1,063,852 research outputs found
The Pulsar Kick Velocity Distribution
We analyse the sample of pulsar proper motions, taking detailed account of
the selection effects of the original surveys. We treat censored data using
survival statistics. From a comparison of our results with Monte Carlo
simulations, we find that the mean birth speed of a pulsar is 250-300 km/s,
rather than the 450 km/s foundby Lyne & Lorimer (1994). The resultant
distribution is consistent with a maxwellian with dispersion . Despite the large birth velocities, we find that the pulsars with long
characteristic ages show the asymmetric drift, indicating that they are
dynamically old. These pulsars may result from the low velocity tail of the
younger population, although modified by their origin in binaries and by
evolution in the galactic potential.Comment: Latex, 10 pages, and 11 postscript figures. Accepted by Monthly
Notice
Single-point velocity distribution in turbulence
We show that the tails of the single-point velocity probability distribution
function (PDF) are generally non-Gaussian in developed turbulence. By using
instanton formalism for the Navier-Stokes equation, we establish the relation
between the PDF tails of the velocity and those of the external forcing. In
particular, we show that a Gaussian random force having correlation scale
and correlation time produces velocity PDF tails at . For a short-correlated forcing
when there is an intermediate asymptotics at .Comment: 9 pages, revtex, no figure
Stellar Velocity Distribution in Galactic Disks
We present numerical studies of the properties of the stellar velocity
distribution in galactic disks which have developed a saturated, two-armed
spiral structure. In previous papers we used the Boltzmann moment equations
(BME) up to second order for our studies of the velocity structure in
self-gravitating stellar disks. A key assumption of our BME approach is the
zero-heat flux approximation, i.e. the neglection of third order velocity
terms. We tested this assumption by performing test particle simulations for
stars in a disk galaxy subject to a rotating spiral perturbation. As a result
we corroborated qualitatively the complex velocity structure found in the BME
approach. It turned out that an equilibrium configuration in velocity space is
only slowly established on a typical timescale of 5 Gyrs or more. Since many
dynamical processes in galaxies (like the growth of spirals or bars)act on
shorter timescales, pure equilibrium models might not be fully appropriate for
a detailed comparison with observations like the local Galactic velocity
distribution. Third order velocity moments were typically small and
uncorrelated over almost all of the disk with the exception of the 4:1
resonance region (UHR). Near the UHR (normalized) fourth and fifth order
velocity moments are still of the same order as the second and third order
terms. Thus, at the UHR higher order terms are not negligible.Comment: 8 pages, 3 figures, to appear in the Proceedings of "Chaos in
Astronomy", Athens, G. Contopoulos & P.A. Patsis (eds.
Velocity and Distribution of Primordial Neutrinos
The Cosmic Neutrinos Background (\textbf{CNB}) are Primordial Neutrinos
decoupled when the Universe was very young. Its detection is complicated,
especially if we take into account neutrino mass and a possible breaking of
Lorentz Invariance at high energy, but has a fundamental relevance to study the
Big-Bang. In this paper, we will see that a Lorentz Violation does not produce
important modification, but the mass does. We will show how the neutrinos
current velocity, with respect to comobile system to Universe expansion, is of
the order of 1065 , much less than light velocity. Besides, we
will see that the neutrinos distribution is complex due to Planetary motion.
This prediction differs totally from the usual massless case, where we would
get a correction similar to the Dipolar Moment of the \textbf{CMB}.Comment: 16 pages, latex, 7 figure
Characterizing a cluster's dynamic state using a single epoch of radial velocities
Radial velocity measurements can be used to constrain the dynamical state of
a stellar cluster. However, for clusters with velocity dispersions smaller than
a few km/s the observed radial velocity distribution tends to be dominated by
the orbital motions of binaries rather than the stellar motions through the
potential well of the cluster. Our goal is to characterize the intrinsic
velocity distribution of a cluster from a single epoch of radial velocity data,
even for a cluster with a velocity dispersion of a fraction of a km/s, using a
maximum likelihood procedure. Assuming a period, mass ratio, and eccentricity
distribution for the binaries in the observed cluster this procedure fits a
dynamical model describing the velocity distribution for the single stars and
center of masses of the binaries, simultaneously with the radial velocities
caused by binary orbital motions, using all the information available in the
observed velocity distribution. We find that the fits to the intrinsic velocity
distribution depend only weakly on the binary properties assumed, so the
uncertainty in the fitted parameters tends to be dominated by statistical
uncertainties. Based on Monte Carlo simulations we provide an estimate of how
these statistical uncertainties vary with the velocity dispersion, binary
fraction, and the number of observed stars, which can be used to estimate the
sample size needed to reach a specific accuracy. Finally we test the method on
the well-studied open cluster NGC 188, showing that it can reproduce a velocity
dispersion of only 0.5 km/s using a single epoch of the multi-epoch radial
velocity data. If the binary period, mass ratio, and eccentricity distribution
of the observed stars are roughly known, this procedure can be used to correct
for the effect of binary orbital motions on an observed velocity distribution.
[Abridged]Comment: 11 pages, 6 figures, accepted by A&
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