20,348 research outputs found
Flame propagation and extinction in particle clouds
Two phase flame propagation and extinction theory required to support the corresponding experiments planned for the space shuttle is being developed. Also being planned are specialized collaborative, experimental and theoretical NASA UCSD studies needed to support the ongoing definition of needed experimental hardware, experimental procedures, data acquisition philosophy, and other ground based support activities required to assure the success of space shuttle based experiments concerned with combustion of clouds of particulates at reduced gravitational conditions. The further development of relations delineating premixed particle cloud and premixed gaseous systems as well as burner stabilized and freely propagating flame systems is considered
Quantum evaporation of a naked singularity
We investigate here quantum effects in gravitational collapse of a scalar
field model which classically leads to a naked singularity. We show that
non-perturbative semi-classical modifications near the singularity, based on
loop quantum gravity, give rise to a strong outward flux of energy. This leads
to the dissolution of the collapsing cloud before the singularity can form.
Quantum gravitational effects thus censor naked singularities by avoiding their
formation. Further, quantum gravity induced mass flux has a distinct feature
which may lead to a novel observable signature in astrophysical bursts.Comment: 4 pages, 2 figures. Minor changes to match published version in
Physical Review Letter
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
Finite Temperature Behavior of Small Silicon and Tin Clusters: An Ab Initio Molecular Dynamics Study
The finite temperature behavior of small Silicon (Si, Si, and
Si) and Tin (Sn and Sn) clusters is studied using
isokinetic Born-Oppenheimer molecular dynamics. The lowest equilibrium
structures of all the clusters are built upon a highly stable tricapped
trigonal prism unit which is seen to play a crucial role in the finite
temperature behavior of these clusters. Thermodynamics of small tin clusters
(Sn and Sn) is revisited in light of the recent experiments on
tin clusters of sizes 18-21 [G. A. Breaux et. al. Phys. Rev. B {\bf 71} 073410
(2005)]. We have calculated heat capacities using multiple histogram technique
for Si, Sn and Si clusters. Our calculated specific heat
curves have a main peak around 2300 K and 2200 K for Si and Sn
clusters respectively. However, various other melting indicators such as root
mean square bond length fluctuations, mean square displacements show that
diffusive motion of atoms within the cluster begins around 650 K. The finite
temperature behavior of Si and Sn is dominated by isomerization
and it is rather difficult to discern the temperature range for transition
region. On the other hand, Si does show a liquid like behavior over a
short temperature range followed by the fragmentation observed around 1800 K.
Finite temperature behavior of Si and Sn show that these clusters
do not melt but fragment around 1200 K and 650 K respectively.Comment: 9 figure
Wave propagation through a coherently amplifying random medium
We report a detailed and systematic numerical study of wave propagation
through a coherently amplifying random one-dimensional medium. The coherent
amplification is modeled by introducing a uniform imaginary part in the site
energies of the disordered single-band tight binding Hamiltonian. Several
distinct length scales (regimes), most of them new, are identified from the
behavior of transmittance and reflectance as a function of the material
parameters. We show that the transmittance is a non-self-averaging quantity
with a well defined mean value. The stationary distribution of the super
reflection differs qualitatively from the analytical results obtained within
the random phase approximation in strong disorder and amplification regime. The
study of the stationary distribution of the phase of the reflected wave reveals
the reason for this discrepancy. The applicability of random phase
approximation is discussed. We emphasize the dual role played by the lasing
medium, as an amplifier as well as a reflector.Comment: 33 pages RevTex, 14 EPS figures included, Accepted for publication in
IJMP-
Dopant Induced Stabilization of Silicon Cluster at Finite Temperature
With the advances in miniaturization, understanding and controlling
properties of significant technological systems like silicon in nano regime
assumes considerable importance. It turns out that small silicon clusters in
the size range of 15-20 atoms are unstable upon heating and in fact fragment in
the temperature range of 1200 K to 1500 K. In the present work we demonstrate
that it is possible to stabilize such clusters by introducing appropriate
dopant (in this case Ti). Specifically, by using the first principle density
functional simulations we show that Ti doped Si, having the Frank-Kasper
geometry, remains stable till 2200 K and fragments only above 2600 K. The
observed melting transition is a two step process. The first step is initiated
by the surface melting around 600 K. The second step is the destruction of the
cage which occurs around 2250 K giving rise to a peak in the heat capacity
curve.Comment: 6 pages, 8 Figs. Submitted to PR
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
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
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