5,961 research outputs found
From HIV infection to AIDS: A dynamically induced percolation transition?
The origin of the unusual incubation period distribution in the development
of AIDS is largely unresolved. A key factor in understanding the observed
distribution of latency periods, as well as the occurrence of infected
individuals not developing AIDS at all, is the dynamics of the long lasting
struggle between HIV and the immune system. Using a computer simulation, we
study the diversification of viral genomes under mutation and the selective
pressure of the immune system.In common infections vast spreading of viral
genomes usually does not takes place. In the case of an HIV infection this may
occur, as the virus successively weakens the immune system by depletion of CD4+
cells.In a sequence space framework, this leads to a dynamically induced
percolation transition, corresponding to the onset of AIDS. As a result, we
obtain the prolongated shape of the incubation period distribution, as well as
a finite fraction of non-progressors that do not develop AIDS, comparing well
with results from recent clinical research.Comment: 7 pages RevTeX, 4 figure
A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti
The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a
substantial quantity of molecular gas while exhibiting dust properties similar
to those of a debris disk. We present resolved observations of the disk around
49 Ceti from the Submillimeter Array in the J=2-1 rotational transition of CO
with a resolution of 1.0x1.2 arcsec. The observed emission reveals an extended
rotating structure viewed approximately edge-on and clear of detectable CO
emission out to a distance of ~90 AU from the star. No 1.3 millimeter continuum
emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk
structure and chemistry indicate that the inner disk is devoid of molecular
gas, while the outer gas disk between 40 and 200 AU from the star is dominated
by photochemistry from stellar and interstellar radiation. We determine
parameters for a model that reproduces the basic features of the spatially
resolved CO J=2-1 emission, the spectral energy distribution, and the
unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and
observable properties for a range of structural parameters. 49 Ceti appears to
be a rare example of a system in a late stage of transition between a gas-rich
protoplanetary disk and a tenuous, virtually gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap
THE MENTAL HEALTH OF MOTHERS AND FATHERS BEFORE AND AFTER COHABITATION AND MARITAL DISSOLUTION
Using data from years one and three of the Fragile Families and Child Well-being Study, changes in depressive and anxious symptoms are compared for mothers and fathers who: 1) dissolve a cohabitating union versus remain intact; 2) dissolve a marital union versus remain intact; and 3) dissolve a cohabiting as compared to a marital union. In order to take into account potential sources of third variable bias from selection factors that differentiate those who are in cohabitations from those who are in marriages, mothers and fathers were matched on several sociodemographic control variables that research has demonstrated to be related to union formation and mental health outcomes. Results indicated that fathers who dissolve cohabitating or marital unions have greater increases in depressive and anxious symptoms over time than those who remain in their unions. In contrast, mothers increased in depressive and anxious symptoms, regardless of the type or stability of the union. For both mothers and fathers, no differences were found in change in mental health by type of union dissolution. In this low income sample of parents, results suggest that the impact of cohabitation and marital dissolution on mental health are similar in magnitude.Depression, fragile families, marriage, cohabitation, income, mental health
IMPLICATIONS OF VIOLENT AND CONTROLLING UNIONS FOR MOTHERS’ MENTAL HEALTH AND LEAVING
We used two waves of the Fragile Families Study (N = 2639) to examine links between control and violence with maternal mental health and relationship dissolution. Mothers in controlling-only or controlling and violent unions had more symptoms of depression and anxiety and greater odds of dissolution than mothers not experiencing violence or control. Over time, all mothers increased in depressive symptoms, but the magnitude of the increase in depressive symptoms was greatest for mothers in violent and controlling stable unions followed by those in controlling-only stable unions. Mothers dissolving violent and/or controlling unions also experienced increases depressive symptoms over time. Results indicate negative consequences for both mothers who remain in and leave violent and controlling unions.
Patterns of Late Cenozoic exhumation deduced from apatite and zircon U-He ages from Fiordland, New Zealand
New apatite and zircon (U-Th)/He ages from the Fiordland region of New Zealand's South Island expand on earlier results and provide new constraints on patterns of Late Cenozoic exhumation and cooling across this region. Zircon (U-Th)/He cooling ages, in combination with increased density of apatite ages, show that in addition to a gradual northward decrease in cooling ages that was seen during an earlier phase of this study, there is also a trend toward younger cooling ages to the east. Distinct breaks in cooling age patterns on southwestern Fiordland appear to be correlated to the location of previously mapped faults. The northward decrease in ages may reflect asynchronous cooling related to migration in the locus of exhumation driven by subduction initiation, or it may reflect synchronous regional exhumation that exposed different structural levels across Fiordland, or some combination of these effects. In either case, differential exhumation accommodated by major and minor faults that dissect Fiordland basement rocks apparently played an important role in producing the resulting age patterns
Two-dimensional anisotropic Heisenberg antiferromagnet in a field
The classical, square lattice, uniaxially anisotropic Heisenberg
antiferromagnet in a magnetic field parallel to the easy axis is studied using
Monte Carlo techniques. The model displays a long-range ordered
antiferromagnetic, an algebraically ordered spin-flop, and a paramagnetic
phase. The simulations indicate that a narrow disordered phase intervenes
between the ordered phases down to quite low temperatures. Results are compared
to previous, partially conflicting findings on related classical models as well
as the quantum variant with spin S=1/2.Comment: 8 pages, 9 figure
The staircase method: integrals for periodic reductions of integrable lattice equations
We show, in full generality, that the staircase method provides integrals for
mappings, and correspondences, obtained as traveling wave reductions of
(systems of) integrable partial difference equations. We apply the staircase
method to a variety of equations, including the Korteweg-De Vries equation, the
five-point Bruschi-Calogero-Droghei equation, the QD-algorithm, and the
Boussinesq system. We show that, in all these cases, if the staircase method
provides r integrals for an n-dimensional mapping, with 2r<n, then one can
introduce q<= 2r variables, which reduce the dimension of the mapping from n to
q. These dimension-reducing variables are obtained as joint invariants of
k-symmetries of the mappings. Our results support the idea that often the
staircase method provides sufficiently many integrals for the periodic
reductions of integrable lattice equations to be completely integrable. We also
study reductions on other quad-graphs than the regular 2D lattice, and we prove
linear growth of the multi-valuedness of iterates of high-dimensional
correspondences obtained as reductions of the QD-algorithm.Comment: 40 pages, 23 Figure
Phase diagrams of a classical two-dimensional Heisenberg antiferromagnet with single-ion anisotropy
A classical variant of the two-dimensional anisotropic Heisenberg model
reproducing inelastic neutron scattering experiments on La_5 Ca_9 Cu_24 O_41
[M. Matsuda et al., Phys.Rev. B 68, 060406(R) (2003)] is analysed using mostly
Monte Carlo techniques. Phase diagrams with external fields parallel and
perpendicular to the easy axis of the anisotropic interactions are determined,
including antiferromagnetic and spin-flop phases. Mobile spinless defects, or
holes, are found to form stripes which bunch, debunch and break up at a phase
transition. A parallel field can lead to a spin-flop phase.Comment: 9 pages, 9 figures; final version as accepted by Phys. Rev. B (Fig. 5
replaced, added remarks in Secs. I, III, and V
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