8,919 research outputs found
Generalized Du Fort-Frankel methods for parabolic initial boundary value problems
The Du Fort-Frankel difference scheme is generalized to difference operators of arbitrary high order accuracy in space and to arbitrary order of the parabolic differential operator. Spectral methods can also be used to approximate the spatial part of the differential operator. The scheme is explicit, and it is unconditionally stable for the initial value problem. Stable boundary conditions are given for two different fourth order accurate space approximations
On the Navier-Stokes equations with constant total temperature
For various applications in fluid dynamics, it is assumed that the total temperature is constant. Therefore, the energy equation can be replaced by an algebraic relation. The resulting set of equations in the inviscid case is analyzed. It is shown that the system is strictly hyperbolic and well posed for the initial value problems. Boundary conditions are described such that the linearized system is well posed. The Hopscotch method is investigated and numerical results are presented
Observation of coherent electroproduction on deuterons at large momentum transfer
The first experimental results for coherent -electroproduction on the
deuteron, , at large momentum transfer, are reported. The
experiment was performed at Jefferson Laboratory at an incident electron energy
of 4.05 GeV. A large pion production yield has been observed in a kinematical
region for 1.11.8 GeV, from threshold to 200 MeV excitation energy
in the system. The -dependence is compared with theoretical
predictions.Comment: 26 page
The nature of turbulence in OMC1 at the star forming scale: observations and simulations
Aim: To study turbulence in the Orion Molecular Cloud (OMC1) by comparing
observed and simulated characteristics of the gas motions.
Method: Using a dataset of vibrationally excited H2 emission in OMC1
containing radial velocity and brightness which covers scales from 70AU to
30000AU, we present the transversal structure functions and the scaling of the
structure functions with their order. These are compared with the predictions
of two-dimensional projections of simulations of supersonic hydrodynamic
turbulence.
Results: The structure functions of OMC1 are not well represented by power
laws, but show clear deviations below 2000AU. However, using the technique of
extended self-similarity, power laws are recovered at scales down to 160AU. The
scaling of the higher order structure functions with order deviates from the
standard scaling for supersonic turbulence. This is explained as a selection
effect of preferentially observing the shocked part of the gas and the scaling
can be reproduced using line-of-sight integrated velocity data from subsets of
supersonic turbulence simulations. These subsets select regions of strong flow
convergence and high density associated with shock structure. Deviations of the
structure functions in OMC1 from power laws cannot however be reproduced in
simulations and remains an outstanding issue.Comment: 12 pages, 8 figures, accepted A&A. Revised in response to referee.
For higher resolution, see http://www.astro.phys.au.dk/~maikeng/sim_paper
Extragalactic gamma-ray signal from Dark Matter annihilation: a power spectrum based computation
We revisit the computation of the extragalactic gamma-ray signal from
cosmological dark matter annihilations. The prediction of this signal is
notoriously model dependent, due to different descriptions of the clumpiness of
the dark matter distribution at small scales, responsible for an enhancement
with respect to the smoothly distributed case. We show how a direct computation
of this "flux multiplier" in terms of the nonlinear power spectrum offers a
conceptually simpler approach and may ease some problems, such as the
extrapolation issue. In fact very simple analytical recipes to construct the
power spectrum yield results similar to the popular Halo Model expectations,
with a straightforward alternative estimate of errors. For this specific
application, one also obviates to the need of identifying (often
literature-dependent) concepts entering the Halo Model, to compare different
simulations.Comment: 6 pages, 2 figures; minor changes, additional references, matches
published version; Mon. Not. R. Astron. Soc. Letters, Feb. 7 (2012
Observations of spatial and velocity structure in the Orion Molecular Cloud
Observations are reported of H2 IR emission in the S(1) v=1-0 line at 2.121
microns in the Orion Molecular Cloud, OMC1, using the GriF instrument on the
Canada-France-Hawaii Telescope. GriF is a combination of adaptive optics and
Fabry-Perot interferometry, yielding a spatial resolution of 0.15" to 0.18" and
a velocity discrimination as high as 1 km/s. Thanks to the high spatial and
velocity resolution of the GriF data, 193 bright H2 emission regions can be
identified in OMC1. The general characteristics of these features are described
in terms of radial velocities, brightness and spatial displacement of maxima of
velocity and brightness, the latter to yield the orientation of flows in the
plane of the sky. Strong spatial correlation between velocity and bright H2
emission is found and serves to identify many features as shocks. Important
results are: (i) velocities of the excited gas illustrate the presence of a
zone to the south of BN-IRc2 and Peak 1, and the west of Peak 2, where there is
a powerful blue-shifted outflow with an average velocity of -18 km/s. This is
shown to be the NIR counterpart of an outflow identified in the radio from
source I, a very young O-star. (ii) There is a band of weak velocity features
(<5 km/s) in Peak 1 which may share a common origin through an explosive event,
in the BN-IRc2 region, with the fast-moving fingers (or bullets) to the NW of
OMC1. (iii) A proportion of the flows are likely to represent sites of low mass
star formation and several regions show multiple outflows, probably indicative
of multiple star formation within OMC1. The high spatial and velocity
resolution of the GriF data show these and other features in more detail than
has previously been possible.Comment: 27 pages, 19 figures, submitted to A&A Version 2: Several additions,
including a section on protostellar candidates in OMC1, have been made based
on the referee's suggestions v3: corrected typograph
On the exponential transform of lemniscates
It is known that the exponential transform of a quadrature domain is a
rational function for which the denominator has a certain separable form. In
the present paper we show that the exponential transform of lemniscate domains
in general are not rational functions, of any form. Several examples are given
to illustrate the general picture. The main tool used is that of polynomial and
meromorphic resultants.Comment: 19 pages, to appear in the Julius Borcea Memorial Volume, (eds.
Petter Branden, Mikael Passare and Mihai Putinar), Trends in Mathematics,
Birkhauser Verla
Extragalactic gamma-ray signal from dark matter annihilation: an appraisal
We re-evaluate the extragalactic gamma-ray flux prediction from dark matter
annihilation in the approach of integrating over the nonlinear matter power
spectrum, extrapolated to the free-streaming scale. We provide an estimate of
the uncertainty based entirely on available N-body simulation results and
minimal theoretical assumptions. We illustrate how an improvement in the
simulation resolution, exemplified by the comparison between the Millennium and
Millennium II simulations, affects our estimate of the flux uncertainty and we
provide a "best guess" value for the flux multiplier, based on the assumption
of stable clustering for the dark matter perturbations described as a
collision-less fluid. We achieve results comparable to traditional Halo Model
calculations, but with a much simpler procedure and a more general approach, as
it relies only on one, directly measurable quantity. In addition we discuss the
extension of our calculation to include baryonic effects as modeled in
hydrodynamical cosmological simulations and other possible sources of
uncertainty that would in turn affect indirect dark matter signals. Upper limit
on the integrated power spectrum from supernovae lensing magnification are also
derived and compared with theoretical expectations.Comment: 20 pages, 9 figures, 1 table. Updated to match the published version.
New material and figures added, conclusions unchange
Disentangling the Hercules stream
Using high-resolution spectra of nearby F and G dwarf stars, we have
investigated the detailed abundance and age structure of the Hercules stream.
We find that the stars in the stream have a wide range of stellar ages,
metallicities, and element abundances. By comparing to existing samples of
stars in the solar neighbourhood with kinematics typical of the Galactic thin
and thick disks we find that the properties of the Hercules stream distinctly
separate into the abundance and age trends of the two disks. Hence, we find it
unlikely that the Hercules stream is a unique Galactic stellar population, but
rather a mixture of thin and thick disk stars. This points toward a dynamical
origin for the Hercules stream, probably caused by the Galactic bar.Comment: Accepted for publication in ApJ Letter
A method for detection of structure
Context. In order to understand the evolution of molecular clouds it is
important to identify the departures from self-similarity associated with the
scales of self-gravity and the driving of turbulence.
Aims. A method is described based on structure functions for determining
whether a region of gas, such as a molecular cloud, is fractal or contains
structure with characteristic scale sizes.
Methods. Using artificial data containing structure it is shown that
derivatives of higher order structure functions provide a powerful way to
detect the presence of characteristic scales should any be present and to
estimate the size of such structures. The method is applied to observations of
hot H2 in the Kleinman-Low nebula, north of the Trapezium stars in the Orion
Molecular Cloud, including both brightness and velocity data. The method is
compared with other techniques such as Fourier transform and histogram
techniques.
Results. It is found that the density structure, represented by H2 emission
brightness in the K-band (2-2.5micron), exhibits mean characteristic sizes of
110, 550, 1700 and 2700AU. The velocity data show the presence of structure at
140, 1500 and 3500AU. Compared with other techniques such as Fourier transform
or histogram, the method appears both more sensitive to characteristic scales
and easier to interpret.Comment: Astronomy and Astrophysics, in pres
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