21,578 research outputs found
Beatwave Excitation of Plasma Waves Based on Relativistic Bi-Stability
A nonlinear beatwave regime of plasma wave excitation is considered. Two
beatwave drivers are considered: intensity-modulated laser pulse and
density-modulated (microbunched) electron beam. It is shown that a long
beatwave pulse can excite strong plasma waves in its wake even when the
beatwave frequency is detuned from the electron plasma frequency. The wake is
caused by the dynamic bi-stability of the nonlinear plasma wave if the beatwave
amplitude exceeds the analytically calculated threshold. In the context of a
microbunched beam driven plasma wakefield accelerator, this excitation regime
can be applied to developing a femtosecond electron injector.Comment: 5 pages, 4 figure
Mass segregation trends in SDSS galaxy groups
It has been shown that galaxy properties depend strongly on their host
environment. In order to understand the relevant physical processes driving
galaxy evolution it is important to study the observed properties of galaxies
in different environments. Mass segregation in bound galaxy structures is an
important indicator of evolutionary history and dynamical friction timescales.
Using group catalogues derived from the Sloan Digital Sky Survey Data Release 7
(SDSS DR7) we investigate mass segregation trends in galaxy groups at low
redshift. We investigate average galaxy stellar mass as a function of
group-centric radius and find evidence for weak mass segregation in SDSS
groups. The magnitude of the mass segregation depends on both galaxy stellar
mass limits and group halo mass. We show that the inclusion of low mass
galaxies tends to strengthen mass segregation trends, and that the strength of
mass segregation tends to decrease with increasing group halo mass. We find the
same trends if we use the fraction of massive galaxies as a function of
group-centric radius as an alternative probe of mass segregation. The magnitude
of mass segregation that we measure, particularly in high-mass haloes,
indicates that dynamical friction is not acting efficiently.Comment: 6 pages, 2 figures, accepted for publication in MNRAS Letter
The Final Fate of Spherical Inhomogeneous Dust Collapse
We examine the role of the initial density and velocity distribution in the
gravitational collapse of a spherical inhomogeneous dust cloud. Such a collapse
is described by the Tolman-Bondi metric which has two free functions: the
`mass-function' and the `energy function', which are determined by the initial
density and velocity profile of the cloud. The collapse can end in a black-hole
or a naked singularity, depending on the initial parameters characterizing
these profiles. In the marginally bound case, we find that the collapse ends in
a naked singularity if the leading non-vanishing derivative of the density at
the center is either the first one or the second one. If the first two
derivatives are zero, and the third derivative non-zero, the singularity could
either be naked or covered, depending on a quantity determined by the third
derivative and the central density. If the first three derivatives are zero,
the collapse ends in a black hole. In particular, the classic result of
Oppenheimer and Snyder, that homogeneous dust collapse leads to a black hole,
is recovered as a special case. Analogous results are found when the cloud is
not marginally bound, and also for the case of a cloud starting from rest. We
also show how the strength of the naked singularity depends on the density and
velocity distribution. Our analysis generalizes and simplifies the earlier work
of Christodoulou and Newman [4,5] by dropping the assumption of evenness of
density functions. It turns out that relaxing this assumption allows for a
smooth transition from the naked singularity phase to the black-hole phase, and
also allows for the occurrence of strong curvature naked singularities.Comment: 23 pages; Plain Tex; TIFR-TAP preprin
On the Role of Initial Data in the Gravitational Collapse of Inhomogeneous Dust
We consider here the gravitational collapse of a spherically symmetric
inhomogeneous dust cloud described by the Tolman-Bondi models. By studying a
general class of these models, we find that the end state of the collapse is
either a black hole or a naked singularity, depending on the parameters of the
initial density distribution, which are , the initial central density
of the massive body, and , the initial boundary. The collapse ends in a
black hole if the dimensionless quantity constructed out of this
initial data is greater than 0.0113, and it ends in a naked singularity if
is less than this number. A simple interpretation of this result can be
given in terms of the strength of the gravitational potential at the starting
epoch of the collapse.Comment: Original title changed, numerical range of naked singularity
corrected. Plain Tex File. 14 pages. To appear in Physical Review
Role of initial data in spherical collapse
We bring out here the role of initial data in causing the black hole and
naked singularity phases as the final end state of a continual gravitational
collapse. The collapse of a type I general matter field is considered, which
includes most of the known physical forms of matter. It is shown that given the
distribution of the density and pressure profiles at the initial surface from
which the collapse evolves, there is a freedom in choosing rest of the free
functions, such as the velocities of the collapsing shells, so that the end
state could be either a black hole or a naked singularity depending on this
choice. It is thus seen that it is the initial data that determines the end
state of spherical collapse in terms of these outcomes, and we get a good
picture of how these phases come about.Comment: 5 pages, Revtex4, Revised version, To appear in Physical Review
Prediction of long and short time rheological behavior in soft glassy materials
We present an effective time approach to predict long and short time
rheological behavior of soft glassy materials from experiments carried out over
practical time scales. Effective time approach takes advantage of relaxation
time dependence on aging time that allows time-aging time superposition even
when aging occurs over the experimental timescales. Interestingly experiments
on variety of soft materials demonstrate that the effective time approach
successfully predicts superposition for diverse aging regimes ranging from
sub-aging to hyper-aging behaviors. This approach can also be used to predict
behavior of any response function in molecular as well as spin glasses.Comment: 13 pages, 4 figure
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