3,459 research outputs found
Improved Algorithms for Radar-based Reconstruction of Asteroid Shapes
We describe our implementation of a global-parameter optimizer and Square
Root Information Filter (SRIF) into the asteroid-modelling software SHAPE. We
compare the performance of our new optimizer with that of the existing
sequential optimizer when operating on various forms of simulated data and
actual asteroid radar data. In all cases, the new implementation performs
substantially better than its predecessor: it converges faster, produces shape
models that are more accurate, and solves for spin axis orientations more
reliably. We discuss potential future changes to improve SHAPE's fitting speed
and accuracy.Comment: 12 pages, 9 figure
Multi-Dimensional Explorations in Supernova Theory
In this paper, we bring together various of our published and unpublished
findings from our recent 2D multi-group, flux-limited radiation hydrodynamic
simulations of the collapse and explosion of the cores of massive stars. Aided
by 2D and 3D graphical renditions, we motivate the acoustic mechanism of
core-collapse supernova explosions and explain, as best we currently can, the
phases and phenomena that attend this mechanism. Two major foci of our
presentation are the outer shock instability and the inner core g-mode
oscillations. The former sets the stage for the latter, which damp by the
generation of sound. This sound propagates outward to energize the explosion
and is relevant only if the core has not exploded earlier by some other means.
Hence, it is a more delayed mechanism than the traditional neutrino mechanism
that has been studied for the last twenty years since it was championed by
Bethe and Wilson. We discuss protoneutron star convection,
accretion-induced-collapse, gravitational wave emissions, pulsar kicks, the
angular anisotropy of the neutrino emissions, a subset of numerical issues, and
a new code we are designing that should supercede our current supernova code
VULCAN/2D. Whatever ideas last from this current generation of numerical
results, and whatever the eventual mechanism(s), we conclude that the breaking
of spherical symmetry will survive as one of the crucial keys to the supernova
puzzle.Comment: To be published in the "Centennial Festschrift for Hans Bethe,"
Physics Reports (Elsevier: Holland), ed. G.E. Brown, E. van den Heuvel, and
V. Kalogera, 200
Bi-class classification of humpback whale sound units against complex background noise with Deep Convolution Neural Network
Automatically detecting sound units of humpback whales in complex
time-varying background noises is a current challenge for scientists. In this
paper, we explore the applicability of Convolution Neural Network (CNN) method
for this task. In the evaluation stage, we present 6 bi-class classification
experimentations of whale sound detection against different background noise
types (e.g., rain, wind). In comparison to classical FFT-based representation
like spectrograms, we showed that the use of image-based pretrained CNN
features brought higher performance to classify whale sounds and background
noise.Comment: arXiv admin note: text overlap with arXiv:1702.02741 by other author
A GPU-based multi-criteria optimization algorithm for HDR brachytherapy
Currently in HDR brachytherapy planning, a manual fine-tuning of an objective
function is necessary to obtain case-specific valid plans. This study intends
to facilitate this process by proposing a patient-specific inverse planning
algorithm for HDR prostate brachytherapy: GPU-based multi-criteria optimization
(gMCO).
Two GPU-based optimization engines including simulated annealing (gSA) and a
quasi-Newton optimizer (gL-BFGS) were implemented to compute multiple plans in
parallel. After evaluating the equivalence and the computation performance of
these two optimization engines, one preferred optimization engine was selected
for the gMCO algorithm. Five hundred sixty-two previously treated prostate HDR
cases were divided into validation set (100) and test set (462). In the
validation set, the number of Pareto optimal plans to achieve the best plan
quality was determined for the gMCO algorithm. In the test set, gMCO plans were
compared with the physician-approved clinical plans.
Over 462 cases, the number of clinically valid plans was 428 (92.6%) for
clinical plans and 461 (99.8%) for gMCO plans. The number of valid plans with
target V100 coverage greater than 95% was 288 (62.3%) for clinical plans and
414 (89.6%) for gMCO plans. The mean planning time was 9.4 s for the gMCO
algorithm to generate 1000 Pareto optimal plans.
In conclusion, gL-BFGS is able to compute thousands of SA equivalent
treatment plans within a short time frame. Powered by gL-BFGS, an ultra-fast
and robust multi-criteria optimization algorithm was implemented for HDR
prostate brachytherapy. A large-scale comparison against physician approved
clinical plans showed that treatment plan quality could be improved and
planning time could be significantly reduced with the proposed gMCO algorithm.Comment: 18 pages, 7 figure
Yarkovsky Drift Detections for 247 Near-Earth Asteroids
The Yarkovsky effect is a thermal process acting upon the orbits of small
celestial bodies, which can cause these orbits to slowly expand or contract
with time. The effect is subtle (da/dt ~ 10^-4 au/My for a 1 km diameter
object) and is thus generally difficult to measure. We analyzed both optical
and radar astrometry for 600 near-Earth asteroids (NEAs) for the purpose of
detecting and quantifying the Yarkovsky effect. We present 247 NEAs with
measured drift rates, which is the largest published set of Yarkovsky
detections. This large sample size provides an opportunity to examine the
Yarkovsky effect in a statistical manner. In particular, we describe two
independent population-based tests that verify the measurement of Yarkovsky
orbital drift. First, we provide observational confirmation for the Yarkovsky
effect's theoretical size dependence of 1/D, where D is diameter. Second, we
find that the observed ratio of negative to positive drift rates in our sample
is 2.34, which, accounting for bias and sampling uncertainty, implies an actual
ratio of . This ratio has a vanishingly small probability of
occurring due to chance or statistical noise. The observed ratio of retrograde
to prograde rotators is two times lower than the ratio expected from numerical
predictions from NEA population studies and traditional assumptions about the
sense of rotation of NEAs originating from various main belt escape routes. We
also examine the efficiency with which solar energy is converted into orbital
energy and find a median efficiency in our sample of 12%. We interpret this
efficiency in terms of NEA spin and thermal properties.Comment: 27 pages, 9 figures, published in the Astronomical Journal, 159, 92,
202
Structural robustness assessment of concrete frames considering membrane action effects
One way to increase the structural robustness is to take into account membrane action effects in beams and slabs. This membrane action generates an additional load transfer to neighbouring supports, which can considerably increase the load-carrying capacity of the member under consideration. However, the effect of membrane action on commonly used robustness indicators is still unknown. In previous contributions of the authors, a numerical model for reinforced concrete slabs and beams under large deformations was developed and validated. In this contribution, a framework is developed in order to incorporate this numerical model in the analysis of a simple concrete frame in case of column loss, in order to assess the influence of membrane action on commonly used robustness indicators
A Two-Dimensional MagnetoHydrodynamics Scheme for General Unstructured Grids
We report a new finite-difference scheme for two-dimensional
magnetohydrodynamics (MHD) simulations, with and without rotation, in
unstructured grids with quadrilateral cells. The new scheme is implemented
within the code VULCAN/2D, which already includes radiation-hydrodynamics in
various approximations and can be used with arbitrarily moving meshes (ALE).
The MHD scheme, which consists of cell-centered magnetic field variables,
preserves the nodal finite difference representation of div(\bB) by
construction, and therefore any initially divergence-free field remains
divergence-free through the simulation. In this paper, we describe the new
scheme in detail and present comparisons of VULCAN/2D results with those of the
code ZEUS/2D for several one-dimensional and two-dimensional test problems. The
code now enables two-dimensional simulations of the collapse and explosion of
the rotating, magnetic cores of massive stars. Moreover, it can be used to
simulate the very wide variety of astrophysical problems for which multi-D
radiation-magnetohydrodynamics (RMHD) is relevant.Comment: 22 pages, including 11 figures; Accepted to the Astrophysical
Journal. Higher resolution figures available at
http://zenith.as.arizona.edu/~burrows/mhd-code
Prospecting Period Measurements with LSST - Low Mass X-ray Binaries as a Test Case
The Large Synoptic Survey Telescope (LSST) will provide for unbiased sampling
of variability properties of objects with mag 24. This should allow for
those objects whose variations reveal their orbital periods (), such
as low mass X-ray binaries (LMXBs) and related objects, to be examined in much
greater detail and with uniform systematic sampling. However, the baseline LSST
observing strategy has temporal sampling that is not optimised for such work in
the Galaxy. Here we assess four candidate observing strategies for measurement
of in the range 10 minutes to 50 days. We simulate multi-filter
quiescent LMXB lightcurves including ellipsoidal modulation and stochastic
flaring, and then sample these using LSST's operations simulator (OpSim) over
the (mag, ) parameter space, and over five sightlines sampling a range
of possible reddening values. The percentage of simulated parameter space with
correctly returned periods ranges from 23 %, for the current baseline
strategy, to 70 % for the two simulated specialist strategies. Convolving
these results with a distribution, a modelled Galactic spatial
distribution and reddening maps, we conservatively estimate that the most
recent version of the LSST baseline strategy will allow determination
for 18 % of the Milky Way's LMXB population, whereas strategies that do
not reduce observations of the Galactic Plane can improve this dramatically to
32 %. This increase would allow characterisation of the full binary
population by breaking degeneracies between suggested distributions
in the literature. Our results can be used in the ongoing assessment of the
effectiveness of various potential cadencing strategies.Comment: Replacement after addressing minor corrections from the referee -
mainly improvements in clarificatio
- …