3,136 research outputs found
Astrocomp: a web service for the use of high performance computers in Astrophysics
Astrocomp is a joint project, developed by the INAF-Astrophysical Observatory
of Catania, University of Roma La Sapienza and Enea. The project has the goal
of providing the scientific community of a web-based user-friendly interface
which allows running parallel codes on a set of high-performance computing
(HPC) resources, without any need for specific knowledge about parallel
programming and Operating Systems commands. Astrocomp provides, also, computing
time on a set of parallel computing systems, available to the authorized user.
At present, the portal makes a few codes available, among which: FLY, a
cosmological code for studying three-dimensional collisionless self-gravitating
systems with periodic boundary conditions; ATD, a parallel tree-code for the
simulation of the dynamics of boundary-free collisional and collisionless
self-gravitating systems and MARA, a code for stellar light curves analysis.
Other codes are going to be added to the portal.Comment: LaTeX with elsart.cls and harvard.sty (included). 7 pages. To be
submitted to a specific journa
Direct Integration of the Collisionless Boltzmann Equation in Six-dimensional Phase Space: Self-gravitating Systems
We present a scheme for numerical simulations of collisionless
self-gravitating systems which directly integrates the Vlasov--Poisson
equations in six-dimensional phase space. By the results from a suite of
large-scale numerical simulations, we demonstrate that the present scheme can
simulate collisionless self-gravitating systems properly. The integration
scheme is based on the positive flux conservation method recently developed in
plasma physics. We test the accuracy of our code by performing several test
calculations including the stability of King spheres, the gravitational
instability and the Landau damping. We show that the mass and the energy are
accurately conserved for all the test cases we study. The results are in good
agreement with linear theory predictions and/or analytic solutions. The
distribution function keeps the property of positivity and remains
non-oscillatory. The largest simulations are run on 64^6 grids. The computation
speed scales well with the number of processors, and thus our code performs
efficiently on massively parallel supercomputers.Comment: 35 pages, 19 figures. Submitted to the Astrophysical Journa
Numerical Models of Binary Neutron Star System Mergers. I.: Numerical Methods and Equilibrium Data for Newtonian Models
The numerical modeling of binary neutron star mergers has become a subject of
much interest in recent years. While a full and accurate model of this
phenomenon would require the evolution of the equations of relativistic
hydrodynamics along with the Einstein field equations, a qualitative study of
the early stages on inspiral can be accomplished by either Newtonian or
post-Newtonian models, which are more tractable. In this paper we offer a
comparison of results from both rotating and non-rotating (inertial) frame
Newtonian calculations. We find that the rotating frame calculations offer
significantly improved accuracy as compared with the inertial frame models.
Furthermore, we show that inertial frame models exhibit significant and
erroneous angular momentum loss during the simulations that leads to an
unphysical inspiral of the two neutron stars. We also examine the dependence of
the models on initial conditions by considering initial configurations that
consist of spherical neutron stars as well as stars that are in equilibrium and
which are tidally distorted. We compare our models those of Rasio & Shapiro
(1992,1994a) and New & Tohline (1997). Finally, we investigate the use of the
isolated star approximation for the construction of initial data.Comment: 32 pages, 19 gif figures, manuscript with postscript figures
available at http://www.astro.sunysb.edu/dswesty/docs/nspap1.p
Turbulent Linewidths as a Diagnostic of Self-Gravity in Protostellar Discs
We use smoothed particle hydrodynamics simulations of massive protostellar
discs to investigate the predicted broadening of molecular lines from discs in
which self-gravity is the dominant source of angular momentum transport. The
simulations include radiative transfer, and span a range of disc-to-star mass
ratios between 0.25 and 1.5. Subtracting off the mean azimuthal flow velocity,
we compute the distribution of the in-plane and perpendicular peculiar velocity
due to large scale structure and turbulence induced by self-gravity. For the
lower mass discs, we show that the characteristic peculiar velocities scale
with the square root of the effective turbulent viscosity parameter, as
expected from local turbulent-disc theory. The derived velocities are
anisotropic, with substantially larger in-plane than perpendicular values. As
the disc mass is increased, the validity of the locally determined turbulence
approximation breaks down, and this is accompanied by anomalously large
in-plane broadening. There is also a high variance due to the importance of
low-m spiral modes. For low-mass discs, the magnitude of in-plane broadening
is, to leading order, equal to the predictions from local disc theory and
cannot constrain the source of turbulence. However, combining our results with
prior evaluations of turbulent broadening expected in discs where the
magnetorotational instability (MRI) is active, we argue that self-gravity may
be distinguishable from the MRI in these systems if it is possible to measure
the anisotropy of the peculiar velocity field with disc inclination.
Furthermore, for large mass discs, the dominant contribution of large-scale
modes is a distinguishing characteristic of self-gravitating turbulence versus
MRI driven turbulence.Comment: 8 pages, 13 figures, accepted for publication in MNRA
High performance astrophysics computing
The application of high end computing to astrophysical problems, mainly in
the galactic environment, is under development since many years at the Dep. of
Physics of Sapienza Univ. of Roma. The main scientific topic is the physics of
self gravitating systems, whose specific subtopics are: i) celestial mechanics
and interplanetary probe transfers in the solar system; ii) dynamics of
globular clusters and of globular cluster systems in their parent galaxies;
iii) nuclear clusters formation and evolution; iv) massive black hole formation
and evolution; v) young star cluster early evolution. In this poster we
describe the software and hardware computational resources available in our
group and how we are developing both software and hardware to reach the
scientific aims above itemized.Comment: 2 pages paper presented at the Conference "Advances in Computational
Astrophysics: methods, tools and outcomes", to be published in the ASP
Conference Series, 2012, vol. 453, R. Capuzzo-Dolcetta, M. Limongi and A.
Tornambe' ed
Spectral element modeling of three dimensional wave propagation in a self-gravitating Earth with an arbitrarily stratified outer core
This paper deals with the spectral element modeling of seismic wave
propagation at the global scale. Two aspects relevant to low-frequency studies
are particularly emphasized. First, the method is generalized beyond the
Cowling approximation in order to fully account for the effects of
self-gravitation. In particular, the perturbation of the gravity field outside
the Earth is handled by a projection of the spectral element solution onto the
basis of spherical harmonics. Second, we propose a new formulation inside the
fluid which allows to account for an arbitrary density stratification. It is
based upon a decomposition of the displacement into two scalar potentials, and
results in a fully explicit fluid-solid coupling strategy. The implementation
of the method is carefully detailed and its accuracy is demonstrated through a
series of benchmark tests.Comment: Sent to Geophysical Journal International on July 29, 200
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