17,226 research outputs found
Clearing residual planetesimals by sweeping secular resonances in transitional disks: a lone-planet scenario for the wide gaps in debris disks around Vega and Fomalhaut
Extended gaps in the debris disks of both Vega and Fomalhaut have been
observed. These structures have been attributed to tidal perturbations by
multiple super-Jupiter gas giant planets. Within the current observational
limits, however, no such massive planets have been detected. Here we propose a
less stringent `lone-planet' scenario to account for the observed structure
with a single eccentric gas giant and suggest that clearing of these wide gaps
is induced by its sweeping secular resonance. During the depletion of the disk
gas, the planet's secular resonance propagates inward and clears a wide gap
over an extended region of the disk. Although some residual intermediate-size
planetesimals may remain in the gap, their surface density is too low to either
produce super-Earths or lead to sufficiently frequent disruptive collisions to
generate any observable dusty signatures. The main advantage of this
lone-planet sweeping-secular-resonance model over the previous multiple gas
giant tidal truncation scenario is the relaxed requirement on the number of gas
giants. The observationally inferred upper mass limit can also be satisfied
provided the hypothetical planet has a significant eccentricity. A significant
fraction of solar or more massive stars bear gas giant planets with significant
eccentricities. If these planets acquired their present-day kinematic
properties prior to the depletion of their natal disks, their sweeping secular
resonance would effectively impede the retention of neighboring planets and
planetesimals over a wide range of orbital semi-major axes.Comment: 20 pages, 12 figures. Accepted for publication in Ap
From integrated to expedient: an adaptive framework for river basin management in developing countries
Water resource management / River basin management / Water allocation / Case studies / Africa South of Sahara / Great Ruaha River Basin
Reverse geometric engineering of singularities
One can geometrically engineer supersymmetric field theories theories by
placing D-branes at or near singularities. The opposite process is described,
where one can reconstruct the singularities from quiver theories. The
description is in terms of a noncommutative quiver algebra which is constructed
from the quiver diagram and the superpotential. The center of this
noncommutative algebra is a commutative algebra, which is the ring of
holomorphic functions on a variety V. If certain algebraic conditions are met,
then the reverse geometric engineering produces V as the geometry that D-branes
probe. It is also argued that the identification of V is invariant under
Seiberg dualities.Comment: 17 pages, Latex. v2: updates reference
Cylindrical gravitational waves in expanding universes: Models for waves from compact sources
New boundary conditions are imposed on the familiar cylindrical gravitational
wave vacuum spacetimes. The new spacetime family represents cylindrical waves
in a flat expanding (Kasner) universe. Space sections are flat and nonconical
where the waves have not reached and wave amplitudes fall off more rapidly than
they do in Einstein-Rosen solutions, permitting a more regular null inifinity.Comment: Minor corrections to references. A note added in proo
Depletion potentials in highly size-asymmetric binary hard-sphere mixtures: Comparison of accurate simulation results with theory
We report a detailed study, using state-of-the-art simulation and theoretical
methods, of the depletion potential between a pair of big hard spheres immersed
in a reservoir of much smaller hard spheres, the size disparity being measured
by the ratio of diameters q=\sigma_s/\sigma_b. Small particles are treated
grand canonically, their influence being parameterized in terms of their
packing fraction in the reservoir, \eta_s^r. Two specialized Monte Carlo
simulation schemes --the geometrical cluster algorithm, and staged particle
insertion-- are deployed to obtain accurate depletion potentials for a number
of combinations of q\leq 0.1 and \eta_s^r. After applying corrections for
simulation finite-size effects, the depletion potentials are compared with the
prediction of new density functional theory (DFT) calculations based on the
insertion trick using the Rosenfeld functional and several subsequent
modifications. While agreement between the DFT and simulation is generally
good, significant discrepancies are evident at the largest reservoir packing
fraction accessible to our simulation methods, namely \eta_s^r=0.35. These
discrepancies are, however, small compared to those between simulation and the
much poorer predictions of the Derjaguin approximation at this \eta_s^r. The
recently proposed morphometric approximation performs better than Derjaguin but
is somewhat poorer than DFT for the size ratios and small sphere packing
fractions that we consider. The effective potentials from simulation, DFT and
the morphometric approximation were used to compute the second virial
coefficient B_2 as a function of \eta_s^r. Comparison of the results enables an
assessment of the extent to which DFT can be expected to correctly predict the
propensity towards fluid fluid phase separation in additive binary hard sphere
mixtures with q\leq 0.1.Comment: 16 pages, 9 figures, revised treatment of morphometric approximation
and reordered some materia
Chain Inflation in the Landscape: "Bubble Bubble Toil and Trouble"
In the model of Chain Inflation, a sequential chain of coupled scalar fields
drives inflation. We consider a multidimensional potential with a large number
of bowls, or local minima, separated by energy barriers: inflation takes place
as the system tunnels from the highest energy bowl to another bowl of lower
energy, and so on until it reaches the zero energy ground state. Such a
scenario can be motivated by the many vacua in the stringy landscape, and our
model can apply to other multidimensional potentials. The ''graceful exit''
problem of Old Inflation is resolved since reheating is easily achieved at each
stage. Coupling between the fields is crucial to the scenario. The model is
quite generic and succeeds for natural couplings and parameters. Chain
inflation succeeds for a wide variety of energy scales -- for potentials
ranging from 10MeV scale inflation to GeV scale inflation.Comment: 31 pages, 3 figures, one reference adde
Deconstructing Noncommutativity with a Giant Fuzzy Moose
We argue that the worldvolume theories of D-branes probing orbifolds with
discrete torsion develop, in the large quiver limit, new non-commutative
directions. This provides an explicit `deconstruction' of a wide class of
noncommutative theories. This also provides insight into the physical meaning
of discrete torsion and its relation to the T-dual B field. We demonstrate that
the strict large quiver limit reproduces the matrix theory construction of
higher-dimensional D-branes, and argue that finite `fuzzy moose' theories
provide novel regularizations of non-commutative theories and explicit string
theory realizations of gauge theories on fuzzy tori. We also comment briefly on
the relation to NCOS, (2,0) and little string theories.Comment: 22 pages, 3 figures, typos caught and refs added; expanded
interpretation of discrete torsio
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