3,344 research outputs found
A Hybrid N-body--Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planets from an
initial ensemble of planetesimals. The algorithm uses a coagulation code to
treat the growth of planetesimals into oligarchs and explicit N-body
calculations to follow the evolution of oligarchs into planets. To validate the
N-body portion of the algorithm, we use a battery of tests in planetary
dynamics. Several complete calculations of terrestrial planet formation with
the hybrid code yield good agreement with previously published calculations.
These results demonstrate that the hybrid code provides an accurate treatment
of the evolution of planetesimals into planets.Comment: Astronomical Journal, accepted; 33 pages + 11 figure
The Merging History of Massive Black Holes
We investigate a hierarchical structure formation scenario describing the
evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the
local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first
POPIII stars. As these pregalactic holes become incorporated through a series
of mergers into larger and larger halos, they sink to the center owing to
dynamical friction, accrete a fraction of the gas in the merger remnant to
become supermassive, form a binary system, and eventually coalesce. A simple
model in which the damage done to a stellar cusps by decaying BH pairs is
cumulative is able to reproduce the observed scaling relation between galaxy
luminosity and core size. An accretion model connecting quasar activity with
major mergers and the observed BH mass-velocity dispersion correlation
reproduces remarkably well the observed luminosity function of
optically-selected quasars in the redshift range 1<z<5. We finally asses the
potential observability of the gravitational wave background generated by the
cosmic evolution of SMBH binaries by the planned space-born interferometer
LISA.Comment: 4 pages, 2 figures, Contribute to "Multiwavelength Cosmology",
Mykonos, Greece, June 17-20, 200
Human pulmonary responses to experimental inhalation of high concentration fine and ultrafine magnesium oxide particles.
Exposure to air polluted with particles less than 2.5 micron in size is associated epidemiologically with adverse cardiopulmonary health consequences in humans. The goal of this study was to characterize human pulmonary responses to controlled experimental high-dose exposure to fine and ultrafine magnesium oxide particles. We quantified bronchoalveolar lavage (BAL) cell and cytokine concentrations, pulmonary function, and peripheral blood neutrophil concentrations in six healthy volunteers 18 to 20 hr after inhalation of fine and ultrafine magnesium oxide particles produced from a furnace system model. We compared postexposure studies with control studies from the same six subjects. Mean +/- standard deviation (SD) cumulative magnesium dose was 4,138 +/- 2,163 min x mg/m3. By weight, 28% of fume particles were ultrafine (<0.1 micron in diameter) and over 98% of fume particles were fine (<2.5 micron in diameter). There were no significant differences in BAL inflammatory cell concentrations, BAL interleukin (IL)-1, IL-6, IL-8, tumor necrosis factor, pulmonary function, or peripheral blood neutrophil concentrations postexposure compared with control. Our findings suggest that high-dose fine and ultrafine magnesium oxide particle exposure does not produce a measurable pulmonary inflammatory response. These findings are in marked contrast with the well-described pulmonary inflammatory response following zinc oxide particle inhalation. We conclude that fine and ultrafine particle inhalation does not result in toxicity in a generic manner independent of particle composition. Our findings support the concept that particle chemical composition, in addition to particle size, is an important determinant of respiratory effects
Collisional Dark Matter and the Origin of Massive Black Holes
If the cosmological dark matter is primarily in the form of an elementary
particle which has cross section and mass for self-interaction having a ratio
similar to that of ordinary nuclear matter, then seed black holes (formed in
stellar collapse) will grow in a Hubble time, due to accretion of the dark
matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence
of the final black hole mass on the galaxy velocity dispersion will be
approximately as observed and the growth rate will show a time dependence
consistent with observations. Other astrophysical consequences of collisional
dark matter and tests of the idea are noted.Comment: 7 pages, no figures, LaTeX2e, Accepted for publication in Phys. Rev.
Lett. Changed conten
Robust Machine Learning Applied to Astronomical Datasets I: Star-Galaxy Classification of the SDSS DR3 Using Decision Trees
We provide classifications for all 143 million non-repeat photometric objects
in the Third Data Release of the Sloan Digital Sky Survey (SDSS) using decision
trees trained on 477,068 objects with SDSS spectroscopic data. We demonstrate
that these star/galaxy classifications are expected to be reliable for
approximately 22 million objects with r < ~20. The general machine learning
environment Data-to-Knowledge and supercomputing resources enabled extensive
investigation of the decision tree parameter space. This work presents the
first public release of objects classified in this way for an entire SDSS data
release. The objects are classified as either galaxy, star or nsng (neither
star nor galaxy), with an associated probability for each class. To demonstrate
how to effectively make use of these classifications, we perform several
important tests. First, we detail selection criteria within the probability
space defined by the three classes to extract samples of stars and galaxies to
a given completeness and efficiency. Second, we investigate the efficacy of the
classifications and the effect of extrapolating from the spectroscopic regime
by performing blind tests on objects in the SDSS, 2dF Galaxy Redshift and 2dF
QSO Redshift (2QZ) surveys. Given the photometric limits of our spectroscopic
training data, we effectively begin to extrapolate past our star-galaxy
training set at r ~ 18. By comparing the number counts of our training sample
with the classified sources, however, we find that our efficiencies appear to
remain robust to r ~ 20. As a result, we expect our classifications to be
accurate for 900,000 galaxies and 6.7 million stars, and remain robust via
extrapolation for a total of 8.0 million galaxies and 13.9 million stars.
[Abridged]Comment: 27 pages, 12 figures, to be published in ApJ, uses emulateapj.cl
Effects of dilute Zn impurities on the uniform magnetic susceptibility of YBa2Cu3O{7-delta}
The effects of dilute Zn impurities on the uniform magnetic susceptibility
are calculated in the normal metallic state for a model of the spin
fluctuations of the layered cuprates. It is shown that scatterings from
extended impurity potentials can lead to a coupling of the q~(pi,pi) and the
q~0 components of the magnetic susceptibility chi(q). Within the presence of
antiferromagnetic correlations, this coupling can enhance the uniform
susceptibility. The implications of this result for the experimental data on Zn
substituted YBa2Cu3O{7-delta} are discussed.Comment: 4 pages, 4 figure
Swift J164449.3+573451 event: generation in the collapsing star cluster?
We discuss the multiband energy release in a model of a collapsing galactic
nucleus, and we try to interpret the unique super-long cosmic gamma-ray event
Swift J164449.3+573451 (GRB 110328A by early classification) in this scenario.
Neutron stars and stellar-mass black holes can form evolutionary a compact
self-gravitating subsystem in the galactic center. Collisions and merges of
these stellar remnants during an avalanche contraction and collapse of the
cluster core can produce powerful events in different bands due to several
mechanisms. Collisions of neutron stars and stellar-mass black holes can
generate gamma-ray bursts (GRBs) similar to the ordinary models of short GRB
origin. The bright peaks during the first two days may also be a consequence of
multiple matter supply (due to matter release in the collisions) and accretion
onto the forming supermassive black hole. Numerous smaller peaks and later
quasi-steady radiation can arise from gravitational lensing, late accretion of
gas onto the supermassive black hole, and from particle acceleration by shock
waves. Even if this model will not reproduce exactly all the Swift
J164449.3+573451 properties in future observations, such collapses of galactic
nuclei can be available for detection in other events.Comment: 7 pages, replaced by the final versio
Online Human Activity Recognition using Low-Power Wearable Devices
Human activity recognition~(HAR) has attracted significant research interest
due to its applications in health monitoring and patient rehabilitation. Recent
research on HAR focuses on using smartphones due to their widespread use.
However, this leads to inconvenient use, limited choice of sensors and
inefficient use of resources, since smartphones are not designed for HAR. This
paper presents the first HAR framework that can perform both online training
and inference. The proposed framework starts with a novel technique that
generates features using the fast Fourier and discrete wavelet transforms of a
textile-based stretch sensor and accelerometer. Using these features, we design
an artificial neural network classifier which is trained online using the
policy gradient algorithm. Experiments on a low power IoT device (TI-CC2650
MCU) with nine users show 97.7% accuracy in identifying six activities and
their transitions with less than 12.5 mW power consumption.Comment: This is in proceedings of ICCAD 2018. The datasets are available at
https://github.com/gmbhat/human-activity-recognitio
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