3,043 research outputs found
REBOUND: An open-source multi-purpose N-body code for collisional dynamics
REBOUND is a new multi-purpose N-body code which is freely available under an
open-source license. It was designed for collisional dynamics such as planetary
rings but can also solve the classical N-body problem. It is highly modular and
can be customized easily to work on a wide variety of different problems in
astrophysics and beyond.
REBOUND comes with three symplectic integrators: leap-frog, the symplectic
epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open,
periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut
tree to calculate both self-gravity and collisions. These modules are fully
parallelized with MPI as well as OpenMP. The former makes use of a static
domain decomposition and a distributed essential tree. Two new collision
detection modules based on a plane-sweep algorithm are also implemented. The
performance of the plane-sweep algorithm is superior to a tree code for
simulations in which one dimension is much longer than the other two and in
simulations which are quasi-two dimensional with less than one million
particles.
In this work, we discuss the different algorithms implemented in REBOUND, the
philosophy behind the code's structure as well as implementation specific
details of the different modules. We present results of accuracy and scaling
tests which show that the code can run efficiently on both desktop machines and
large computing clusters.Comment: 10 pages, 9 figures, accepted by A&A, source code available at
https://github.com/hannorein/reboun
Formation of trapped surfaces for the spherically symmetric Einstein-Vlasov system
We consider the spherically symmetric, asymptotically flat, non-vacuum
Einstein equations, using as matter model a collisionless gas as described by
the Vlasov equation. We find explicit conditions on the initial data which
guarantee the formation of a trapped surface in the evolution which in
particular implies that weak cosmic censorship holds for these data. We also
analyze the evolution of solutions after a trapped surface has formed and we
show that the event horizon is future complete. Furthermore we find that the
apparent horizon and the event horizon do not coincide. This behavior is
analogous to what is found in certain Vaidya spacetimes. The analysis is
carried out in Eddington-Finkelstein coordinates.Comment: 2
Extended Rein-Sehgal model for tau lepton production
The polarization density matrix formalism is employed to include the final
lepton mass and spin into the popular model by Rein and Sehgal for single pion
neutrinoproduction. We investigate the effect of the lepton mass on the
differential cross sections. The lepton polarization evaluated within the
extended RS model is compared against that follows from the single resonance
production model based upon the Rarita-Schwinger formalism with
phenomenological transition form factors.Comment: Contribution to the 3rd International Workshop on Neutrino-Nucleus
Interactions in the Few-GeV Region, 17-21 March, Gran Sasso (Italy
Global existence and asymptotic behaviour in the future for the Einstein-Vlasov system with positive cosmological constant
The behaviour of expanding cosmological models with collisionless matter and
a positive cosmological constant is analysed. It is shown that under the
assumption of plane or hyperbolic symmetry the area radius goes to infinity,
the spacetimes are future geodesically complete, and the expansion becomes
isotropic and exponential at late times. This proves a form of the cosmic no
hair theorem in this class of spacetimes
Long distance contribution to , - a searching ground mode for new physics
The decay has been sugested as a test for minimal
supersymmetric standard model and for supersymmetric models with R-parity
violating couplings, in view of its extreme smallnesss in the standard model.
We calculate two long distance contributions to this decay, that associated
with and intermediate states and that induced by virtual ,
mesons. The branching ratio due to these contributions is ,
which is somewhat smaller than the standard model short distance result,
leaving this decay free for the search of new physics.Comment: 13 pages, 2 figures, revised versio
Spherically symmetric steady states of galactic dynamics in scalar gravity
The kinetic motion of the stars of a galaxy is considered within the
framework of a relativistic scalar theory of gravitation. This model, even
though unphysical, may represent a good laboratory where to study in a
rigorous, mathematical way those problems, like the influence of the
gravitational radiation on the dynamics, which are still beyond our present
understanding of the physical model represented by the Einstein--Vlasov system.
The present paper is devoted to derive the equations of the model and to prove
the existence of spherically symmetric equilibria with finite radius.Comment: 13 pages, mistypos correcte
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