15,430 research outputs found
Restoration of isotropy on fractals
We report a new type of restoration of macroscopic isotropy (homogenization)
in fractals with microscopic anisotropy. The phenomenon is observed in various
physical setups, including diffusions, random walks, resistor networks, and
Gaussian field theories. The mechanism is unique in that it is absent in spaces
with translational invariance, while universal in that it is observed in a wide
class of fractals.Comment: 11 pages, REVTEX, 3 postscript figures. (Compressed and encoded
figures archived by "figure" command). To appear in Physical Review Letter
3D Photoionisation Modelling of NGC 6302
We present a three-dimensional photoionisation and dust radiative transfer
model of NGC 6302, an extreme, high-excitation planetary nebula. We use the 3D
photoionisation code Mocassin} to model the emission from the gas and dust. We
have produced a good fit to the optical emission-line spectrum, from which we
derived a density distribution for the nebula. A fit to the infrared coronal
lines places strong constraints on the properties of the unseen ionising
source. We find the best fit comes from using a 220,000 K hydrogen-deficient
central star model atmosphere, indicating that the central star of this PN may
have undergone a late thermal pulse.
We have also fitted the overall shape of the ISO spectrum of NGC 6302 using a
dust model with a shallow power-law size distribution and grains up to 1.0
micron in size. To obtain a good fit to the infrared SED the dust must be
sufficiently recessed within the circumstellar disk to prevent large amounts of
hot dust at short wavelengths, a region where the ISO spectrum is particularly
lacking. These and other discoveries are helping to unveil many properties of
this extreme object and trace it's evolutionary history.Comment: 8 pages, 4 figures; for the proceedings of "Asymmetric Planetary
Nebuale IV," R. L. M. Corradi, A. Manchado, N. Soker ed
Upper estimate of martingale dimension for self-similar fractals
We study upper estimates of the martingale dimension of diffusion
processes associated with strong local Dirichlet forms. By applying a general
strategy to self-similar Dirichlet forms on self-similar fractals, we prove
that for natural diffusions on post-critically finite self-similar sets
and that is dominated by the spectral dimension for the Brownian motion
on Sierpinski carpets.Comment: 49 pages, 7 figures; minor revision with adding a referenc
Transition density of diffusion on Sierpinski gasket and extension of Flory's formula
Some problems related to the transition density u(t,x) of the diffusion on
the Sierpinski gasket are considerd, based on recent rigorous results and
detailed numerical calculations. The main contents are an extension of Flory's
formula for the end-to-end distance exponent of self-avoiding walks on the
fractal spaces, and an evidence of the oscillatory behavior of u(t,x) on the
Sierpinski gasket.Comment: 11 pages, REVTEX, 2 postscript figure
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