8,038 research outputs found
Microgravity studies of aggregation in particulate clouds
Aggregation in clouds of submillimeter quartz and volcanic ash particles was studied in microgravity. Particle clouds generated by pulses of air immediately formed electrostatic filamentary aggregates upon cessation of air turbulence. Manual agitation of experiment chambers produced cm-size loose grain clusters which voraciously scavenged free-floating material in their vicinity. A dipole model accounts for these observations. Experimental results have ramifications for the behavior of natural cloud systems and primary accretion of solids in the early solar nebula
Relativistic Jets and Long-Duration Gamma-ray Bursts from the Birth of Magnetars
We present time-dependent axisymmetric magnetohydrodynamic simulations of the
interaction of a relativistic magnetized wind produced by a proto-magnetar with
a surrounding stellar envelope, in the first seconds after core
collapse. We inject a super-magnetosonic wind with ergs
s into a cavity created by an outgoing supernova shock. A strong
toroidal magnetic field builds up in the bubble of plasma and magnetic field
that is at first inertially confined by the progenitor star. This drives a jet
out along the polar axis of the star, even though the star and the magnetar
wind are each spherically symmetric. The jet has the properties needed to
produce a long-duration gamma-ray burst (GRB). At s after core bounce,
the jet has escaped the host star and the Lorentz factor of the material in the
jet at large radii cm is similar to that in the magnetar wind
near the source. Most of the spindown power of the central magnetar escapes via
the relativistic jet. There are fluctuations in the Lorentz factor and energy
flux in the jet on second timescale. These may contribute to
variability in GRB emission (e.g., via internal shocks).Comment: 5 pages, 3 figures, accepted in MNRAS letter, presented at the
conference "Astrophysics of Compact Objects", 1-7 July, Huangshan, Chin
The Replication Argument for Incompatibilism
In this paper, I articulate an argument for incompatibilism about moral responsibility and determinism. My argument comes in the form of an extended story, modeled loosely on Peter van Inwagen’s “rollback argument” scenario. I thus call it “the replication argument.” As I aim to bring out, though the argument is inspired by so-called “manipulation” and “original design” arguments, the argument is not a version of either such argument—and plausibly has advantages over both. The result, I believe, is a more convincing incompatibilist argument than those we have considered previously
The 2nd order renormalization group flow for non-linear sigma models in 2 dimensions
We show that for two dimensional manifolds M with negative Euler
characteristic there exists subsets of the space of smooth Riemannian metrics
which are invariant and either parabolic or backwards-parabolic for the 2nd
order RG flow. We also show that solutions exists globally on these sets.
Finally, we establish the existence of an eternal solution that has both a UV
and IR limit, and passes through regions where the flow is parabolic and
backwards-parabolic
Radiation Pressure Supported Starburst Disks and AGN Fueling
We consider the structure of marginally Toomre-stable starburst disks under
the assumption that radiation pressure on dust grains provides the dominant
vertical support against gravity. This is particularly appropriate when the
disk is optically thick to its own IR radiation, as in the central regions of
ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for
star formation changes in the optically-thick limit, with the star formation
rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface
density and kappa is the mean opacity. We show that optically thick starburst
disks have a characteristic flux and dust effective temperature of F ~ 10^{13}
L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with
observations and find good agreement. We extend our model from many-hundred
parsec scales to sub-parsec scales and address the problem of fueling AGN. We
assume that angular momentum transport proceeds via global torques rather than
a local viscosity. We account for the radial depletion of gas due to star
formation and find a strong bifurcation between two classes of disk models: (1)
solutions with a starburst on large scales that consumes all of the gas with
little fueling of a central AGN and (2) models with an outer large-scale
starburst accompanied by a more compact starburst on 1-10 pc scales and a
bright central AGN. The luminosity of the latter models is in many cases
dominated by the AGN. We show that the vertical thickness of the starburst disk
on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration
in some Type 2 AGN. We also argue that the disk of young stars in the Galactic
Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes,
discussion tightened, references adde
Computation of thermodynamic and transport properties to predict thermophoretic effects in an argon-krypton mixture
Thermophoresis is the movement of molecules caused by a temperature gradient. Here we report the results of a study of thermophoresis using non-equilibrium molecular dynamics simulations of a confined argon-krypton fluid subject to two different temperatures at thermostated walls. The resulting temperature profile between the walls is used along with the Soret coefficient to predict the concentration profile that develops across the channel. We obtain the Soret coefficient by calculating the mutual diffusion and thermal diffusion coefficients. We report an appropriate method for calculating the transport coefficients for binary systems, using the Green-Kubo integrals and radial distribution functions obtained from equilibrium molecular dynamics simulations of the bulk fluid. Our method has the unique advantage of separating the mutual diffusion and thermal diffusion coef- ficients, and calculating the sign and magnitude of their individual contributions to thermophoresis in binary mixtures
Frontostriatal Maturation Predicts Cognitive Control Failure to Appetitive Cues in Adolescents
Adolescent risk-taking is a public health issue that increases the odds of poor lifetime outcomes. One factor thought to influence adolescents' propensity for risk-taking is an enhanced sensitivity to appetitive cues, relative to an immature capacity to exert sufficient cognitive control. We tested this hypothesis by characterizing interactions among ventral striatal, dorsal striatal, and prefrontal cortical regions with varying appetitive load using fMRI scanning. Child, teen, and adult participants performed a go/no-go task with appetitive (happy faces) and neutral cues (calm faces). Impulse control to neutral cues showed linear improvement with age, whereas teens showed a nonlinear reduction in impulse control to appetitive cues. This performance decrement in teens was paralleled by enhanced activity in the ventral striatum. Prefrontal cortical recruitment correlated with overall accuracy and showed a linear response with age for no-go versus go trials. Connectivity analyses identified a ventral frontostriatal circuit including the inferior frontal gyrus and dorsal striatum during no-go versus go trials. Examining recruitment developmentally showed that teens had greater between-subject ventral-dorsal striatal coactivation relative to children and adults for happy no-go versus go trials. These findings implicate exaggerated ventral striatal representation of appetitive cues in adolescents relative to an intermediary cognitive control response. Connectivity and coactivity data suggest these systems communicate at the level of the dorsal striatum differentially across development. Biased responding in this system is one possible mechanism underlying heightened risk-taking during adolescence
Auditing and Assessing Air Quality in Concentrated Feeding Operations
The potential adverse effects of concentrated animal feeding operations (CAFO) on the environment are a growing concern. The air quality issues of most concern to CAFO vary but generally include ammonia, hydrogen sulfide, particulate matter, volatile organic compounds, greenhouse gases, and odors. Air pollutants may be regulated by federal and state laws or by nuisance complaints. The United States Environmental Protection Agency, and poultry, swine, and dairy industries recently agreed to the National Air Emissions Monitoring System to fund research on atmospheric emissions from production farms in the United States. Air quality regulations may be based on actual emissions, atmospheric concentrations, or human perception, or via limiting the size or location of CAFO. Measuring the concentrations or emissions of most air pollutants is expensive and complex. Because of spatial and temporal variability, concentrations and emissions must be measured continuously over an extended period of time. Because different methods or models can give different results with the same data set, a multitude of methods should be used simultaneously to assure emissions are reasonable. The “best” method to measure concentrations and emissions will depend upon atmospheric concentrations, cost, facility characteristics, objectives, and other factors. In the future, requirements for monitoring of air emissions from CAFO will probably increase. Reliable processbased models need to be developed so that emissions of air pollutants can be estimated from readily obtained diet, animal, facility, and environmental variables. Auditors will need to be trained in a variety of disciplines including animal sciences, chemistry, engineering, micrometeorology, instrumentation, modeling, and logic
Alignments of the Dominant Galaxies in Poor Clusters
We have examined the orientations of brightest cluster galaxies (BCGs) in
poor MKW and AWM clusters and find that, like their counterparts in richer
Abell clusters, poor cluster BCGs exhibit a strong propensity to be aligned
with the principal axes of their host clusters as well as the surrounding
distribution of nearby (< 20/h Mpc) Abell clusters. The processes responsible
for dominant galaxy alignments are therefore independent of cluster richness.
We argue that these alignments most likely arise from anisotropic infall of
material into clusters along large-scale filaments.Comment: 8 pages, 5 figure
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