35,967 research outputs found
Strategies for Optimize Off-Lattice Aggregate Simulations
We review some computer algorithms for the simulation of off-lattice clusters
grown from a seed, with emphasis on the diffusion-limited aggregation,
ballistic aggregation and Eden models. Only those methods which can be
immediately extended to distinct off-lattice aggregation processes are
discussed. The computer efficiencies of the distinct algorithms are compared.Comment: 6 pages, 7 figures and 3 tables; published at Brazilian Journal of
Physics 38, march, 2008 (http://www.sbfisica.org.br/bjp/files/v38_81.pdf
Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height
The formation of mounded surfaces in epitaxial growth is attributed to the
presence of barriers against interlayer diffusion in the terrace edges, known
as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth
using a ES barrier explicitly dependent on the step height. Our model has an
intrinsic topological step barrier even in the absence of an explicit ES
barrier. We show that mounded morphologies can be obtained even for a small
barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma
equation, is observed in absence of an explicit step barrier. The mounded
surfaces are described by a super-roughness dynamical scaling characterized by
locally smooth (faceted) surfaces and a global roughness exponent .
The thin film limit is featured by surfaces with self-assembled
three-dimensional structures having an aspect ratio (height/width) that may
increase or decrease with temperature depending on the strength of step
barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary
materia
Contact process on a Voronoi triangulation
We study the continuous absorbing-state phase transition in the contact
process on the Voronoi-Delaunay lattice. The Voronoi construction is a natural
way to introduce quenched coordination disorder in lattice models. We simulate
the disordered system using the quasistationary simulation method and determine
its critical exponents and moment ratios. Our results suggest that the critical
behavior of the disordered system is unchanged with respect to that on a
regular lattice, i.e., that of directed percolation
Molecule survival in magnetized protostellar disk winds. II. Predicted H2O line profiles versus Herschel/HIFI observations
We investigate whether the broad wings of H2O emission identified with
Herschel towards low-mass Class 0 and Class 1 protostars may be consistent with
an origin in a dusty MHD disk wind, and the constraints it would set on the
underlying disk properties. We present synthetic H2O line profiles predictions
for a typical MHD disk wind solution with various values of disk accretion
rate, stellar mass, extension of the launching area, and view angle. We compare
them in terms of line shapes and intensities with the HIFI profiles observed by
the WISH Key Program. We find that a dusty MHD disk wind launched from 0.2--0.6
AU AU to 3--25 AU can reproduce to a remarkable degree the observed shapes and
intensities of the broad H2O component, both in the fundamental 557 GHz line
and in more excited lines. Such a model also readily reproduces the observed
correlation of 557 GHz line luminosity with envelope density, if the infall
rate at 1000 AU is 1--3 times the disk accretion rate in the wind ejection
region. It is also compatible with the typical disk size and bolometric
luminosity in the observed targets. However, the narrower line profiles in
Class 1 sources suggest that MHD disk winds in these sources, if present, would
have to be slower and/or less water rich than in Class 0 sources. In
conclusion, MHD disk winds appear as a valid (though not unique) option to
consider for the origin of the broad H2O component in low-mass protostars. ALMA
appears ideally suited to further test this model by searching for resolved
signatures of the warm and slow wide-angle molecular wind that would be
predicted.Comment: accepted for publication in A&
Information profiles for DNA pattern discovery
Finite-context modeling is a powerful tool for compressing and hence for
representing DNA sequences. We describe an algorithm to detect genomic
regularities, within a blind discovery strategy. The algorithm uses information
profiles built using suitable combinations of finite-context models. We used
the genome of the fission yeast Schizosaccharomyces pombe strain 972 h- for
illustration, unveilling locations of low information content, which are
usually associated with DNA regions of potential biological interest.Comment: Full version of DCC 2014 paper "Information profiles for DNA pattern
discovery
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