6,213 research outputs found
Gamma-Ray Bursts in Circumstellar Shells: A Possible Explanation for Flares
It is now generally accepted that long-duration gamma ray bursts (GRBs) are
due to the collapse of massive rotating stars. The precise collapse process
itself, however, is not yet fully understood. Strong winds, outbursts, and
intense ionizing UV radiation from single stars or strongly interacting
binaries are expected to destroy the molecular cloud cores that give birth to
them and create highly complex circumburst environments for the explosion. Such
environments might imprint features on GRB light curves that uniquely identify
the nature of the progenitor and its collapse. We have performed numerical
simulations of realistic environments for a variety of long-duration GRB
progenitors with ZEUS-MP, and have developed an analytical method for
calculating GRB light curves in these profiles. Though a full,
three-dimensional, relativistic magnetohydrodynamical computational model is
required to precisely describe the light curve from a GRB in complex
environments, our method can provide a qualitative understanding of these
phenomena. We find that, in the context of the standard afterglow model,
massive shells around GRBs produce strong signatures in their light curves, and
that this can distinguish them from those occurring in uniform media or steady
winds. These features can constrain the mass of the shell and the properties of
the wind before and after the ejection. Moreover, the interaction of the GRB
with the circumburst shell is seen to produce features that are consistent with
observed X-ray flares that are often attributed to delayed energy injection by
the central engine. Our algorithm for computing light curves is also applicable
to GRBs in a variety of environments such as those in high-redshift
cosmological halos or protogalaxies, both of which will soon be targets of
future surveys such as JANUS or Lobster.Comment: 12 pages, 5 figures, Accepted by Ap
Compatibility experiments of facilities, materials, and propellants for electrothermal thrusters
Experiments were performed to determine the compatibility of materials and propellants for electro-thermal thrusters. Candidate propellants for resistojet propulsion include carbon dioxide, methane, hydrogen, ammonia, and hydrazine. The materials being examined are grain stabilized platinum for resistojets for Space station and rhenium for high performance resistojets for satellites. Heater mass loss and deterioration of materials were evaluated. A coiled tube of platinum, with yttria dispersed throughout the base material to inhibit grain growth, was tested in carbon dioxide at 1300 C for 2000 hr. Post-test examination indicated the platinum-yttria heater would last over 100 000 hr with less than 10 percent mass loss. Short-term compatibility tests were conducted to test the integrity of the platinum-yttria in hydrogen, methane, carbon dioxide/methane mixtures and ammonia environments. In each of these 100 hr tests, the platinum-yttria mass change indicated a minimum coil life of 100 000 hr. Facility related effects were investigated in materials tests using rhenium heated to high tempertures. Vacuum facility water reduction was monitored using a mass spectrometer. In vacuum environments obtained using only diffusion pumping and those obtained with the assistance of cryogenic equipment there were mass gains in the rhenium heaters. These mass gains were the result of the high amount of oxygen and water contained in the gas. Propellant purity and preferred test facility environments are discussed
Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts
Ionization fronts, the sharp radiation fronts behind which H/He ionizing
photons from massive stars and galaxies propagate through space, were
ubiquitous in the universe from its earliest times. The cosmic dark ages ended
with the formation of the first primeval stars and galaxies a few hundred Myr
after the Big Bang. Numerical simulations suggest that stars in this era were
very massive, 25 - 500 solar masses, with H II regions of up to 30,000
light-years in diameter. We present three-dimensional radiation hydrodynamical
calculations that reveal that the I-fronts of the first stars and galaxies were
prone to violent instabilities, enhancing the escape of UV photons into the
early intergalactic medium (IGM) and forming clumpy media in which supernovae
later exploded. The enrichment of such clumps with metals by the first
supernovae may have led to the prompt formation of a second generation of
low-mass stars, profoundly transforming the nature of the first protogalaxies.
Cosmological radiation hydrodynamics is unique because ionizing photons coupled
strongly to both gas flows and primordial chemistry at early epochs,
introducing a hierarchy of disparate characteristic timescales whose relative
magnitudes can vary greatly throughout a given calculation. We describe the
adaptive multistep integration scheme we have developed for the self-consistent
transport of both cosmological and galactic ionization fronts.Comment: 6 pages, 4 figures, accepted for proceedings of HEDLA2010, Caltech,
March 15 - 18, 201
Toward the assessment of the susceptibility of a digital system to lightning upset
Accomplishments and directions for further research aimed at developing methods for assessing a candidate design of an avionic computer with respect to susceptability to lightning upset are reported. Emphasis is on fault tolerant computers. Both lightning stress and shielding are covered in a review of the electromagnetic environment. Stress characterization, system characterization, upset detection, and positive and negative design features are considered. A first cut theory of comparing candidate designs is presented including tests of comparative susceptability as well as its analysis and simulation. An approach to lightning induced transient fault effects is included
Vacuum chamber pressure effects on thrust measurements of low Reynolds number nozzles
Tests were conducted to investigate the effect of vacuum facility pressure on the performance of small thruster nozzles. Thrust measurements of two converging-diverging nozzles with an area ratio of 140 and an orifice plate flowing unheated nitrogen and hydrogen were taken over a wide range of vacuum facility pressures and nozzle throat Reynolds numbers. In the Reynolds number range of 2200 to 12 000 there was no discernable viscous effect on thrust below an ambient to total pressure ratio of 1000. In nearly all cases, flow separation occurred at a pressure ratio of about 1000. This was the upper limit for obtaining an accurate thrust measurement for a conical nozzle with an area ratio of 140
Exposures to the environmental toxicants pentachlorophenol (PCP) and dichlorodiphenyltrichloroethane (DDT) modify secretion of interleukin 1-beta (IL-1β) from human immune cells
Pentachlorophenol (PCP) and Dichlorodiphenyltrichloroethane (DDT) are environmental contaminants found in human blood. Previous studies have shown that PCP and DDT inhibit the lytic function of highly purified human natural killer (NK) lymphocytes and decrease the expression of several surface proteins on NK cells. Interleukin-1 βeta (IL-1β) is a cytokine produced by lymphocytes and monocytes, and anything that elevates its levels inappropriately can lead to chronic inflammation, which among other consequences can increase tumor development and invasiveness. Here, PCP and DDT were examined for their ability to alter secretion of IL-1β from immune cell preparations of various complexity: NK cells; monocyte-depleted (MD) peripheral blood mononuclear cells (PBMCS); and PBMCs. Cells were exposed to concentrations of PCP ranging from 5 to 0.05 µM and DDT concentrations of 2.5–0.025 μM for 24, 48 h, and 6 days. Results showed that both PCP and DDT increased IL-1β secretion from all of the immune cell preparations. The specific concentrations of PCP and DDT that increased IL-1β secretion varied by donor. Immune cells from all donors showed compound-induced increases in IL-1β secretion at one or more concentration at one or more length of exposure. The mechanism of PCP stimulation of IL1-β secretion was also addressed, and it appears that the MAPKs, ERK1/2 and p38, may be utilized by PCP to stimulate secretion of IL-1β
How the First Stars Regulated Star Formation. II. Enrichment by Nearby Supernovae
Metals from Population III (Pop III) supernovae led to the formation of less
massive Pop II stars in the early universe, altering the course of evolution of
primeval galaxies and cosmological reionization. There are a variety of
scenarios in which heavy elements from the first supernovae were taken up into
second-generation stars, but cosmological simulations only model them on the
largest scales. We present small-scale, high-resolution simulations of the
chemical enrichment of a primordial halo by a nearby supernova after partial
evaporation by the progenitor star. We find that ejecta from the explosion
crash into and mix violently with ablative flows driven off the halo by the
star, creating dense, enriched clumps capable of collapsing into Pop II stars.
Metals may mix less efficiently with the partially exposed core of the halo, so
it might form either Pop III or Pop II stars. Both Pop II and III stars may
thus form after the collision if the ejecta do not strip all the gas from the
halo. The partial evaporation of the halo prior to the explosion is crucial to
its later enrichment by the supernova.Comment: Accepted to Ap
Effects of butyltin exposures on MAP kinase-dependent transcription regulators in human natural killer cells
Natural killer (NK) cells are a major immune defense mechanism against cancer development and viral infection. The butyltins (BTs), tributyltin (TBT) and dibutyltin (DBT), have been widely used in industrial and other applications and significantly contaminate the environment. Both TBT and DBT have been detected in human blood. These compounds inhibit the lytic and binding function of human NK cells and thus could increase the incidence of cancer and viral infections. Butyltin (BT)-induced loss of NK function is accompanied by activation of mitogen activated protein kinases (MAPKs) and decreases in expression of cell-surface and cytolytic proteins. MAPKs activate components of the transcription regulator AP-1 and activate the transcription regulator Elk-1. Based on the fact that BTs activate MAPKs and alter protein expression, the current study examined the effect of BT exposures on the levels and phosphorylation states of the components of AP-1 and the phosphorylation state of Elk-1. Exposure to 300 nM TBT for 10 min increased the phosphorylation of c-Jun in NK cells. One hour exposures to 300 nM and 200 nM TBT increased the phosphorylation and overall level of c-Jun. During a 300 nM treatment with TBT for 1 h the binding activity of AP-1 was significantly decreased. There were no significant alterations of AP-1 components or of Elk-1 with DBT exposures. Thus, it appears that TBT-induced alterations on phosphorylation, total levels, and binding activity of c-Jun might contribute to, but are not fully responsible for, TBT-induced alterations of NK protein expression
Taking Interdependence And Production More Seriously: Toward Mutual Rationality And A More Useful Law And Economics
Taking Interdependence And Production More Seriously: Toward Mutual Rationality And A More Useful Law And Economics
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