2,174 research outputs found
Continuum and Spectral Line Radiation from a Random Clumpy Medium
We present a formalism for continuum and line emission from random clumpy
media together with its application to problems of current interest, including
CO spectral lines from ensembles of clouds and radio emission from HII regions,
supernovae and star-forming regions. For line emission we find that the effects
of clump opacity on observed line ratios can be indistinguishable from
variations of intrinsic line strengths, adding to the difficulties in
determining abundances from line observations. Our formalism is applicable to
arbitrary distributions of cloud properties, provided the cloud volume filling
factor is small; numerical simulations show it to hold up to filling factors of
about 10%. We show that irrespective of the complexity of the cloud ensemble,
the radiative effect of clumpiness can be parametrized at each frequency by a
single multiplicative correction to the overall optical depth; this multiplier
is derived from appropriate averaging over individual cloud properties. Our
main finding is that cloud shapes have only a negligible effect on radiation
propagation in clumpy media; the results of calculations employing point-like
clouds are practically indistinguishable from those for finite-size clouds with
arbitrary geometrical shapes.Comment: ApJ, to be publishe
A Circumstellar Disc in a High-Mass Star Forming Region
We present an edge-on Keplerian disc model to explain the main component of
the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N.
The brightness distribution and spectrum of the line of bright masers are
successfully modeled with high amplification of background radio continuum
emission along velocity coherent paths through a maser disc. The bend seen in
the position-velocity diagram is a characteristic signature of differentially
rotating discs. For a central mass of 30 solar masses, suggested by other
observations, our model fixes the masing disc to have inner and outer radii of
about 350 AU and 1000 AU.Comment: 11 pages, accepted for publication in ApJ Letter
The decreasing percentile residual life aging notion
Earlier researchers have studied some aspects of the classes of distribution functions with decreasing ?-percentile residual life (DPRL(?)), 0Reliability theory, Hazard rate, Stochastic orders, Aging notions, Nonparametric estimation, Strongly uniform consistency
Upper critical dimension of the KPZ equation
Numerical results for the Directed Polymer model in 1+4 dimensions in various
types of disorder are presented. The results are obtained for system size
considerably larger than that considered previously. For the extreme strong
disorder case (Min-Max system), associated with the Directed Percolation model,
the expected value of the meandering exponent, zeta = 0.5 is clearly revealed,
with very week finite size effects. For the week disorder case, associated with
the KPZ equation, finite size effects are stronger, but the value of seta is
clearly seen in the vicinity of 0.57. In systems with "strong disorder" it is
expected that the system will cross over sharply from Min-Max behavior at short
chains to weak disorder behavior at long chains. This is indeed what we find.
These results indicate that 1+4 is not the Upper Critical Dimension (UCD) in
the week disorder case, and thus 4+1 does not seem to be the upper critical
dimension for the KPZ equation
Significant g-factor values of a two-electron ground state in quantum dots with spin-orbit coupling
The magnetization of semiconductor quantum dots in the presence of spin-orbit
coupling and interactions is investigated numerically. When the dot is occupied
by two electrons we find that a level crossing between the two lowest many-body
eigenstates may occur as a function of the spin-orbit coupling strength. This
level crossing is accompanied by a non-vanishing magnetization of the
ground-state. Using first order perturbation theory as well as exact numerical
diagonalization of small clusters we show that the tendency of interactions to
cause Stoner-like instability is enhanced by the SO coupling. The resulting
g-factor can have a significant value, and thus may influence g-factor
measurements. Finally we propose an experimental method by which the predicted
phenomenon can be observed.Comment: 7+ pages, 7 figure
Dynamical Inequality in Growth Models
A recent exponent inequality is applied to a number of dynamical growth
models. Many of the known exponents for models such as the Kardar-Parisi-Zhang
(KPZ) equation are shown to be consistent with the inequality. In some cases,
such as the Molecular Beam Equation, the situation is more interesting, where
the exponents saturate the inequality. As the acid test for the relative
strength of four popular approximation schemes we apply the inequality to the
exponents obtained for two Non Local KPZ systems. We find that all methods but
one, the Self Consistent Expansion, violate the inequality in some regions of
parameter space. To further demonstrate the usefulness of the inequality, we
apply it to a specific model, which belongs to a family of models in which the
inequality becomes an equality. We thus show that the inequality can easily
yield results, which otherwise have to rely either on approximations or general
beliefs.Comment: 6 pages, 4 figure
RANDOM MATRIX THEORY APPROACH TO THE INTENSITY DISTRIBUTIONS OF WAVES PROPAGATING IN A RANDOM MEDIUM
Statistical properties of coherent radiation propagating in a quasi - 1D
random media is studied in the framework of random matrix theory. Distribution
functions for the total transmission coefficient and the angular transmission
coefficient are obtained.Comment: 8 pages, latex, no figures. Submitted to Phys.Rev.
Perturbation of null spaces with application to the eigenvalue problem and generalized inverses
AbstractWe consider properties of a null space of an analytically perturbed matrix. In particular, we obtain Taylor expansions for the eigenvectors which constitute a basis for the perturbed null space. Furthermore, we apply these results to the calculation of Puiseux expansion of the perturbed eigenvectors in the case of general eigenvalue problem as well as to the calculation of Laurent series expansions for the perturbed group inverse and pseudoinverse matrices
Pulsed Adiabatic Photoassociation via Scattering Resonances
We develop the theory for the Adiabatic Raman Photoassociation (ARPA) of
ultracold atoms to form ultracold molecules in the presence of scattering
resonances. Based on a computational method in which we replace the continuum
with a discrete set of "effective modes", we show that the existence of
resonances greatly aids in the formation of deeply bound molecular states. We
illustrate our general theory by computationally studying the formation of
Rb molecules from pairs of colliding ultracold Rb atoms. The
single-event transfer yield is shown to have a near-unity value for wide
resonances, while the ensemble-averaged transfer yield is shown to be higher
for narrow resonances. The ARPA yields are compared with that of (the
experimentally measured) "Feshbach molecule" magneto-association. Our findings
suggest that an experimental investigation of ARPA at sub-K temperatures
is warranted.Comment: 20 pages, 11 figure
Holography and Fermions at a Finite Chemical Potential
We review the Sakai-Sugimoto model of holographic QCD at zero temperature and
finite chemical potential, comparing the results to those expected at
large- QCD, and those in a closely related holographic model. We find that
as the baryon chemical potential is increased above a critical value, there is
a phase transition to a nuclear matter phase, the details of which depend on
the model. We argue that the nuclear matter phase is necessarily inhomogeneous
to arbitrarily high density, which suggests an explanation of the "chiral
density wave" instability of the quark Fermi surface in large- QCD. Some
details of the instanton distribution in the holographic dual are reminiscent
of a Fermi surface. This short manuscript summarizes a talk given by M.R. at
"Theory Canada 4" conference, and is based largely (but not entirely) on the
results of \cite{Rozalietal2008}.Comment: 11 pages, 1 figur
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