14,109 research outputs found
Mean field and pairing properties in the crust of neutron stars
Properties of the matter in the inner crust of a neutron star are
investigated in a Hartree-Fock plus BCS approximation employing schematic
effective forces of the type of the Skyrme forces. Special attention is paid to
differences between a homogenous and inhomogeneous description of the matter
distribution. For that purpose self-consistent Hartree Fock calculations are
performed in a spherical Wigner-Seitz cell. The results are compared to
predictions of corresponding Thomas Fermi calculations. The influence of the
shell structure on the formation of pairing correlations in inhomogeneous
matter are discussed.Comment: 11 pages, 9 figure
Species Abundance Patterns in Complex Evolutionary Dynamics
An analytic theory of species abundance patterns (SAPs) in biological
networks is presented. The theory is based on multispecies replicator dynamics
equivalent to the Lotka-Volterra equation, with diverse interspecies
interactions. Various SAPs observed in nature are derived from a single
parameter. The abundance distribution is formed like a widely observed
left-skewed lognormal distribution. As the model has a general form, the result
can be applied to similar patterns in other complex biological networks, e.g.
gene expression.Comment: 4 pages, 3 figures. Physical Review Letters, in pres
Complementarity and diversity in a soluble model ecosystem
Complementarity among species with different traits is one of the basic
processes affecting biodiversity, defined as the number of species in the
ecosystem. We present here a soluble model ecosystem in which the species are
characterized by binary traits and their pairwise interactions follow a
complementarity principle. Manipulation of the species composition, and so the
study of its effects on the species diversity is achieved through the
introduction of a bias parameter favoring one of the traits. Using statistical
mechanics tools we find explicit expressions for the allowed values of the
equilibrium species concentrations in terms of the control parameters of the
model
The open cluster NGC 6520 and the nearby dark molecular cloud Barnard 86
Wide field BVI photometry and CO(10) observations are presen ted
in the region of the open cluster NGC 6520 and the dark molecular cloud
Barnard~86. From the analysis of the optical data we find that the cluster is
rather compact, with a radius of 1.00.5 arcmin, smaller than previous
estimates. The cluster age is 15050 Myr and the reddening
E=0.420.10. The distance from the Sun is estimated to be
1900100 pc, and it is larger than previous estimates. We finally derive
basic properties of the dark nebula Barnard 86 on the assumption that it lies
at the same distance of the cluster.Comment: 21 pages, 8 eps figures (a few degraded in resolution), accepted for
publication in the Astronomical Journa
Excision for simplicial sheaves on the Stein site and Gromov's Oka principle
A complex manifold satisfies the Oka-Grauert property if the inclusion
\Cal O(S,X) \hookrightarrow \Cal C(S,X) is a weak equivalence for every Stein
manifold , where the spaces of holomorphic and continuous maps from to
are given the compact-open topology. Gromov's Oka principle states that if
has a spray, then it has the Oka-Grauert property. The purpose of this
paper is to investigate the Oka-Grauert property using homotopical algebra. We
embed the category of complex manifolds into the model category of simplicial
sheaves on the site of Stein manifolds. Our main result is that the Oka-Grauert
property is equivalent to representing a finite homotopy sheaf on the Stein
site. This expresses the Oka-Grauert property in purely holomorphic terms,
without reference to continuous maps.Comment: Version 3 contains a few very minor improvement
The S(0) structure in highly compressed hydrogen and the orientational transition
A calculation of the rotational S(0) frequencies in high pressure solid
para-hydrogen is performed. Convergence of the perturbative series at high
density is demonstrated by the calculation of second and third order terms. The
results of the theory are compared with the available experimental data to
derive the density behaviour of structural parameters. In particular, a strong
increase of the value of the lattice constant ratio and of the
internuclear distance is determined. Also a decrease of the anisotropic
intermolecular potential is observed which is attributed to charge transfer
effects. The structural parameters determined at the phase transition may be
used to calculate quantum properties of the rotationally ordered phase.Comment: accepted Europhysics Letter
Transmission of severe acute respiratory syndrome in dynamical small-world networks
The outbreak of severe acute respiratory syndrome (SARS) is still threatening
the world because of a possible resurgence. In the current situation that
effective medical treatments such as antiviral drugs are not discovered yet,
dynamical features of the epidemics should be clarified for establishing
strategies for tracing, quarantine, isolation, and regulating social behavior
of the public at appropriate costs. Here we propose a network model for SARS
epidemics and discuss why superspreaders emerged and why SARS spread especially
in hospitals, which were key factors of the recent outbreak. We suggest that
superspreaders are biologically contagious patients, and they may amplify the
spreads by going to potentially contagious places such as hospitals. To avoid
mass transmission in hospitals, it may be a good measure to treat suspected
cases without hospitalizing them. Finally, we indicate that SARS probably
propagates in small-world networks associated with human contacts and that the
biological nature of individuals and social group properties are factors more
important than the heterogeneous rates of social contacts among individuals.
This is in marked contrast with epidemics of sexually transmitted diseases or
computer viruses to which scale-free network models often apply.Comment: 4 figure
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