39,861 research outputs found
Reionization Through the Lens of Percolation Theory
The reionization of intergalactic hydrogen has received intense theoretical
scrutiny over the past two decades. Here, we approach the process formally as a
percolation process and phase transition. Using semi-numeric simulations, we
demonstrate that an infinitely-large ionized region abruptly appears at an
ionized fraction of ~0.1 and quickly grows to encompass most of the ionized
gas: by an ionized fraction of 0.3, nearly ninety percent of the ionized
material is part of this region. Throughout most of reionization, nearly all of
the intergalactic medium is divided into just two regions, one ionized and one
neutral, and both infinite in extent. We also show that the discrete ionized
regions that exist before and near this transition point follow a near-power
law distribution in volume, with equal contributions to the total filling
factor per logarithmic interval in size up to a sharp cutoff in volume. These
qualities are generic to percolation processes, with the detailed behavior a
result of long-range correlations in the underlying density field. These
insights will be crucial to understanding the distribution of ionized and
neutral gas during reionization and provide precise meaning to the intuitive
description of reionization as an "overlap" process.Comment: 16 pages, version accepted by MNRAS (conclusions unchanged from
original
Maneuvering strategies using CMGs
This paper considers control strategies for maneuvering spacecraft using Single-Gimbal Control Momentum Gyros (CMGs). A pyramid configuration using four gyros is utilized. Preferred initial gimbal angles for maximum utilization of CMG momentum are obtained for some known torque commands. Feedback control laws are derived from the stability point of view by using the Liapunov's Second Theorem. The gyro rates are obtained by the pseudo-inverse technique. The effect of gimbal rate bounds on controllability are studied for an example maneuver. Singularity avoidance is based on limiting the gyro rates depending on a singularity index
Photo-heating and the fate of hard photons during the reionisation of HeII by quasars
We use a combination of analytic and numerical arguments to consider the
impact of quasar photo-heating during HeII reionisation on the thermal
evolution of the intergalactic medium (IGM). We demonstrate that rapid (\Delta
z 10^4 K) photo-heating is difficult to achieve
across the entire IGM unless quasar spectra are significantly harder than
implied by current observational constraints. Although filtering of intrinsic
quasar radiation through dense regions in the IGM does increase the mean excess
energy per HeII photo-ionisation, it also weakens the radiation intensity and
lowers the photo-ionisation rate, preventing rapid heating over time intervals
shorter than the local photo-ionisation timescale. Moreover, the hard photons
responsible for the strongest heating are more likely to deposit their energy
inside dense clumps. The abundance of such clumps is, however, uncertain and
model-dependent, leading to a fairly large uncertainty in the photo-heating
rates. Nevertheless, although some of the IGM may be exposed to a hardened and
weakened ionising background for long periods, most of the IGM must instead be
reionised by the more abundant, softer photons and with accordingly modest
heating rates (\Delta T < 10^4 K). The repeated ionisation of fossil quasar
HeIII regions does not increase the net heating because the recombination times
in these regions typically exceed the IGM cooling times and the average time
lag between successive rounds of quasar activity. Detailed line-of-sight
radiative transfer simulations confirm these expectations and predict a rich
thermal structure in the IGM during HeII reionisation. [Abridged]Comment: 20 pages, 6 figures, accepted by MNRA
-kaon cross section in meson exchange model
We calculate the cross section for the dissociation of by kaons
within the framework of a meson exchange model including anomalous parity
interactions. Off-shell effects at the vertices were handled with QCD sum rule
estimates for the running coupling constants. The total -kaon cross
section was found to be mb for 4.1\leq\sqrt{s}\leq5 \GeV.Comment: 13 pages, 4 eps figure
Molecular beam epitaxial growth of high-quality InSb on InP and GaAs substrates
Epitaxial layers of InSb were grown on InP and GaAs substrates by molecular beam epitaxy. The dependence of the epilayer quality on flux ratio, J sub Sb4/J sub In, was studied. Deviation from an optimum value of J sub Sb4/J sub In (approx. 2) during growth led to deterioration in the surface morphology and the electrical and crystalline qualities of the films. Room temperature electron mobilities as high as 70,000 and 53,000 sq cm /V-s were measured in InSb layers grown on InP and GaAs substrates, respectively. Unlike the previous results, the conductivity in these films is n-type even at T = 13 K, and no degradation of the electron mobility due to the high density of dislocations was observed. The measured electron mobilities (and carrier concentrations) at 77 K in InSb layers grown on InP and GaAs substrates are 110,000 sq cm/V-s (3 x 10(15) cm(-3)) and 55,000 sq cm/V-s (4.95 x 10(15) cm(-3)), respectively, suggesting their application to electronic devices at cryogenic temperatures
Nonvolatile memories using deep traps formed in HfOā by Nb ion implantation
We report nonvolatile memories (NVMs) based on deep-energy trap levels formed in HfOā by metal ion implantation. A comparison of Nb- and Ta-implanted samples shows that suitable charge-trapping centers are formed in Nb-implanted samples, but not in Ta-implanted samples. This is consistent with density-functional theory calculations which predict that only Nb will form deep-energy levels in the bandgap of HfOā. Photocurrent spectroscopy exhibits characteristics consistent with one of the trap levels predicted in these calculations. Nb-implanted samples showing memory windows in capacitanceāvoltage (V) curves always exhibit current (I) peaks in IāV curves, indicating that NVM effects result from deep traps in HfOā. In contrast, Ta-implanted samples show dielectric breakdowns during the IāV sweeps between 5 and 11 V, consistent with the fact that no trap levels are present. For a sample implanted with a fluence of 10Ā¹Ā³Nb cmā»Ā², the charge losses after 10ā“ s are ā¼9.8 and ā¼25.5% at room temperature (RT) and 85Ā°C, respectively, and the expected charge loss after 10 years is ā¼34% at RT, very promising for commercial NVMs
The Peculiar Velocity Function of Galaxy Clusters
The peculiar velocity function of clusters of galaxies is determined using an
accurate sample of cluster velocities based on Tully-Fisher distances of Sc
galaxies (Giovanelli et al 1995b). In contrast with previous results based on
samples with considerably larger velocity uncertainties, the observed velocity
function does not exhibit a tail of high velocity clusters. The results
indicate a low probability of \,5\% of finding clusters with
one-dimensional velocities greater than 600 {\kms}. The root-mean-square
one-dimensional cluster velocity is 29328 {\kms}. The observed cluster
velocity function is compared with expectations from different cosmological
models. The absence of a high velocity tail in the observed function is most
consistent with a low mass-density (0.3) CDM model, and is
inconsistent at level with = 1.0 CDM and HDM models.
The root-mean-square one-dimensional cluster velocities in these models
correspond, respectively, to 314, 516, and 632 {\kms} (when convolved with the
observational uncertainties). Comparison with the observed RMS cluster velocity
of 29328 {\kms} further supports the low-density CDM model.Comment: revised version accepted for publication in ApJ Letters, 18 pages,
uuencoded PostScript with 3 figures included; complete paper available
through WWW at http://www.astro.princeton.edu/~library/prep.htm
Quantisation of Conformal Fields in Three-dimensional Anti-de Sitter Black Hole Spacetime
Utilizing the conformal-flatness nature of 3-dim. Anti-de Sitter (AdS_3)
black hole solution of Banados, Teitelboim and Zanelli, the quantisation of
conformally-coupled scalar and spinor fields in this background spacetime is
explicitly carried out. In particular, mode expansion forms and propagators of
the fields are obtained in closed forms. The vacuum in this conformally-coupled
field theories in AdS_3 black hole spacetime, which is conformally-flat, is the
conformal vacuum which is unique and has global meaning. This point
particularly suggests that now the particle production by AdS_3 black hole
spacetime should be absent. General argument establishing the absence of real
particle creation by AdS_3 black hole spacetime for this case of conformal
triviality is provided. Then next, using the explicit mode expansion forms for
conformally-coupled scalar and spinor fields, the bosonic and fermionic
superradiances are examined and found to be absent confirming the expectation.Comment: 51 pages, Revtex, version to appear in Int. J. Mod. Phys.
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