5,810 research outputs found
Merger as Intermittent Accretion
The Self-Similar Secondary Infall Model (SSIM) is modified to simulate a
merger event. The model encompass spherical versions of tidal stripping and
dynamical friction that agrees with the Syer & White merger paradigm's
behaviour. The SSIM shows robustness in absorbing even comparable mass
perturbations and returning to its original state. It suggests the approach to
be invertible and allows to consider accretion as smooth mass inflow merging
and mergers as intermittent mass inflow accretion.Comment: letter accepted by A&A 29/09/08, 4 pages, colour figure
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
Relaxing and Virializing a Dark Matter Halo
Navarro, Frenk, and White have suggested that the density profiles of
simulated dark matter halos have a ``universal'' shape so that a given halo can
be characterized by a single free parameter which fixes its mass. In this
paper, we revisit the spherical infall model in the hope of recognizing in
detail the existence and origin of any such universality. A system of particles
is followed from linear perturbation, through first shell crossing, then
through an accretion or infall phase, and finally to virialization. During the
accretion phase, the system relaxes through a combination of phase mixing,
phase space instability, and moderate violent relation. It is driven quickly,
by the flow of mass through its surface, toward self-similar evolution. The
self-similar solution plays its usual role of intermediate attractor and can be
recognized from a virial-type theorem in scaled variables and from our
numerical simulations. The transition to final equilibrium state once infall
has ceased is relatively gentle, an observation which leads to an approximate
form for the distribution function of the final system. The infall phase fixes
the density profile in intermediate regions of the halo to be close to r^{-2}.
We make contact with the standard hierarchical clustering scenario and explain
how modifications of the self-similar infall model might lead to density
profiles in agreement with those found in numerical simulations.Comment: 26 pages, Latex, plus 11 figure
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
Spectral constraints on models of gas in clusters of galaxies
The HEAO 1A2 spectra of clusters of galaxies are used to determine the temperature profile which characterizes the X-ray emitting gas. Strong evidence of nonisothermality is found for the Coma, A85, and A1795 clusters. Properties of the cluster potential which binds the gas are calculated for a range of model parameters. The typical binding mass, if the gas is adiabatic, is 2-4E14 solar masses and is quite centrally concentrated. In addition, the Fe abundance in Coma is .26 + or - .06 solar, less than the typical value (.5) found for rich clusters. The results for the gas in Coma may imply a physical description of the cluster which is quite different from what was previously believed
Possible mechanism for changes in glycogen metabolism in unloaded soleus muscle
Carbohydrate metabolism has been shown to be affected in a number of ways by different models of hypokinesia. In vivo glycogen levels in the soleus muscle are known to be increased by short-term denervation and harness suspension. In addition, exposure to 7 days of hypogravity also caused a dramatic increase in glycogen concentration in this muscle. The biochemical alterations caused by unloading that may bring about these increases in glycogen storage in the soleus were sought
Wavelet Analysis of Inhomogeneous Data with Application to the Cosmic Velocity Field
In this article we give an account of a method of smoothing spatial
inhomogeneous data sets by using wavelet reconstruction on a regular grid in an
auxilliary space onto which the original data is mapped. In a previous paper by
the present authors, we devised a method for inferring the velocity potential
from the radial component of the cosmic velocity field assuming an ideal
sampling. Unfortunately the sparseness of the real data as well as errors of
measurement require us to first smooth the velocity field as observed on a
3-dimensional support (i.e. the galaxy positions) inhomogeneously distributed
throughout the sampled volume. The wavelet formalism permits us to introduce a
minimal smoothing procedure that is characterized by the variation in size of
the smothing window function. Moreover the output smoothed radial velocity
field can be shown to correspond to a well defined theoretical quantity as long
as the spatial sampling support satisfies certain criteria. We argue also that
one should be very cautious when comparing the velocity potential derived from
such a smoothed radial component of the velocity field with related quantities
derived from other studies (e.g : of the density field).Comment: 19 pages, Latex file, figures are avaible under requests, published
in Inverse Problems, 11 (1995) 76
Black holes and Galactic density cusps -- I. Radial orbit cusps and bulges
In this paper, we study the distribution functions that arise naturally
during self-similar radial infall of collisionless matter. Such matter may be
thought of either as stars or as dark matter particles. If a rigorous steady
state is assumed, then the system is infinite and is described by a universal
distribution function given the self-similar index. The steady logarithmic
potential case is exceptional and yields the familiar Gaussian for an infinite
system with an inverse-square density profile. We show subsequently that for
time-dependent radial self-similar infall, the logarithmic case is accurately
described by the Fridmann and Polyachenko distribution function. The system in
this case is finite but growing. We are able to embed a central mass in the
universal steady distribution only by iteration, except in the case of massless
particles. The iteration yields logarithmic corrections to the massless
particle case and requires a `renormalization' of the central mass. A central
spherical mass may be accurately embedded in the Fridmann and Polyachenko
growing distribution however. Some speculation is given concerning the
importance of radial collisionless infall in actual galaxy formation.Comment: 10 pp, 3 fig
- âŠ