8,836 research outputs found
Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments
We present analytical approximations for calculating the scattering,
absorption and escape of nonionizing photons from a spherically symmetric
two-phase clumpy medium, with either a central point source of isotropic
radiation, a uniform distribution of isotropic emitters, or uniformly
illuminated by external sources. The analytical approximations are based on the
mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman,
combined with escape and absorption probability formulae for homogeneous media.
The accuracy of the approximations is examined by comparison with 3D Monte
Carlo simulations of radiative transfer, including multiple scattering. Our
studies show that the combined mega-grains and escape/absorption probability
formulae provide a good approximation of the escaping and absorbed radiation
fractions for a wide range of parameters characterizing the medium. A realistic
test is performed by modeling the absorption of a starlike source of radiation
by interstellar dust in a clumpy medium, and by calculating the resulting
equilibrium dust temperatures and infrared emission spectrum of both the clumps
and the interclump medium. In particular, we find that the temperature of dust
in clumps is lower than in the interclump medium if clumps are optically thick.
Comparison with Monte Carlo simulations of radiative transfer in the same
environment shows that the analytic model yields a good approximation of dust
temperatures and the emerging UV to FIR spectrum of radiation for all three
types of source distributions mentioned above. Our analytical model provides a
numerically expedient way to estimate radiative transfer in a variety of
interstellar conditions and can be applied to a wide range of astrophysical
environments, from star forming regions to starburst galaxies.Comment: 55 pages, 27 figures. ApJ 523 (1999), in press. Corrected equations
and text so as to be same as ApJ versio
A New Way to Make Waves
I describe a new algorithm for solving nonlinear wave equations. In this
approach, evolution takes place on characteristic hypersurfaces. The algorithm
is directly applicable to electromagnetic, Yang-Mills and gravitational fields
and other systems described by second differential order hyperbolic equations.
The basic ideas should also be applicable to hydrodynamics. It is an especially
accurate and efficient way for simulating waves in regions where the
characteristics are well behaved. A prime application of the algorithm is to
Cauchy-characteristic matching, in which this new approach is matched to a
standard Cauchy evolution to obtain a global solution. In a model problem of a
nonlinear wave, this proves to be more accurate and efficient than any other
present method of assigning Cauchy outer boundary conditions. The approach was
developed to compute the gravitational wave signal produced by collisions of
two black holes. An application to colliding black holes is presented.Comment: In Proceeding of CIMENICS 2000, The Vth International Congress on
Numerical Methods in Engineering and Applied Science (Puerto La Cruz,
Venezuela, March 2000
Development and application of a three dimensional numerical model for predicting pollutant and sediment transport using an Eulerian-Lagrangian marker particle technique
A computer coded Lagrangian marker particle in Eulerian finite difference cell solution to the three dimensional incompressible mass transport equation, Water Advective Particle in Cell Technique, WAPIC, was developed, verified against analytic solutions, and subsequently applied in the prediction of long term transport of a suspended sediment cloud resulting from an instantaneous dredge spoil release. Numerical results from WAPIC were verified against analytic solutions to the three dimensional incompressible mass transport equation for turbulent diffusion and advection of Gaussian dye releases in unbounded uniform and uniformly sheared uni-directional flow, and for steady-uniform plug channel flow. WAPIC was utilized to simulate an analytic solution for non-equilibrium sediment dropout from an initially vertically uniform particle distribution in one dimensional turbulent channel flow
WFIRST Ultra-Precise Astrometry II: Asteroseismology
WFIRST microlensing observations will return high-precision parallaxes,
sigma(pi) < 0.3 microarcsec, for the roughly 1 million stars with H<14 in its
2.8 deg^2 field toward the Galactic bulge. Combined with its 40,000 epochs of
high precision photometry (~0.7 mmag at H_vega=14 and ~0.1 mmag at H=8), this
will yield a wealth of asteroseismic data of giant stars, primarily in the
Galactic bulge but including a substantial fraction of disk stars at all
Galactocentric radii interior to the Sun. For brighter stars, the astrometric
data will yield an external check on the radii derived from the two
asteroseismic parameters, and nu_max, while for the fainter ones, it
will enable a mass measurement from the single measurable asteroseismic
parameter nu_max. Simulations based on Kepler data indicate that WFIRST will be
capable of detecting oscillations in stars from slightly less luminous than the
red clump to the tip of the red giant branch, yielding roughly 1 million
detections.Comment: 13 pages, 6 figures, submitted to JKA
A fast and explicit algorithm for simulating the dynamics of small dust grains with smoothed particle hydrodynamics
We describe a simple method for simulating the dynamics of small grains in a
dusty gas, relevant to micron-sized grains in the interstellar medium and
grains of centimetre size and smaller in protoplanetary discs. The method
involves solving one extra diffusion equation for the dust fraction in addition
to the usual equations of hydrodynamics. This "diffusion approximation for
dust" is valid when the dust stopping time is smaller than the computational
timestep. We present a numerical implementation using Smoothed Particle
Hydrodynamics (SPH) that is conservative, accurate and fast. It does not
require any implicit timestepping and can be straightforwardly ported into
existing 3D codes.Comment: 15 pages, 10 figures, accepted to MNRAS. Code implementation (ndspmhd
v2.1) and setup of test problems available at:
http://users.monash.edu.au/~dprice/ndspmhd/. v3: sign errors fixed as per
erratum to published pape
Simulating the gamma-ray emission from galaxy clusters: a universal cosmic ray spectrum and spatial distribution
Entering a new era of high-energy gamma-ray experiments, there is an exciting
quest for the first detection of gamma-ray emission from clusters of galaxies.
To complement these observational efforts, we use high-resolution simulations
of a broad sample of galaxy clusters, and follow self-consistent cosmic ray
(CR) physics using an improved spectral description. We study CR proton spectra
as well as the different contributions of the pion decay and inverse Compton
emission to the total flux and present spectral index maps. We find a universal
spectrum of the CR component in clusters with surprisingly little scatter
across our cluster sample. The spatial CR distribution also shows approximate
universality; it depends however on the cluster mass. This enables us to derive
a semi-analytic model for both, the distribution of CRs as well as the
pion-decay gamma-ray emission that results from hadronic CR interactions with
ambient gas protons. In addition, we provide an analytic framework for the
inverse Compton emission that is produced by shock-accelerated CR electrons and
valid in the full gamma-ray energy range. Combining the complete sample of the
brightest X-ray clusters observed by ROSAT with our gamma-ray scaling
relations, we identify the brightest clusters for the gamma-ray space telescope
Fermi and current imaging air Cherenkov telescopes (MAGIC, HESS, VERITAS). We
reproduce the result in Pfrommer (2008), but provide somewhat more conservative
predictions for the fluxes in the energy regimes of Fermi and imaging air
Cherenkov telescopes when accounting for the bias of `artificial galaxies' in
cosmological simulations. We find that it will be challenging to detect cluster
gamma-ray emission with Fermi after the second year but this mission has the
potential of constraining interesting values of the shock acceleration
efficiency after several years of surveying.Comment: 33 pages, 25 figures. Accepted for publication in MNRAS: Typos
corrected and primary IC analysis now includes the Klein-Nishina effect and a
simple-to-use semi-analytic formul
Vortex Quantum Nucleation and Tunneling in Superconducting Thin Films: Role of Dissipation and Periodic Pinning
We investigate the phenomenon of decay of a supercurrent in a superconducting
thin film in the absence of an applied magnetic field. The resulting
zero-temperature resistance derives from two equally possible mechanisms: 1)
quantum tunneling of vortices from the edges of the sample; and 2) homogeneous
quantum nucleation of vortex-antivortex pairs in the bulk of the sample,
arising from the instability of the Magnus field's ``vacuum''. We study both
situations in the case where quantum dissipation dominates over the inertia of
the vortices. We find that the vortex tunneling and nucleation rates have a
very rapid dependence on the current density driven through the sample.
Accordingly, whilst normally the superconductor is essentially resistance-free,
for the high current densities that can be reached in high- films a
measurable resistance might develop. We show that edge-tunneling appears
favoured, but the presence of pinning centres and of thermal fluctuations leads
to an enhancement of the nucleation rates. In the case where a periodic pinning
potential is artificially introduced in the sample, we show that
current-oscillations will develop indicating an effect specific to the
nucleation mechanism where the vortex pair-production rate, thus the
resistance, becomes sensitive to the corrugation of the pinning substrate. In
all situations, we give estimates for the observability of the studied
phenomena.Comment: 8 pages (LaTeX), 2 postscript figures. Invited talk to the SATT8 (8th
Italian Meeting on High-T_c Superconductivity), Como (Italy), Villa Olmo, 1-4
October 1996, to be published in La Rivista del Nuovo Cimento
Dynamics of Primordial Black Hole Formation
We present a numerical investigation of the gravitational collapse of
horizon-size density fluctuations to primordial black holes (PBHs) during the
radiation-dominated phase of the Early Universe. The collapse dynamics of three
different families of initial perturbation shapes, imposed at the time of
horizon crossing, is computed. The perturbation threshold for black hole
formation, needed for estimations of the cosmological PBH mass function, is
found to be rather than the generally employed
, if is defined as \Delta M/\mh, the
relative excess mass within the initial horizon volume. In order to study the
accretion onto the newly formed black holes, we use a numerical scheme that
allows us to follow the evolution for long times after formation of the event
horizon. In general, small black holes (compared to the horizon mass at the
onset of the collapse) give rise to a fluid bounce that effectively shuts off
accretion onto the black hole, while large ones do not. In both cases, the
growth of the black hole mass owing to accretion is insignificant. Furthermore,
the scaling of black hole mass with distance from the formation threshold,
known to occur in near-critical gravitational collapse, is demonstrated to
apply to primordial black hole formation.Comment: 10 pages, 8 figures, revtex style, submitted to PR
A subsonic transonic and supersonic nozzle flow by the inverse technique
Inverse solution of two dimensional gas dynamic flow fields of rotational or irrotational characte
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