fe and
B/Fe ratios requires roughly 3 times 10^{47} ergs of accelerated protons and
alphas. This is much less than the 10^{51} ergs available in a supernova
explosion.Comment: 5 pages, Latex, to be published in the 4th Compton Symposium
Conference Proceedin
High-energy astrophysics: A theoretical analysis of thermal radiation from neutron stars
The unambiguous detection of thermal radiation from the surface of a cooling neutron star was one of the most anxiously awaited results in neutron star physics. This particular Holy Grail was found by Halpern and Holt, who used ROSAT to detect pulsed X-rays from the gamma-ray source Geminga and demonstrate that it was a neutron star, probably a radio pulsar beamed away from us. At an age of approximately 3.4 x 10(exp 5) years, Geminga is in the photon cooling era. Its surface temperature of 5.2 x 10(exp 5) K can be explained within the contexts of both the slow and fast cooling scenarios. In the slow cooling scenario, the surface temperature is too high unless the specific heat of the interior is reduced by extensive baryon pairing. In the fast cooling scenario, the surface temperature will be much too low unless the fast neutrino cooling is shut off by baryon pairing. Two other pulsars, PSR 0656+14 and PSR 1055-52, have also been detected in thermal X-rays by ROSAT. They are also in the photon cooling era. All of this research's neutron star cooling models to date have used the unmagnetized effective temperature-interior temperature relation for the outer boundary condition. Models are being improved by using published magnetic envelope calculations and assumed geometried for the surface magnetic field to determine local interior temperature-emitted flux relations for the surface of the star
Diffusive behavior of a greedy traveling salesman
Using Monte Carlo simulations we examine the diffusive properties of the
greedy algorithm in the d-dimensional traveling salesman problem. Our results
show that for d=3 and 4 the average squared distance from the origin is
proportional to the number of steps t. In the d=2 case such a scaling is
modified with some logarithmic corrections, which might suggest that d=2 is the
critical dimension of the problem. The distribution of lengths also shows
marked differences between d=2 and d>2 versions. A simple strategy adopted by
the salesman might resemble strategies chosen by some foraging and hunting
animals, for which anomalous diffusive behavior has recently been reported and
interpreted in terms of Levy flights. Our results suggest that broad and
Levy-like distributions in such systems might appear due to dimension-dependent
properties of a search space.Comment: accepted in Phys. Rev.
Investigations of an urban area and its locale using ERTS-1 data supported by U-photography
An urban area in central Pennsylvania and the surrounding locality were investigated separately at first by photointerpretation of ERTS-1 imagery and by computer processing of MSS tapes. Next the photointerpretation and processing were coordinated. The results of the cooperative effort of photointerpreters and computer processing analysts were much improved over independent efforts. It was found that single frames of U-2 photography could be projected onto printer output maps with little recognizable distortion in areas 10 to 25 cm square. In this way targets could be identified for use as training areas for computer processed signature identification. In addition, at any stage of category mapping, the level of success in correct classification could be assessed by this method. The results of the classification of the study area are discussed
Mechanical Translation
Contains reports on two research projects.National Science Foundatio
Phase I and Phase II Therapies for Acute Ischemic Stroke: An Update on Currently Studied Drugs in Clinical Research.
Acute ischemic stroke is a devastating cause of death and disability, consequences of which depend on the time from ischemia onset to treatment, the affected brain region, and its size. The main targets of ischemic stroke therapy aim to restore tissue perfusion in the ischemic penumbra in order to decrease the total infarct area by maintaining blood flow. Advances in research of pathological process and pathways during acute ischemia have resulted in improvement of new treatment strategies apart from restoring perfusion. Additionally, limiting the injury severity by manipulating the molecular mechanisms during ischemia has become a promising approach, especially in animal research. The purpose of this article is to review completed and ongoing phases I and II trials for the treatment of acute ischemic stroke, reviewing studies on antithrombotic, thrombolytic, neuroprotective, and antineuroinflammatory drugs that may translate into more effective treatments
Mechanical Translation
Contains research objectives and reports one two research projects.National Science Foundatio
The role of Volatile Anesthetics in Cardioprotection: a systematic review.
This review evaluates the mechanism of volatile anesthetics as cardioprotective agents in both clinical and laboratory research and furthermore assesses possible cardiac side effects upon usage. Cardiac as well as non-cardiac surgery may evoke perioperative adverse events including: ischemia, diverse arrhythmias and reperfusion injury. As volatile anesthetics have cardiovascular effects that can lead to hypotension, clinicians may choose to administer alternative anesthetics to patients with coronary artery disease, particularly if the patient has severe preoperative ischemia or cardiovascular instability. Increasing preclinical evidence demonstrated that administration of inhaled anesthetics - before and during surgery - reduces the degree of ischemia and reperfusion injury to the heart. Recently, this preclinical data has been implemented clinically, and beneficial effects have been found in some studies of patients undergoing coronary artery bypass graft surgery. Administration of volatile anesthetic gases was protective for patients undergoing cardiac surgery through manipulation of the potassium ATP (KATP) channel, mitochondrial permeability transition pore (mPTP), reactive oxygen species (ROS) production, as well as through cytoprotective Akt and extracellular-signal kinases (ERK) pathways. However, as not all studies have demonstrated improved outcomes, the risks for undesirable hemodynamic effects must be weighed against the possible benefits of using volatile anesthetics as a means to provide cardiac protection in patients with coronary artery disease who are undergoing surgery
- …