10,512 research outputs found
Methodology development for evaluation of selective-fidelity rotorcraft simulation
This paper addressed the initial step toward the goal of establishing performance and handling qualities acceptance criteria for realtime rotorcraft simulators through a planned research effort to quantify the system capabilities of 'selective fidelity' simulators. Within this framework the simulator is then classified based on the required task. The simulator is evaluated by separating the various subsystems (visual, motion, etc.) and applying corresponding fidelity constants based on the specific task. This methodology not only provides an assessment technique, but also provides a technique to determine the required levels of subsystem fidelity for a specific task
Preliminary catalog of pictures taken on the lunar surface during the Apollo 15 mission
Catalog of all pictures taken from lunar module or lunar surface during Apollo 15 missio
The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System
As large optical quasar surveys for damped Lya become a reality and the study
of star forming gas in the early Universe achieves statistical robustness, it
is now vital to identify and quantify the sources of systematic error. Because
the nature of optically-selected quasar surveys makes them vulnerable to dust
obscuration, we have undertaken a radio-selected quasar survey for damped Lya
systems to address this bias. We present the definition and results of this
survey. We then combine our sample with the CORALS dataset to investigate the
HI column density distribution function f(N) of damped Lya systems toward
radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1
N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in
excellent agreement with that of optically-selected samples at low N(HI), an
important yet expected result given that obscuration should have negligible
effect at these gas columns. However, because of the relatively small size of
the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not
well constrained at large N(HI) and the first moment of the HI distribution
function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep
enough, however, that extrapolating it to higher column densities implies only
a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g
= 1.15 x 10^-3, agrees well with the results of optically-selected surveys.
While our results indicate that dust obscuration is likely not a major issue
for surveys of damped Lya systems, we estimate that a radio-selected sample of
approximately 100 damped Lya systems will be required to obtain the precision
necessary to absolutely confirm an absence of dust bias.Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 200
Electron-phonon scattering at the intersection of two Landau levels
We predict a double-resonant feature in the magnetic field dependence of the
phonon-mediated longitudinal conductivity of a two-subband
quasi-two-dimensional electron system in a quantizing magnetic field. The two
sharp peaks in appear when the energy separation between two
Landau levels belonging to different size-quantization subbands is favorable
for acoustic-phonon transitions. One-phonon and two-phonon mechanisms of
electron conductivity are calculated and mutually compared. The phonon-mediated
interaction between the intersecting Landau levels is considered and no avoided
crossing is found at thermal equilibrium.Comment: 13 pages, 8 figure
The Broadband Spectrum of Galaxy Clusters
We examine whether nonthermal protons energized during a cluster merger are
simultaneously responsible for the Coma cluster's diffuse radio flux (via
secondary decay) and the departure of its intra-cluster medium (ICM) from a
thermal profile via Coulomb collisions between the quasithermal electrons and
the hadrons. Rather than approximating the influence of nonthermal
proton/thermal electron collisions as extremely rare events which cause an
injection of nonthermal, power-law electrons (the `knock-on' approximation), we
self-consistently solve (to our knowledge, for the first time) the covariant
kinetic equations for the two populations. The electron population resulting
from these collisions is out of equilibrium, yet not a power law, and
importantly displays a higher bremsstrahlung radiative efficiency than a pure
power law. Observations with GLAST will test this model directly.Comment: Accepted for publication in Ap
Heat exchange mediated by a quantum system
We consider heat transfer between two thermal reservoirs mediated by a
quantum system using the generalized quantum Langevin equation. The thermal
reservoirs are treated as ensembles of oscillators within the framework of the
Drude-Ullersma model. General expressions for the heat current and thermal
conductance are obtained for arbitrary coupling strength between the reservoirs
and the mediator and for different temperature regimes. As an application of
these results we discuss the origin of Fourier's law in a chain of large, but
finite subsystems coupled to each other by the quantum mediators. We also
address a question of anomalously large heat current between the STM tip and
substrate found in a recent experiment. The question of minimum thermal
conductivity is revisited in the framework of scaling theory as a potential
application of the developed approach.Comment: 16 pages, 6 figure
Neutrinos and Gamma Rays from Galaxy Clusters
The next generation of neutrino and gamma-ray detectors should provide new
insights into the creation and propagation of high-energy protons within galaxy
clusters, probing both the particle physics of cosmic rays interacting with the
background medium and the mechanisms for high-energy particle production within
the cluster. In this paper we examine the possible detection of gamma-rays (via
the GLAST satellite) and neutrinos (via the ICECUBE and Auger experiments) from
the Coma cluster of galaxies, as well as for the gamma-ray bright clusters
Abell 85, 1758, and 1914. These three were selected from their possible
association with unidentified EGRET sources, so it is not yet entirely certain
that their gamma-rays are indeed produced diffusively within the intracluster
medium, as opposed to AGNs. It is not obvious why these inconspicuous
Abell-clusters should be the first to be seen in gamma-rays, but a possible
reason is that all of them show direct evidence of recent or ongoing mergers.
Their identification with the EGRET gamma-ray sources is also supported by the
close correlation between their radio and (purported) gamma-ray fluxes. Under
favorable conditions (including a proton spectral index of 2.5 in the case of
Abell 85, and sim 2.3 for Coma, and Abell 1758 and 1914), we expect ICECUBE to
make as many as 0.3 neutrino detections per year from the Coma cluster of
galaxies, and as many as a few per year from the Abell clusters 85, 1758, and
1914. Also, Auger may detect as many as 2 events per decade at ~ EeV energies
from these gamma-ray bright clusters.Comment: Accepted for publication in Ap
Complete chloroplast genome sequence of Holoparasite Cistanche Deserticola (Orobanchaceae) reveals gene loss and horizontal gene transfer from Its host Haloxylon Ammodendron (Chenopodiaceae)
The central function of chloroplasts is to carry out photosynthesis, and its gene content and structure are highly conserved across land plants. Parasitic plants, which have reduced photosynthetic ability, suffer gene losses from the chloroplast (cp) genome accompanied by the relaxation of selective constraints. Compared with the rapid rise in the number of cp genome sequences of photosynthetic organisms, there are limited data sets from parasitic plants. The authors report the complete sequence of the cp genome of Cistanche deserticola, a holoparasitic desert species belonging to the family Orobanchaceae
The Borrego Mountain, California, earthquake of 9 April 1968: A preliminary report
The largest earthquake to hit California in more than 15 years occurred at
02:28:58.9 GCT on 9 April 1968 near Borrego Mountain, on the western edge of
the Imperial Valley. The Seismological Laboratory at Pasadena has tentatively
assigned the shock a magnitude of 6.5, an epicentral location of 33 ° 08.8' N, 116 °
07.5' W, and a focal depth of 20 km. The earthquake was felt throughout most
of southern California and adjacent areas, but the absence of severe damage and
casualties was in large part due to the relatively undeveloped nature of the epicentral
region. Indeed, it would have been difficult to pick a location in the southernmost
part of the State more remote from centers of population
An 84 microGauss Magnetic Field in a Galaxy at Redshift z=0.692
The magnetic field pervading our Galaxy is a crucial constituent of the
interstellar medium: it mediates the dynamics of interstellar clouds, the
energy density of cosmic rays, and the formation of stars. The field associated
with ionized interstellar gas has been determined through observations of
pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the
rotation of the plane of linear polarization, i.e., Faraday rotation, yield an
average value B ~ 3 microGauss. The possible detection of Faraday rotation of
linearly polarized photons emitted by high-redshift quasars suggests similar
magnetic fields are present in foreground galaxies with redshifts z > 1. As
Faraday rotation alone, however, determines neither the magnitude nor the
redshift of the magnetic field, the strength of galactic magnetic fields at
redshifts z > 0 remains uncertain. Here we report a measurement of a magnetic
field of B ~ 84 microGauss in a galaxy at z =0.692, using the same
Zeeman-splitting technique that revealed an average value of B = 6 microGauss
in the neutral interstellar gas of our Galaxy. This is unexpected, as the
leading theory of magnetic field generation, the mean-field dynamo model,
predicts large-scale magnetic fields to be weaker in the past rather than
stronger
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