60 research outputs found
Long-Range Correlations in Self-Gravitating N-Body Systems
Observed self-gravitating systems reveal often fragmented non-equilibrium
structures that feature characteristic long-range correlations. However, models
accounting for non-linear structure growth are not always consistent with
observations and a better understanding of self-gravitating -body systems
appears necessary. Because unstable gravitating systems are sensitive to
non-gravitational perturbations we study the effect of different dissipative
factors as well as different small and large scale boundary conditions on
idealized -body systems. We find, in the interval of negative specific heat,
equilibrium properties differing from theoretical predictions made for
gravo-thermal systems, substantiating the importance of microscopic physics and
the lack of consistent theoretical tools to describe self-gravitating gas.
Also, in the interval of negative specific heat, yet outside of equilibrium,
unforced systems fragment and establish transient long-range correlations. The
strength of these correlations depends on the degree of granularity, suggesting
to make the resolution of mass and force coherent. Finally, persistent
correlations appear in model systems subject to an energy flow.Comment: 20 pages, 21 figures. Accepted for publication in A&
Substellar fragmentation in self-gravitating fluids with a major phase transition
The existence of substellar cold H2 globules in planetary nebulae and the
mere existence of comets suggest that the physics of cold interstellar gas
might be much richer than usually envisioned.
We study the case of a cold gaseous medium in ISM conditions which is subject
to a gas-liquid/solid phase transition.
First the equilibrium of general non-ideal fluids is studied using the virial
theorem and linear stability analysis. Then the non-linear dynamics is studied
by using simulations to characterize the expected formation of solid bodies
analogous to comets. The simulations are run with a state of the art molecular
dynamics code (LAMMPS). The long-range gravitational forces can be taken into
account with short-range molecular forces with finite limited computational
resources by using super-molecules, provided the right scaling is followed.
The concept of super-molecule is tested with simulations, allowing us to
correctly satisfy the Jeans instability criterion for one-phase fluids. The
simulations show that fluids presenting a phase transition are gravitationally
unstable as well, independent of the strength of the gravitational potential,
producing two distinct kinds of sub-stellar bodies, those dominated by gravity
("planetoids") and those dominated by molecular attractive force ("comets").
Observations, formal analysis and computer simulations suggest the
possibility of the formation of substellar H2 clumps in cold molecular clouds
due to the combination of phase transition and gravity. Fluids presenting a
phase transition are gravitationally unstable, independent of the strength of
the gravitational potential. Arbitrarily small H2 clumps may form even at
relatively high temperatures up to 400 - 600K, according to virial analysis.
The combination of phase transition and gravity may be relevant for a wider
range of astrophysical situations, such as proto-planetary disks.Comment: 24 pages, 44 figures. accepted for publication in A&
Solid H2 in the interstellar medium
Condensation of H 2 in the interstellar medium (ISM) has long been seen as a
possibility, either by deposition on dust grains or thanks to a phase
transition combined with self-gravity. H 2 condensation might explain the
observed low efficiency of star formation and might help to hide baryons in
spiral galaxies.
Our aim is to quantify the solid fraction of H 2 in the ISM due to a phase
transition including self-gravity for different densities and temperatures in
order to use the results in more complex simulations of the ISM as subgrid
physics.
We used molecular dynamics simulations of fluids at different temperatures
and densities to study the formation of solids. Once the simulations reached a
steady state, we calculated the solid mass fraction, energy increase, and
timescales. By determining the power laws measured over several orders of
magnitude, we extrapolated to lower densities the higher density fluids that
can be simulated with current computers.
The solid fraction and energy increase of fluids in a phase transition are
above 0.1 and do not follow a power law. Fluids out of a phase transition are
still forming a small amount of solids due to chance encounters of molecules.
The solid mass fraction and energy increase of these fluids are linearly
dependent on density and can easily be extrapolated. The timescale is below one
second, the condensation can be considered instantaneous.
The presence of solid H 2 grains has important dynamic implications on the
ISM as they may be the building blocks for larger solid bodies when gravity is
included. We provide the solid mass fraction, energy increase, and timescales
for high density fluids and extrapolation laws for lower densities.Comment: accepted for publication in A&
Bifurcation at Complex Instability
The properties of motion close to the transition of a stable family of
periodic orbits to complex instability is investigated with two symplectic 4D
mappings, natural extensions of the standard mapping. As for the other types of
instabilities new families of periodic orbits may bifurcate at the transition;
but, more generally, families of {\sl isolated invariant curves} bifurcate,
similar to but distinct from a Hopf bifurcation. The evolution of the stable
invariant curves and their bifurcations are described.Comment: 5 pages, self-unpacking uuencoded compressed Postscript, Contribution
at the NATO ASI Conference on "Hamiltonian Systems with Three or More Degrees
of Freedom, Barcelona, Spain, June 19-30, 199
- …