126 research outputs found
Phase mixing in MOND
Dissipationless collapses in Modified Newtonian Dynamics (MOND) have been
studied by using our MOND particle-mesh N-body code, finding that the projected
density profiles of the final virialized systems are well described by Sersic
profiles with index m<4 (down to m~2 for a deep-MOND collapse). The simulations
provided also strong evidence that phase mixing is much less effective in MOND
than in Newtonian gravity. Here we describe "ad hoc" numerical simulations with
the force angular components frozen to zero, thus producing radial collapses.
Our previous findings are confirmed, indicating that possible differences in
radial orbit instability under Newtonian and MOND gravity are not relevant in
the present context.Comment: 10 pages, 3 figures. To appear in the Proceedings of the
International Workshop "Collective Phenomena in Macroscopic Systems", G.
Bertin, R. Pozzoli, M. Rome, and K.R. Sreenivasan, eds., World Scientific,
Singapor
ECHO: an Eulerian Conservative High Order scheme for general relativistic magnetohydrodynamics and magnetodynamics
We present a new numerical code, ECHO, based on an Eulerian Conservative High
Order scheme for time dependent three-dimensional general relativistic
magnetohydrodynamics (GRMHD) and magnetodynamics (GRMD). ECHO is aimed at
providing a shock-capturing conservative method able to work at an arbitrary
level of formal accuracy (for smooth flows), where the other existing GRMHD and
GRMD schemes yield an overall second order at most. Moreover, our goal is to
present a general framework, based on the 3+1 Eulerian formalism, allowing for
different sets of equations, different algorithms, and working in a generic
space-time metric, so that ECHO may be easily coupled to any solver for
Einstein's equations. Various high order reconstruction methods are implemented
and a two-wave approximate Riemann solver is used. The induction equation is
treated by adopting the Upwind Constrained Transport (UCT) procedures,
appropriate to preserve the divergence-free condition of the magnetic field in
shock-capturing methods. The limiting case of magnetodynamics (also known as
force-free degenerate electrodynamics) is implemented by simply replacing the
fluid velocity with the electromagnetic drift velocity and by neglecting the
matter contribution to the stress tensor. ECHO is particularly accurate,
efficient, versatile, and robust. It has been tested against several
astrophysical applications, including a novel test on the propagation of large
amplitude circularly polarized Alfven waves. In particular, we show that
reconstruction based on a Monotonicity Preserving filter applied to a fixed
5-point stencil gives highly accurate results for smooth solutions, both in
flat and curved metric (up to the nominal fifth order), while at the same time
providing sharp profiles in tests involving discontinuities.Comment: 20 pages, revised version submitted to A&
Laser ion acceleration using a solid target coupled with a low density layer
We investigate by particle-in-cell simulations in two and three dimensions
the laser-plasma interaction and the proton acceleration in multilayer targets
where a low density "near-critical" layer of a few micron thickness is added on
the illuminated side of a thin, high density layer. This target design can be
obtained by depositing a "foam" layer on a thin metallic foil. The presence of
the near-critical plasma strongly increases both the conversion efficiency and
the energy of electrons and leads to enhanced acceleration of proton from a
rear side layer via the Target Normal Sheath Acceleration mechanism. The
electrons of the foam are strongly accelerated in the forward direction and
propagate on the rear side of the target building up a high electric field with
a relatively flat longitudinal profile. In these conditions the maximum proton
energy is up to three times higher than in the case of the bare solid target.Comment: 9 pages, 11 figures. Submitted to Physical Review
Galactic fountains and gas accretion
Star-forming disc galaxies such as the Milky Way need to accrete \gsim 1
of gas each year to sustain their star formation. This gas
accretion is likely to come from the cooling of the hot corona, however it is
still not clear how this process can take place. We present simulations
supporting the idea that this cooling and the subsequent accretion are caused
by the passage of cold galactic-fountain clouds through the hot corona. The
Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas
causes coronal gas to condense in the cloud's wake. For likely parameters of
the Galactic corona and of typical fountain clouds we obtain a global accretion
rate of the order of that required to feed the star formation.Comment: 2 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista &
C.C. Popescu, AIP Conf. Se
Self-consistent stellar dynamical tori
We present preliminary results on a new family of distribution functions that
are able to generate axisymmetric, truncated (i.e., finite size) stellar
dynamical models characterized by toroidal shapes. The relevant distribution
functions generalize those that are known to describe polytropic spheres, for
which all the dynamical and structural properties of the system can be
expressed in explicit form as elementary functions of the system gravitational
potential. The model construction is then completed by a numerical study of the
associated Poisson equation. We note that our axisymmetric models can also
include the presence of an external gravitational field, such as that produced
by a massive disk or by a central mass concentration (e.g., a supermassive
black hole).Comment: 4 pages, 2 figures, to appear in "Plasmas in the Laboratory and in
the Universe: new insights and new challenges", G. Bertin, D. Farina, R.
Pozzoli eds., AIP Conference Proceeding
Fountain-driven gas accretion by the Milky Way
Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in
star-forming disc galaxies, such as the Milky Way, in order to sustain their
star-formation rates. In this work we present the results of a new hydrodynamic
simulation supporting the scenario in which the gas required for star formation
is drawn from the hot corona that surrounds the star-forming disc. In
particular, the cooling of this hot gas and its accretion on to the disc are
caused by the passage of cold galactic fountain clouds through the corona.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand 17-22 April 2011,
European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schulthei
Dissipationless collapse, weak homology and central cores of elliptical galaxies
By means of high-resolution N-body simulations we revisited the
dissipationless collapse scenario for galaxy formation. We considered both
single-component collapses and collapses of a cold stellar distribution in a
live dark matter halo. Single-component collapses lead to stellar systems whose
projected profiles are fitted very well by the Sersic R^(1/m) law with 3.6 < m
< 8. The stellar end-products of collapses in a dark matter halo are still well
described by the R^(1/m) law, but with 1.9 < m < 12, where the lowest m values
are obtained when the halo is dominant. In all the explored cases the profiles
at small radii deviate from their global best-fit R^(1/m) model, being
significantly flatter. The break-radius values are comparable with those
measured in `core' elliptical galaxies, and are directly related to the
coldness of the initial conditions. The dissipationless collapse of initially
cold stellar distributions in pre-existing dark matter haloes may thus have a
role in determining the observed weak homology of elliptical galaxies.Comment: Accepted for publication in MNRAS (11 pages, 10 figures
Dissipationless collapses in MOND
Dissipationless collapses in Modified Newtonian Dynamics (MOND) are studied by using a new particle-mesh N-body code based on our numerical MOND potential solver. We found that low surface-density end-products have shallower inner density profile, flatter radial velocity-dispersion profile, and more radially anisotropic orbital distribution than high surface-density end-products. The projected density profiles of the final virialized systems are well described by Sersic profiles with index m~4, down to m~2 for a deep-MOND collapse. Consistently with observations of elliptical galaxies, the MOND end-products, if interpreted in the context of Newtonian gravity, would appear to have little or no dark matter within the effective radius. However, we found impossible (under the assumption of constant mass-to-light ratio) to simultaneously place the resulting systems on the observed Kormendy, Faber-Jackson and Fundamental Plane relations of elliptical galaxies. Finally, the simulations provide strong evidence that phase mixing is less effective in MOND than in Newtonian gravity
Parametric decay of circularly polarized Alfvén waves: Multidimensional simulations in periodic and open domains
The nonlinear evolution of monochromatic large-amplitude circularly polarized
Alfvén waves subject to the decay instability is studied via numerical
simulations in one, two, and three spatial dimensions.
The asymptotic value of the cross helicity depends strongly on the plasma beta:
in the low beta case multiple decays are observed, with about half of the
energy being transferred to waves propagating in the opposite direction at
lower wave numbers, for each saturation step.
Correspondingly, the other half of the total transverse energy (kinetic and
magnetic) goes into energy carried by the daughter compressive waves and to
the associated shock heating.
In higher beta conditions we find instead that the cross helicity decreases
monotonically with time towards zero, implying an asymptotic balance between
inward and outward Alfvénic modes, a feature similar to the observed
decrease with distance in the solar wind.
Although the instability mainly takes place along the propagation direction,
in the two and three-dimensional case a turbulent cascade occurs also
transverse to the field.
The asymptotic state of density fluctuations appears to be
rather isotropic, whereas a slight preferential cascade
in the transverse direction is seen in magnetic field spectra.
Finally, parametric decay is shown to occur also in a non-periodic domain
with open boundaries, when the mother wave is continuously injected from one
side. In two and three dimensions a strong transverse filamentation
is found at long times, reminiscent of density ray-like
features observed in the extended solar corona and pressure-balanced
structures found in solar wind data
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