1,195 research outputs found
Promissory Note, 9 May 1938
https://egrove.olemiss.edu/aldrichcorr_a/1094/thumbnail.jp
The second Palomar Sky Survey
We describe the main characteristics of the second Palomar Observatory Sky Survey, currently being taken using the Oschin telescope on Palomar Mountain. The limiting magnitudes of the POSS II plates are directly comparable with those of the SERC/ESO southern surveys, at Β_j = 22.5 mag, R_c = 20.8 mag, and I_c = 19.5 mag. We discuss the main modifications made to the telescope and the photographic methods employed in the course of the survey and compare the POSS I and POSS II plates
Spectral analysis of Gene co-expression network of Zebrafish
We analyze the gene expression data of Zebrafish under the combined framework
of complex networks and random matrix theory. The nearest neighbor spacing
distribution of the corresponding matrix spectra follows random matrix
predictions of Gaussian orthogonal statistics. Based on the eigenvector
analysis we can divide the spectra into two parts, first part for which the
eigenvector localization properties match with the random matrix theory
predictions, and the second part for which they show deviation from the theory
and hence are useful to understand the system dependent properties. Spectra
with the localized eigenvectors can be characterized into three groups based on
the eigenvalues. We explore the position of localized nodes from these
different categories. Using an overlap measure, we find that the top
contributing nodes in the different groups carry distinguished structural
features. Furthermore, the top contributing nodes of the different localized
eigenvectors corresponding to the lower eigenvalue regime form different
densely connected structure well separated from each other. Preliminary
biological interpretation of the genes, associated with the top contributing
nodes in the localized eigenvectors, suggests that the genes corresponding to
same vector share common features.Comment: 6 pages, four figures (accepted in EPL
Two-way multi-lane traffic model for pedestrians in corridors
We extend the Aw-Rascle macroscopic model of car traffic into a two-way
multi-lane model of pedestrian traffic. Within this model, we propose a
technique for the handling of the congestion constraint, i.e. the fact that the
pedestrian density cannot exceed a maximal density corresponding to contact
between pedestrians. In a first step, we propose a singularly perturbed
pressure relation which models the fact that the pedestrian velocity is
considerably reduced, if not blocked, at congestion. In a second step, we carry
over the singular limit into the model and show that abrupt transitions between
compressible flow (in the uncongested regions) to incompressible flow (in
congested regions) occur. We also investigate the hyperbolicity of the two-way
models and show that they can lose their hyperbolicity in some cases. We study
a diffusive correction of these models and discuss the characteristic time and
length scales of the instability
Formation of the compact jets in the black hole GX 339-4
Galactic black hole binaries produce powerful outflows with emit over almost
the entire electromagnetic spectrum. Here, we report the first detection with
the Herschel observatory of a variable far-infrared source associated with the
compact jets of the black hole transient GX 339-4 during the decay of its
recent 2010-2011 outburst, after the transition to the hard state. We also
outline the results of very sensitive radio observations conducted with the
Australia Telescope Compact Array, along with a series of near-infrared,
optical (OIR) and X-ray observations, allowing for the first time the
re-ignition of the compact jets to be observed over a wide range of
wavelengths. The compact jets first turn on at radio frequencies with an
optically thin spectrum that later evolves to optically thick synchrotron
emission. An OIR reflare is observed about ten days after the onset of radio
and hard X-ray emission, likely reflecting the necessary time to build up
enough density, as well as to have acceleration (e.g. through shocks) along an
extended region in the jets. The Herschel measurements are consistent with an
extrapolation of the radio inverted power-law spectrum, but they highlight a
more complex radio to OIR spectral energy distribution for the jets.Comment: Accepted for publication in MNRAS Letter, 6 pages, 3 Figures + 1
online Tabl
On discretization in time in simulations of particulate flows
We propose a time discretization scheme for a class of ordinary differential
equations arising in simulations of fluid/particle flows. The scheme is
intended to work robustly in the lubrication regime when the distance between
two particles immersed in the fluid or between a particle and the wall tends to
zero. The idea consists in introducing a small threshold for the particle-wall
distance below which the real trajectory of the particle is replaced by an
approximated one where the distance is kept equal to the threshold value. The
error of this approximation is estimated both theoretically and by numerical
experiments. Our time marching scheme can be easily incorporated into a full
simulation method where the velocity of the fluid is obtained by a numerical
solution to Stokes or Navier-Stokes equations. We also provide a derivation of
the asymptotic expansion for the lubrication force (used in our numerical
experiments) acting on a disk immersed in a Newtonian fluid and approaching the
wall. The method of this derivation is new and can be easily adapted to other
cases
A jump-growth model for predator-prey dynamics: derivation and application to marine ecosystems
This paper investigates the dynamics of biomass in a marine ecosystem. A
stochastic process is defined in which organisms undergo jumps in body size as
they catch and eat smaller organisms. Using a systematic expansion of the
master equation, we derive a deterministic equation for the macroscopic
dynamics, which we call the deterministic jump-growth equation, and a linear
Fokker-Planck equation for the stochastic fluctuations. The McKendrick--von
Foerster equation, used in previous studies, is shown to be a first-order
approximation, appropriate in equilibrium systems where predators are much
larger than their prey. The model has a power-law steady state consistent with
the approximate constancy of mass density in logarithmic intervals of body mass
often observed in marine ecosystems. The behaviours of the stochastic process,
the deterministic jump-growth equation and the McKendrick--von Foerster
equation are compared using numerical methods. The numerical analysis shows two
classes of attractors: steady states and travelling waves.Comment: 27 pages, 4 figures. Final version as published. Only minor change
Observation of contemporaneous optical radiation from a gamma-ray burst
The origin of gamma-ray bursts (GRBs) has been enigmatic since their
discovery. The situation improved dramatically in 1997, when the rapid
availability of precise coordinates for the bursts allowed the detection of
faint optical and radio afterglows - optical spectra thus obtained have
demonstrated conclusively that the bursts occur at cosmological distances. But,
despite efforts by several groups, optical detection has not hitherto been
achieved during the brief duration of a burst. Here we report the detection of
bright optical emission from GRB990123 while the burst was still in progress.
Our observations begin 22 seconds after the onset of the burst and show an
increase in brightness by a factor of 14 during the first 25 seconds; the
brightness then declines by a factor of 100, at which point (700 seconds after
the burst onset) it falls below our detection threshold. The redshift of this
burst, approximately 1.6, implies a peak optical luminosity of 5 times 10^{49}
erg per second. Optical emission from gamma-ray bursts has been generally
thought to take place at the shock fronts generated by interaction of the
primary energy source with the surrounding medium, where the gamma-rays might
also be produced. The lack of a significant change in the gamma-ray light curve
when the optical emission develops suggests that the gamma-rays are not
produced at the shock front, but closer to the site of the original explosion.Comment: 10 pages, 2 figures. Accepted for publication in Nature. For
additional information see http://www.umich.edu/~rotse
The first ALMA view of IRAS 16293-2422: Direct detection of infall onto source B and high-resolution kinematics of source A
Aims: We focus on the kinematical properties of a proto-binary to study the
infall and rotation of gas towards its two protostellar components. Methods: We
present ALMA Science Verification observations with high-spectral resolution of
IRAS 16293-2422 at 220.2 GHz. The wealth of molecular lines in this source and
the very high spectral resolution offered by ALMA allow us to study the gas
kinematics with unprecedented detail. Results: We present the first detection
of an inverse P-Cygni profile towards source B in the three brightest lines.
The line profiles are fitted with a simple two-layer model to derive an infall
rate of 4.5x10^-5 Msun/yr. This infall detection would rule-out the previously
suggested possibility that source B is a T Tauri star. A position velocity
diagram for source A shows evidence for rotation with an axis close to the
line-of-sight.Comment: Accepted by A&A Letters. 4 pages, 3 figures, 3 appendices (one for
Tables, one for additional figures). This second version includes small
language modifications and changes to keep the letter within the 4 page limi
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