75,982 research outputs found
Diagnostic accuracy of tests based on radiologic measurements of dogs and cats: a systematic review
Structure and dynamics of topological defects in a glassy liquid on a negatively curved manifold
We study the low-temperature regime of an atomic liquid on the hyperbolic
plane by means of molecular dynamics simulation and we compare the results to a
continuum theory of defects in a negatively curved hexagonal background. In
agreement with the theory and previous results on positively curved (spherical)
surfaces, we find that the atomic configurations consist of isolated defect
structures, dubbed "grain boundary scars", that form around an irreducible
density of curvature-induced disclinations in an otherwise hexagonal
background. We investigate the structure and the dynamics of these grain
boundary scars
Quantum Holonomies in (2+1)-Dimensional Gravity
We describe an approach to the quantization of (2+1)--dimensional gravity
with topology R x T^2 and negative cosmological constant, which uses two
quantum holonomy matrices satisfying a q--commutation relation. Solutions of
diagonal and upper--triangular form are constructed, which in the latter case
exhibit additional, non--trivial internal relations for each holonomy matrix.
This leads to the notion of quantum matrix pairs. These are pairs of matrices
with non-commuting entries, which have the same pattern of internal relations,
q-commute with each other under matrix multiplication, and are such that
products of powers of the matrices obey the same pattern of internal relations
as the original pair. This has implications for the classical moduli space,
described by ordered pairs of commuting SL(2,R) matrices modulo simultaneous
conjugation by SL(2,R) matrices.Comment: 5 pages, to appear in the proceedings of 10th Marcel Grossmann
Meeting on Recent Developments in Theoretical and Experimental General
Relativity, Gravitation and Relativistic Field Theories (MG X MMIII), Rio de
Janeiro, Brazil, 20-26 Jul 200
Discs and Planetary Formation
The formation, structure and evolution of protoplanetary discs is considered.
The formation of giant planets within the environment of these models is also
discussed.Comment: 22 pages, LaTeX (including 6 figures), uses paspconf.sty, epsf.sty
and rotate.sty, to be published in Proceedings of the EC Summer School on
'Astrophysical Discs', eds J. A. Sellwood and J. Goodman, ASP Conf. Serie
Terrestrial planet formation in low eccentricity warm-Jupiter systems
We examine the effect of giant planet migration on the formation of inner
terrestrial planet systems. We consider situations in which the giant planet
halts migration at semi-major axes in the range 0.13 - 1.7 AU due to gas disk
dispersal. An N-body code is employed that is linked to a viscous gas disk
algorithm capable of simulating: gas loss via accretion onto the central star
and photoevaporation; gap formation by the giant planet; type II migration of
the giant; optional type I migration of protoplanets; gas drag on
planetesimals. We find that most of the inner system planetary building blocks
survive the passage of the giant planet, either by being shepherded inward or
scattered into exterior orbits. Systems of one or more hot-Earths are predicted
to form and remain interior to the giant planet, especially if type II
migration has been limited, or where type I migration has affected
protoplanetary dynamics. Habitable planets in low eccentricity warm-Jupiter
systems appear possible if the giant planet makes a limited incursion into the
outer regions of the habitable zone (HZ), or traverses its entire width and
ceases migrating at a radial distance of less than half that of the HZ's inner
edge. We conclude that Type II migration does not prevent terrestrial planet
formation.Comment: Accepted for publication in A&A; 18 pages, 12 figures, 2 table
Tidally-induced warps in protostellar discs
We review results on the dynamics of warped gaseous discs. We consider tidal
perturbation of a Keplerian disc by a companion star orbiting in a plane
inclined to the disc. The perturbation induces the precession of the disc, and
thus of any jet it could drive. In some conditions the precession rate is
uniform, and as a result the disc settles into a warp mode. The tidal torque
also leads to the truncation of the disc, to the evolution of the inclination
angle (not necessarily towards alignment of the disc and orbital planes) and to
a transport of angular momentum in the disc. We note that the spectral energy
distribution of such a warped disc is different from that of a flat disc. We
conclude by listing observational effects of warps in protostellar discs.Comment: 10 pages, LaTeX (including 1 figure), uses paspconf.sty and epsf.sty,
to be published in Proceedings of the EC Summer School on 'Astrophysical
Discs', eds J. A. Sellwood and J. Goodman, ASP Conf. Serie
Study to investigate the effects of ionizing radiation on transistor surfaces Third quarterly report, period ending, 31 Mar. 1966
Ionizing radiation effects on transistor surface
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