6,445 research outputs found
The Temperature Dependence of Solar Neutrino Fluxes
By comparing neutrino fluxes and central temperatures calculated from 1000
detailed numerical solar models, we derive improved scaling laws which show how
each of the neutrino fluxes depends upon the central temperature (flux ); we also estimate uncertainties for the temperature exponents. With the
aid of a one-zone model of the sun, we derive expressions for the temperature
exponents of the neutrino fluxes. For the most important neutrino fluxes, the
exponents calculated with the one-zone model agree to within 20\% or better
with the exponents extracted from the detailed numerical models. The one-zone
model provides a physical understanding of the temperature dependence of the
neutrino fluxes. For the neutrino flux, the one-zone model explains the
(initially-surprising) dependence of the flux upon a negative power of the
temperature and suggests a new functional dependence. This new function makes
explicit the strong anti-correlation between the Be and neutrino
fluxes. The one-zone model also predicts successfully the average linear
relations between neutrino fluxes, but cannot predict the appreciable scatter
in a versus diagram.Comment: Repaired http URL path for postscript file. 24 pages (RevTeX) + 5
figures (postscript), uuencoded gz-compressed tar file including
text+figures. Postscript file also available at
http://www.sns.ias.edu/~jnb/preprints.html Accepted for publication in
Physical Review
Telluric correction in the near-infrared: Standard star or synthetic transmission?
Context. The atmospheric absorption of the Earth is an important limiting
factor for ground-based spectroscopic observations and the near-infrared and
infrared regions are the most affected. Several software packages that produce
a synthetic atmospheric transmission spectrum have been developed to correct
for the telluric absorption; these are Molecfit, TelFit, and TAPAS. Aims. Our
goal is to compare the correction achieved using these three telluric
correction packages and the division by a telluric standard star. We want to
evaluate the best method to correct near-infrared high-resolution spectra as
well as the limitations of each software package and methodology. Methods. We
applied the telluric correction methods to CRIRES archival data taken in the J
and K bands. We explored how the achieved correction level varies depending on
the atmospheric T-P profile used in the modelling, the depth of the atmospheric
lines, and the molecules creating the absorption. Results. We found that the
Molecfit and TelFit corrections lead to smaller residuals for the water lines.
The standard star method corrects best the oxygen lines. The Molecfit package
and the standard star method corrections result in global offsets always below
0.5% for all lines; the offset is similar with TelFit and TAPAS for the H2O
lines and around 1% for the O2 lines. All methods and software packages result
in a scatter between 3% and 7% inside the telluric lines. The use of a tailored
atmospheric profile for the observatory leads to a scatter two times smaller,
and the correction level improves with lower values of precipitable water
vapour. Conclusions. The synthetic transmission methods lead to an improved
correction compared to the standard star method for the water lines in the J
band with no loss of telescope time, but the oxygen lines were better corrected
by the standard star method.Comment: 18 pages, 13 figures, Accepted to A&
The Giant X-Ray Flare of NGC 5905: Tidal Disruption of a Star, a Brown Dwarf, or a Planet?
We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 as the
tidal disruption of a star by a supermassive black hole. From the observed
rapid decline of the luminosity, over a timescale of a few years, we argue that
the flare was powered by the fallback of debris rather than subsequent
accretion via a thin disk. The fallback model allows constraints to be set on
the black hole mass and the mass of debris. The latter must be very much less
than a solar mass to explain the very low luminosity of the flare. The
observations can be explained either as the partial stripping of the outer
layers of a low-mass main sequence star or as the disruption of a brown dwarf
or a giant planet. We find that the X-ray emission in the flare must have
originated within a small patch rather than over the entire torus of
circularized material surrounding the black hole. We suggest that the patch
corresponds to the ``bright spot'' where the stream of returning debris impacts
the torus. Interestingly, although the peak luminosity of the flare was highly
sub-Eddington, the peak flux from the bright spot was close to the Eddington
limit. We speculate on the implications of this result for observations of
other flare events.Comment: 25 pages, including 5 figure
Spinaler Infarkt
Zusammenfassung: Die durch einen RĂŒckenmarkinfarkt verursachte Symptomatik kann aufgrund der komplexen Blutversorgung des Myelons zu unterschiedlichen neurologischen AusfĂ€llen fĂŒhren. Dabei steht hĂ€ufig die durch eine arterielle Minderperfusion des Myelons bedingte Querschnittssymptomatik im Vordergrund. Venös induzierte Mikrozirkulationsstörungen sind anhand des neurologischen Befundes klinisch nicht immer von arteriellen Infarkten zu unterscheiden. Die moderne Bildgebung unter Einsatz der CT- (CTA) und MR-Angiographie (MRA) dient dem Ausschluss nichtvaskulĂ€rer Ursachen fĂŒr die Symptomatik wie EntzĂŒndungen und Tumoren sowie der prĂ€operativen Planung vor der Aortenchirurgie zum Nachweis der fĂŒr die Myelondurchblutung entscheidenden A.Adamkiewicz. Im Gegensatz zur CT kann mittels MRT ein Infarkt im Myelon mit hoher VerlĂ€sslichkeit nachgewiesen werde
Measurement of the hyperfine structure of antihydrogen in a beam
A measurement of the hyperfine structure of antihydrogen promises one of the
best tests of CPT symmetry. We describe an experiment planned at the Antiproton
Decelerator of CERN to measure this quantity in a beam of slow antihydrogen
atoms.Comment: 5th International Symposium on Symmetries in Subatomic Physics
(SSP2012), Groningen (The Netherlands), June 18 to 22, 201
A commentary on claw deformities in the blue crab
Throughout history mankind has probably reacted to all natural phenomena by some degree of either attraction or repulsion. Certainly the same objects or phenomena are not viewed exactly alike by all. The rhythmic, as contrasted with discordant, motion, or symmetrical versus non-symmetrical patterns of structure may elicit various feelings in different people. Since natural growth generally produces structural patterns which follow some type of symmetry, this is what we usually expect. While sluggish motion and radial symmetry often are associated, usually animals which dart about or are quick in action have evolved along lines of bilateral symmetry, as have fishes and crabs. Any deviation from recognized actions and structural patterns is certain to attract one\u27s attention . In contrast to a fish and other bilaterally symmetrical animals, including most crustaceans, the crab darts sideways, not along its axis of symmetry. In this study we have been attracted to deviations from symmetry in the external structure of crab claws
OSSE observations of galactic 511 keV annihilation radiation
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory has performed several observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation. Preliminary analysis of data collected during the observation of the galactic center region over the period 13-24 Jun. 1991, indicates the presence of a 511 keV line and positronium continuum superimposed on a power-law continuum. The line of flux was found to be (2.7 +/- 0.5) x 10(exp -4) gamma/sq cm sec, with a positronium fraction of (0.9 +/- 0.2). The 3(sigma) upper limit to daily variations in the 511 keV line flux from the mean during the observation interval is 3 x 10(exp -4) gamma/sq cm sec. If all of the observed annihilation radiation is assumed to originate from the x-ray source 1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 +/- 0.6) x 10(exp -4) gamma/sq cm sec. The 3(sigma) upper limit for 511 keV line emission from the x-ray binary GX1+4 is 6 x 10(exp -4) gamma/sq cm sec. Results from the galactic plane observations at galactic longitudes of 25 degrees (16-21 Aug. 1991) and 339 degrees (6-11 Sep. 1991) suggest that the emission is concentrated near the galactic center. The observations and the preliminary results are described
Controls on lithium concentration and diffusion in zircon
This study was supported by ETH Research Grants ETH-34 15-2 (JS), ETH-14 16-1 (FM) and by Swiss National Science Foundation research grant 200020-153112/1 (NK).Thermal annealing of zircons prior to uranium-lead dating by laser ablation inductively coupled plasma mass spectrometry is a commonly-implemented procedure which improves data accuracy and precision by partially repairing radiation damage from the decay of uranium and thorium. However, it also leads to significantly higher concentrations of lithium in the zircon lattice, which become positively correlated with trivalent yttrium and rare earth elements. Prior to such treatments, zircons typically contain lithium below detection limits (typically <0.55 ÎŒg gâ1), unless correlated with lanthanum and aluminum (i.e., melt/mineral inclusion tracer elements). This suggests that lithium in zircon is primarily sequestered within inclusions, and is able to permeate the crystal lattice to couple with yttrium and rare earth elements during the thermal annealing procedure. This process occurs 2â3 orders of magnitude faster than diffusion experiments have previously determined, indicating that another diffusion mechanism may apply. A model is proposed, whereby: (i) charge compensation of stoichiometrically over-abundant trivalent cations under water-rich magmatic conditions is likely accomplished by hydrogen, given the incompatibility of lithium in zircon and the abundance of hydrogen. However, (ii) conditions that are high temperature and low pressure (characteristic of both thermal annealing and the syn-eruptive environment), drive silicate melt inclusions to exsolve water, generating a motive force for both hydrogen and lithium from inclusions to permeate the lattice in order to reestablish electrochemical equilibrium between the interior and exterior of the zircon. To test the pressure dependency of lithium migration in the zircon lattice, thermal annealing experiments were performed at 850 °C and 1 bar, 2 kbar and 6 kbar using zircons from the Fish Canyon Tuff. The experiments demonstrate that thermal annealing at 2 and 6 kbar inhibits lithium mobility, with zircons registering lithium concentrations below detection limits similar to controls. The experimental results suggest that lithium concentrations in zircon are vulnerable to rapid perturbation by decompression (concurrent with high temperatures), which further indicate that lithium-in-zircon diffusion data should be interpreted with caution.Publisher PDFPeer reviewe
Posthumanist Education
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