1,303 research outputs found
The molecular gas in the supernova remnant IC443
Although a few highly perturbed regions characterized by gas motions with velocities larger than 20 km/s have been discovered during the last several years in the supernova remnant (SNR) IC 443, the nature of these perturbed clumps and their relationship to the quiescent molecular gas near the SNR remains unknown. In part, this is due to a lack of large-scale, high angular resolution observations. Therefore, a systematic survey of this SNR in the CO (J=1 yields 0) line has been conducted, covering a roughly 50' x 50' region spaced by 2'. The observations were made with the 14 m telescope of the Five College Radio Astronomy Observatory (FCRAO), which has a resolution of 45" and a single sideband receiver temperature of 200 K at 2.6 mm wavelength. Five new clumps were discovered, bringing the total number of known perturbed regions to eight. To study the physical structure of these clumps in more detail, more complete maps of the clumps have been made in both the CO(J=1 yields 0) and (J=2 yields 1) transitions with the FCRAO telescope. These maps show that the extent of perturbed gas in a typical clump is several arcmin, or a few pc at a distance of 1.5 kpc
Path-integral representation for a stochastic sandpile
We introduce an operator description for a stochastic sandpile model with a
conserved particle density, and develop a path-integral representation for its
evolution. The resulting (exact) expression for the effective action highlights
certain interesting features of the model, for example, that it is nominally
massless, and that the dynamics is via cooperative diffusion. Using the
path-integral formalism, we construct a diagrammatic perturbation theory,
yielding a series expansion for the activity density in powers of the time.Comment: 22 pages, 6 figure
Asymptotic behavior of the order parameter in a stochastic sandpile
We derive the first four terms in a series for the order paramater (the
stationary activity density rho) in the supercritical regime of a
one-dimensional stochastic sandpile; in the two-dimensional case the first
three terms are reported. We reorganize the pertubation theory for the model,
recently derived using a path-integral formalism [R. Dickman e R. Vidigal, J.
Phys. A 35, 7269 (2002)], to obtain an expansion for stationary properties.
Since the process has a strictly conserved particle density p, the Fourier mode
N^{-1} psi_{k=0} -> p, when the number of sites N -> infinity, and so is not a
random variable. Isolating this mode, we obtain a new effective action leading
to an expansion for rho in the parameter kappa = 1/(1+4p). This requires
enumeration and numerical evaluation of more than 200 000 diagrams, for which
task we develop a computational algorithm. Predictions derived from this series
are in good accord with simulation results. We also discuss the nature of
correlation functions and one-site reduced densities in the small-kappa
(large-p) limit.Comment: 18 pages, 5 figure
Series expansion for a stochastic sandpile
Using operator algebra, we extend the series for the activity density in a
one-dimensional stochastic sandpile with fixed particle density p, the first
terms of which were obtained via perturbation theory [R. Dickman and R.
Vidigal, J. Phys. A35, 7269 (2002)]. The expansion is in powers of the time;
the coefficients are polynomials in p. We devise an algorithm for evaluating
expectations of operator products and extend the series to O(t^{16}).
Constructing Pade approximants to a suitably transformed series, we obtain
predictions for the activity that compare well against simulations, in the
supercritical regime.Comment: Extended series and improved analysi
Fermionic field theory for directed percolation in (1+1) dimensions
We formulate directed percolation in (1+1) dimensions in the language of a
reaction-diffusion process with exclusion taking place in one space dimension.
We map the master equation that describes the dynamics of the system onto a
quantum spin chain problem. From there we build an interacting fermionic field
theory of a new type. We study the resulting theory using renormalization group
techniques. This yields numerical estimates for the critical exponents and
provides a new alternative analytic systematic procedure to study
low-dimensional directed percolation.Comment: 20 pages, 2 figure
Diffusion in stochastic sandpiles
We study diffusion of particles in large-scale simulations of one-dimensional
stochastic sandpiles, in both the restricted and unrestricted versions. The
results indicate that the diffusion constant scales in the same manner as the
activity density, so that it represents an alternative definition of an order
parameter. The critical behavior of the unrestricted sandpile is very similar
to that of its restricted counterpart, including the fact that a data collapse
of the order parameter as a function of the particle density is only possible
over a very narrow interval near the critical point. We also develop a series
expansion, in inverse powers of the density. for the collective diffusion
coefficient in a variant of the stochastic sandpile in which the toppling rate
at a site with particles is , and compare the theoretical
prediction with simulation results.Comment: 21 page
Large scale numerical simulations of "ultrametric" long-range depinning
The depinning of an elastic line interacting with a quenched disorder is
studied for long range interactions, applicable to crack propagation or
wetting. An ultrametric distance is introduced instead of the Euclidean
distance, allowing for a drastic reduction of the numerical complexity of the
problem. Based on large scale simulations, two to three orders of magnitude
larger than previously considered, we obtain a very precise determination of
critical exponents which are shown to be indistinguishable from their Euclidean
metric counterparts. Moreover the scaling functions are shown to be unchanged.
The choice of an ultrametric distance thus does not affect the universality
class of the depinning transition and opens the way to an analytic real space
renormalization group approach.Comment: submitted to Phys. Rev.
Scaling in self-organized criticality from interface depinning?
The avalanche properties of models that exhibit 'self-organized criticality'
(SOC) are still mostly awaiting theoretical explanations. A recent mapping
(Europhys. Lett.~53, 569) of many sandpile models to interface depinning is
presented first, to understand how to reach the SOC ensemble and the
differences of this ensemble with the usual depinning scenario. In order to
derive the SOC avalanche exponents from those of the depinning critical point,
a geometric description is discussed, of the quenched landscape in which the
'interface' measuring the integrated activity moves. It turns out that there
are two main alternatives concerning the scaling properties of the SOC
ensemble. These are outlined in one dimension in the light of scaling arguments
and numerical simulations of a sandpile model which is in the quenched
Edwards-Wilkinson universality class.Comment: 7 pages, 3 figures, Statphys satellite meeting in Merida, July 200
Flow properties of driven-diffusive lattice gases: theory and computer simulation
We develop n-cluster mean-field theories (0 < n < 5) for calculating the flow
properties of the non-equilibrium steady-states of the Katz-Lebowitz-Spohn
model of the driven diffusive lattice gas, with attractive and repulsive
inter-particle interactions, in both one and two dimensions for arbitrary
particle densities, temperature as well as the driving field. We compare our
theoretical results with the corresponding numerical data we have obtained from
the computer simulations to demonstrate the level of accuracy of our
theoretical predictions. We also compare our results with those for some other
prototype models, notably particle-hopping models of vehicular traffic, to
demonstrate the novel qualitative features we have observed in the
Katz-Lebowitz-Spohn model, emphasizing, in particular, the consequences of
repulsive inter-particle interactions.Comment: 12 RevTex page
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