4,440 research outputs found
Isolated and non-isolated dark matter halos and the NFW profile
We compare the conclusions reached using the coarse-graining technique
employed by Henriksen (2004) for a one degree of freedom (per particle)
collisionless system, to those presented in a paper by Binney (2004) based on
an exact one degree of freedom model. We find agreement in detail but in
addition we show that the isolated 1D system is self-similar and therefore
unrelaxed. Fine graining of this system recovers much less prominent wave-like
structure than in a spherically symmetric isotropic 3D system. The rate of
central flattening is also reduced in the 1D system. We take this to be
evidence that relaxation of collisionless systems proceeds ultimately by way of
short wavelength Landau damping. N-body systems, both real and simulated, can
be trapped in an incompletely relaxed state because of a break in the cascade
of energy to small scales. This may be due to the rapid dissipation of the
small scale oscillations in an isolated system, to the existence of conserved
quantities such as angular momentum, or to the failure in simulations to
resolve sub-Jeans length scales. Such a partially relaxed state appears to be
the NFW state, and is to be expected especially in young systems. The NFW core
is shown to be isolated. In non-isolated systems continuing coarse-grained
relaxation should be towards a density core in solid body rotation.Comment: 14 pages, MNRAS, submitted 21 June 200
Distribution of Gamma-Ray Bursts in Halo Neutron Star-Comet Models
The motions of comets and neutron stars have been integrated over five
billion years in the Galactic potential to determine a gamma-ray burst
distribution, presuming that bursts are the result of interactions between
these two families of objects. The comets originate in two distinct populations
- one from ejection by stars in the Galactic disk, and the other from ejection
by stars in globular clusters. No choice of the free parameters resulted in
agreement with both the isotropy data and the data.Comment: 4 pages LaTex and two style files, tarred, compressed, and uuencoded.
One postscript figure. To appear in Astrophysics and Space Science as part of
the proceedings of the 29th ESLAB Symposium 'Toward the Source of Gamma-Ray
Bursts' held in Noordwijk, 1995. A postscript version can be found at
http://astro.queensu.ca/~mark/preprints.htm
On Stationary, Self-Similar Distributions of a Collisionless, Self-Gravitating, Gas
We study systematically stationary solutions to the coupled Vlasov and
Poisson equations which have `self-similar' or scaling symmetry in phase space.
In particular, we find analytically {\it all} spherically symmetric
distribution functions where the mass density and gravitational potential are
strict power laws in , the distance from the symmetry point. We treat as
special cases, systems built from purely radial orbits and systems that are
isotropic in velocity space. We then discuss systems with arbitrary velocity
space anisotropy finding a new and very general class of distribution
functions. These distributions may prove useful in modelling galaxies.
Distribution functions in cylindrical and planar geometries are also discussed.
Finally, we study spatially spheroidal systems that again exhibit strict
power-law behaviour for the density and potential and find results in agreement
with results published recently.Comment: 23 pages, regular Tex, figures in separate .uu file to follo
Cored Apple Bipolarity : A Global Instability to Convection in Radial Accretion?
We propose that the prevalence of bipolarity in Young Stellar Objects is due
to the fine tuning that is required for spherical accretion of an ambient
medium onto a central node.It is shown that there are two steady modes that are
more likely than radial accretion, each of which is associated with a
hyperbolic central point in the meridional stream lines, and consequently with
either an equatorial inflow and an axial ejection or vice versa. In each case
the stream lines pass through a thick accretion torus, which is better thought
of as a standing pressure wave rather than as a relatively inert Keplerian
structure.We base our arguments on a simple analytic example,which is topologi
cally generic,wherein each bipolarmode is created by the rebound of accreting
matter under the action of the thermal,magnetic,turbulent and centrifugal
pressures created in the flow. In both bipolar modes the presence of non-zero
angular momentum implies axial regions wherein the pressure is first reduced
below the value at infinity and then becomes negative, where the solution fails
because rotating material can not enter this region without suction.The model
thus has empty stems where the activity of the central source must dominate.So
the basic engine of the bipolar flow discussed here is simply the rebound of
freely falling material from a thick pressure disc into an axial low pressure
region.The low mass,high velocity outflow must be produced in this region by an
additional mechanism. This is reminiscent of the cored apple structure observed
recently in the very young bipolar source VLA 1623.Comment: PostScript, 10 page
The Deutsch Field Gamma-Ray Pulsar - Paper I: The Model Basics
A new model for the high-energy emission from pulsars is developed by
considering charged particle motion in the fields of a spinning, highly
magnetised and conducting sphere in vacuum. A generally applicable
approximation to the particle motion in strong fields is developed and applied
to the numerical modelling, and the radiation emitted by curvature emission is
summed to generate light curves. The model predicts many of the observed
features of pulsar light curves. This paper outlines the basic properties of
the model; a subsequent paper will discuss the statistical properties of a
population of model pulsars and apply the model to the known gamma-ray pulsars.Comment: 11 pages LaTex, 10 postscript figures included with psfig. The paper
can also be found at ftp://astro.queensu.ca/pub/mark/preprints/paper1.ps.Z as
a compressed postscript file. Submitted to MNRA
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