219 research outputs found
The Structure of Isothermal, Self-gravitating Gas Spheres for Softened Gravity
A theory for the structure of isothermal, self-gravitating gas spheres in
pressure equilibrium in a softened gravitational field is developed. The one
parameter spline softening proposed by Hernquist & Katz (1989) is used. We show
that the addition of this extra scale parameter implies that the set of
equilibrium solutions constitute a one-parameter family, rather than the one
and only one isothermal sphere solution for Newtonian gravity. We demonstrate
the perhaps somewhat surprising result that for any finite choice of softening
length and temperature, it is possible to deposit an arbitrarily large mass of
gas in pressure equilibrium and with a non-singular density distribution inside
of r_0 for any r_0 > 0. The theoretical predictions of our models are compared
with the properties of the small, massive, quasi-isothermal gas clumps which
typically form in numerical Tree-SPH simulations of 'passive' galaxy formation
of Milky Way sized galaxies. We find reasonable agreement despite the neglect
of rotational support in the models. We comment on whether the hydrodynamical
resolution in our numerical simulation of galaxy formation is sufficient, and
finally we conclude that one should be cautious, when comparing results of
numerical simulations involving gravitational softening and hydrodynamical
smoothing, with reality.Comment: 22 pages Latex + 12 figure
Protostellar collapse: A comparison between SPH and AMR calculations
The development of parallel supercomputers allows today the detailed study of
the collapse and the fragmentation of prestellar cores with increasingly
accurate numerical simulations. Thanks to the advances in sub-millimeter
observations, a wide range of observed initial conditions enable us to study
the different modes of low-mass star formation. The challenge for the
simulations is to reproduce the observational results. Two main numerical
methods, namely AMR and SPH, are widely used to simulate the collapse and the
fragmentation of prestellar cores. We compare thoroughly these two methods
within their standard framework. We use the AMR code RAMSES and the SPH code
DRAGON. Our physical model is as simple as possible and consists of an
isothermal sphere rotating around the z-axis. We first study the conservation
of angular momentum as a function of the resolution. Then, we explore a wide
range of simulation parameters to study the fragmentation of prestellar cores.
There seems to be a convergence between the two methods, provided resolution in
each case is sufficient. Resolution criteria adapted to our physical cases, in
terms of resolution per Jeans mass, for an accurate description of the
formation of protostellar cores are deduced from the present study. This
convergence is encouraging for future work in simulations of low-mass star
formation, providing the aforementioned criteria are fulfilled.
Higher resolution figures can be downloaded at
http://www-dapnia.cea.fr/Projets/COAST/paper_amrvssph.pdfComment: 16 pages, 16 figures, accepted for publication in A&
COSMOS: A Hybrid N-Body/Hydrodynamics Code for Cosmological Problems
We describe a new hybrid N-body/hydrodynamical code based on the
particle-mesh (PM) method and the piecewise-parabolic method (PPM) for use in
solving problems related to the evolution of large-scale structure, galaxy
clusters, and individual galaxies. The code, named COSMOS, possesses several
new features which distinguish it from other PM-PPM codes. In particular, to
solve the Poisson equation we have written a new multigrid solver which can
determine the gravitational potential of isolated matter distributions and
which properly takes into account the finite-volume discretization required by
PPM. All components of the code are constructed to work with a nonuniform mesh,
preserving second-order spatial differences. The PPM code uses vacuum boundary
conditions for isolated problems, preventing inflows when appropriate. The PM
code uses a second-order variable-timestep time integration scheme. Radiative
cooling and cosmological expansion terms are included. COSMOS has been
implemented for parallel computers using the Parallel Virtual Machine (PVM)
library, and it features a modular design which simplifies the addition of new
physics and the configuration of the code for different types of problems. We
discuss the equations solved by COSMOS and describe the algorithms used, with
emphasis on these features. We also discuss the results of tests we have
performed to establish that COSMOS works and to determine its range of
validity.Comment: 43 pages, 14 figures, submitted to ApJS and revised according to
referee's comment
An algorithm to calculate the transport exponent in strip geometries
An algorithm for solving the random resistor problem by means of the
transfer-matrix approach is presented. Preconditioning by spanning clusters
extraction both reduces the size of the conductivity matrix and speed up the
calculations.Comment: 17 pages, RevTeX2.1, HLRZ - 97/9
Bar Diagnostics in Edge-On Spiral Galaxies. II. Hydrodynamical Simulations
We develop diagnostics based on gas kinematics to identify the presence of a
bar in an edge-on spiral galaxy and determine its orientation. We use
position-velocity diagrams (PVDs) obtained by projecting edge-on
two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy
potential. We show that when a nuclear spiral is formed, the presence of a gap
in the PVDs, between the signature of the nuclear spiral and that of the outer
parts of the disk, reliably indicates the presence of a bar. This gap is due to
the presence of shocks and inflows in the simulations, leading to a depletion
of the gas in the outer bar region. If no nuclear spiral signature is present
in a PVD, only indirect arguments can be used to argue for the presence of a
bar. The shape of the signature of the nuclear spiral, and to a lesser extent
that of the outer bar region, allows to determine the orientation of the bar
with respect to the line-of-sight. The presence of dust can also help to
discriminate between viewing angles on either side of the bar. Simulations
covering a large fraction of parameter space constrain the bar properties and
mass distribution of observed galaxies. The strongest constraint comes from the
presence or absence of the signature of a nuclear spiral in the PVD.Comment: 25 pages (AASTeX, aaspp4.sty), 11 jpg figures. Accepted for
publication in The Astrophysical Journal. Online manuscript with PostScript
figures available at: http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
On the role of shock waves in galaxy cluster evolution
Numerical simulations of galaxy clusters including two species -- baryonic
gas and dark matter particles --are presented. Cold Dark Matter spectrum,
Gaussian statistics and flat universe are assumed. The dark matter component is
evolved numerically by means of a standard particle mesh method. The evolution
of the baryonic component has been studied numerically by using a
multidimensional (3D) hydrodynamical code based on modern high resolution shock
capturing techniques. These techniques are specially designed for treating
accurately complex flows in which shocks appear and interact. With this
picture, the role of shock waves in the formation and evolution of rich galaxy
clusters is analyzed. Our results display two well differenced morphologies of
the shocked baryonic matter: filamentary at early epochs and quasi-spherical at
low redshifts.Comment: 28 pages, LaTex with aasms4.sty, 12 postscript figures, tared,gziped
and uuencoded. Accepted in Ap
Mass transfer in eccentric binaries: the new Oil-on-Water SPH technique
To measure the onset of mass transfer in eccentric binaries we have developed
a two-phase SPH technique. Mass transfer is important in the evolution of close
binaries, and a key issue is to determine the separation at which mass transfer
begins. The circular case is well understood and can be treated through the use
of the Roche formalism. To treat the eccentric case we use a newly-developed
two phase system. The body of the donor star is made up from high-mass "water"
particles, whilst the atmosphere is modelled with low-mass "oil" particles.
Both sets of particles take part fully in SPH interactions. To test the
technique we model circular mass-transfer binaries containing a 0.6 Msun donor
star and a 1 Msun white dwarf; such binaries are thought to form cataclysmic
variable (CV) systems. We find that we can reproduce a reasonable CV
mass-transfer rate, and that our extended atmosphere gives a separation that is
too large by aproximately 16%, although its pressure scale height is
considerably exaggerated. We use the technique to measure the semi-major axis
required for the onset of mass transfer in binaries with a mass ratio of q=0.6
and a range of eccentricities. Comparing to the value obtained by considering
the instantaneous Roche lobe at pericentre we find that the radius of the star
required for mass transfer to begin decreases systematically with increasing
eccentricity.Comment: 9 pages, 8 figures, accepted by MNRA
NeXSPheRIO results on azimuthal anisotropy in Au-Au collisions at 200A GeV
In this work, we present the results obtained by the hydrodynamic code
NeXSPheRIO on anisotropic flows. In our calculation, we made use of
event-by-event fluctuating initial conditions, and chemical freeze-out was
explicitly implemented. We studied directed flow, elliptic flow and forth
harmonic coefficient for various hadrons at different centrality windows for
Au+Au collisions at 200 AGeV. The results are discussed and compared with
experimental data from RHIC.Comment: 6 pages and 6 figures, sqm2008 contributio
Conservation Laws in Smooth Particle Hydrodynamics: the DEVA Code
We describe DEVA, a multistep AP3M-like-SPH code particularly designed to
study galaxy formation and evolution in connection with the global cosmological
model. This code uses a formulation of SPH equations which ensures both energy
and entropy conservation by including the so-called \bn h terms. Particular
attention has also been paid to angular momentum conservation and to the
accuracy of our code. We find that, in order to avoid unphysical solutions, our
code requires that cooling processes must be implemented in a non-multistep
way.
We detail various cosmological simulations which have been performed to test
our code and also to study the influence of the \bn h terms. Our results
indicate that such correction terms have a non-negligible effect on some
cosmological simulations, especially on high density regions associated either
to shock fronts or central cores of collapsed objects. Moreover, they suggest
that codes paying a particular attention to the implementation of conservation
laws of physics at the scales of interest, can attain good accuracy levels in
conservation laws with limited computational resources.Comment: 36 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
Critical Dynamics of Gelation
Shear relaxation and dynamic density fluctuations are studied within a Rouse
model, generalized to include the effects of permanent random crosslinks. We
derive an exact correspondence between the static shear viscosity and the
resistance of a random resistor network. This relation allows us to compute the
static shear viscosity exactly for uncorrelated crosslinks. For more general
percolation models, which are amenable to a scaling description, it yields the
scaling relation for the critical exponent of the shear
viscosity. Here is the thermal exponent for the gel fraction and
is the crossover exponent of the resistor network. The results on the shear
viscosity are also used in deriving upper and lower bounds on the incoherent
scattering function in the long-time limit, thereby corroborating previous
results.Comment: 34 pages, 2 figures (revtex, amssymb); revised version (minor
changes
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