3,753 research outputs found
Chlorella, physiology and taxonomy of forty-one isolates
Physiological and morphological characteristics of taxonomic value in Chlorella isolate
Simulations of Electron Acceleration at Collisionless Shocks: The Effects of Surface Fluctuations
Energetic electrons are a common feature of interplanetary shocks and
planetary bow shocks, and they are invoked as a key component of models of
nonthermal radio emission, such as solar radio bursts. A simulation study is
carried out of electron acceleration for high Mach number, quasi-perpendicular
shocks, typical of the shocks in the solar wind. Two dimensional
self-consistent hybrid shock simulations provide the electric and magnetic
fields in which test particle electrons are followed. A range of different
shock types, shock normal angles, and injection energies are studied. When the
Mach number is low, or the simulation configuration suppresses fluctuations
along the magnetic field direction, the results agree with theory assuming
magnetic moment conserving reflection (or Fast Fermi acceleration), with
electron energy gains of a factor only 2 - 3. For high Mach number, with a
realistic simulation configuration, the shock front has a dynamic rippled
character. The corresponding electron energization is radically different:
Energy spectra display: (1) considerably higher maximum energies than Fast
Fermi acceleration; (2) a plateau, or shallow sloped region, at intermediate
energies 2 - 5 times the injection energy; (3) power law fall off with
increasing energy, for both upstream and downstream particles, with a slope
decreasing as the shock normal angle approaches perpendicular; (4) sustained
flux levels over a broader region of shock normal angle than for adiabatic
reflection. All these features are in good qualitative agreement with
observations, and show that dynamic structure in the shock surface at ion
scales produces effective scattering and can be responsible for making high
Mach number shocks effective sites for electron acceleration.Comment: 26 pages, 12 figure
Surface recombination measurements on IIIâV candidate materials for nanostructure light-emitting diodes
Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes
A Mini-survey of X-ray Point Sources in Starburst and Non-Starburst Galaxies
We present a comparison of X-ray point source luminosity functions of 3
starburst galaxies (the Antennae, M82, and NGC 253) and 4 non-starburst spiral
galaxies (NGC 3184, NGC 1291, M83, and IC 5332). We find that the luminosity
functions of the starbursts are flatter than those of the spiral galaxies; the
starbursts have relatively more sources at high luminosities. This trend
extends to early-type galaxies which have steeper luminosity functions than
spirals. We show that the luminosity function slope is correlated with 60
micron luminosity, a measure of star formation. We suggest that the difference
in luminosity functions is related to the age of the X-ray binary populations
and present a simple model which highlights how the shape of the luminosity
distribution is affected by the age of the underlying X-ray binary population.Comment: 8 pages, 4 figures. accepted for publication in Ap
A Critical Analysis of Techniques and Basic Phenomena Related to Deposition of High Temperature Superconducting Thin Films
The processes involved in plasma and ion beam sputter-, electron evaporation-, and laser ablation-deposition of high temperature superconducting thin films are critically reviewed. Recent advances in the development of these techniques are discussed in relation to basic physical phenomena, specific to each technique, which must be understood before high quality films can be produced. Low temperature processing of films is a common goal for each technique, particularly in relation to integrating high temperature superconducting films with the current microelectronics technology. Research is now demonstrating that the introduction of oxygen into the growing film, simultaneously with the deposition of the film components, is necessary to produce as-deposited superconducting films at relatively low substrate temperatures
Heavy Quark Expansion and Preasymptotic Corrections to Decay Widths in the 't Hooft Model
We address nonperturbative power corrections to inclusive decay widths of
heavy flavor hadrons in the context of the 't Hooft model (two-dimensional QCD
at N_c->oo), with the emphasis on the spectator-dependent effects sensitive to
the flavor of the spectator. The summation of exclusive widths is performed
analytically using the `t Hooft equation. We show that the 1/m_Q expansion of
both the Weak Annihilation and Pauli Interference widths coincides with the OPE
predictions, to the computed orders. Violation of local duality in the
inclusive widths is quantified, and the new example is identified where the OPE
prediction and the actual effect are completely saturated by a single final
state. The qualitative aspects of quark hadronization emerging from the
analysis in the 't Hooft model are discussed. Certain aspects of summation of
spectator-independent hadronic weak decay widths are given in more detail,
which were not spelled out previously. We also give some useful details of the
1/m_Q expansion in the 't~Hooft model.Comment: 54 pages, 8 figures in the text. Version to be published in Phys.
Rev. D. A number of typos are corrected and relevant references added.
Clarifications in Conclusions, Appendices 2.1 and 3 are adde
Supermagnetosonic jets behind a collisionless quasi-parallel shock
The downstream region of a collisionless quasi-parallel shock is structured
containing bulk flows with high kinetic energy density from a previously
unidentified source. We present Cluster multi-spacecraft measurements of this
type of supermagnetosonic jet as well as of a weak secondary shock front within
the sheath, that allow us to propose the following generation mechanism for the
jets: The local curvature variations inherent to quasi-parallel shocks can
create fast, deflected jets accompanied by density variations in the downstream
region. If the speed of the jet is super(magneto)sonic in the reference frame
of the obstacle, a second shock front forms in the sheath closer to the
obstacle. Our results can be applied to collisionless quasi-parallel shocks in
many plasma environments.Comment: accepted to Phys. Rev. Lett. (Nov 5, 2009
Contribution of Long Wavelength Gravitational Waves to the CMB Anisotropy
We present an in depth discussion of the production of gravitational waves
from an inflationary phase that could have occurred in the early universe,
giving derivations for the resulting spectrum and energy density. We also
consider the large-scale anisotropy in the cosmic microwave background
radiation coming from these waves. Assuming that the observed quadrupole
anisotropy comes mostly from gravitational waves (consistent with the
predictions of a flat spectrum of scalar density perturbations and the measured
dipole anisotropy) we describe in detail how to derive a value for the scale of
inflation of GeV, which is at a particularly interesting
scale for particle physics. This upper limit corresponds to a 95\% confidence
level upper limit on the scale of inflation assuming only that the quadrupole
anisotropy from gravitational waves is not cancelled by another source. Direct
detection of gravitational waves produced by inflation near this scale will
have to wait for the next generation of detectors.Comment: (LaTeX 16 pages), 2 figures not included, YCTP-P16-9
The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?
We review five independent techniques which are used to set the distance
scale to globular clusters, including subdwarf main sequence fitting utilizing
the recent Hipparcos parallax catalogue. These data together all indicate that
globular clusters are farther away than previously believed, implying a
reduction in age estimates. This new distance scale estimate is combined with a
detailed numerical Monte Carlo study designed to assess the uncertainty
associated with the theoretical age-turnoff luminosity relationship in order to
estimate both the absolute age and uncertainty in age of the oldest globular
clusters. Our best estimate for the mean age of the oldest globular clusters is
now Gyr, with a one-sided, 95% confidence level lower limit of
9.5 Gyr. This represents a systematic shift of over 2 compared to our
earlier estimate, due completely to the new distance scale---which we emphasize
is not just due to the Hipparcos data. This now provides a lower limit on the
age of the universe which is consistent with either an open universe, or a
flat, matter dominated universe (the latter requiring H_0 \le 67 \kmsmpc).
Our new study also explicitly quantifies how remaining uncertainties in the
distance scale and stellar evolution models translate into uncertainties in the
derived globular cluster ages. Simple formulae are provided which can be used
to update our age estimate as improved determinations for various quantities
become available.Comment: 41 pages, including 10 eps figs, uses aaspp4.sty and flushrt.sty,
submitted to Ap.J., revised to incorporate FULL Hipparcos catalogue dat
The velocity peaks in the cold dark matter spectrum on Earth
The cold dark matter spectrum on earth is expected to have peaks in velocity
space. We obtain estimates for the sizes and locations of these peaks. To this
end we have generalized the secondary infall model of galactic halo formation
to include angular momentum of the dark matter particles. This new model is
still spherically symmetric and it has self-similar solutions. Our results are
relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio
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