16,997 research outputs found
Collisional Grooming Models of the Kuiper Belt Dust Cloud
We modeled the 3-D structure of the Kuiper Belt dust cloud at four different
dust production rates, incorporating both planet-dust interactions and
grain-grain collisions using the collisional grooming algorithm. Simulated
images of a model with a face-on optical depth of ~10^-4 primarily show an
azimuthally-symmetric ring at 40-47 AU in submillimeter and infrared
wavelengths; this ring is associated with the cold classical Kuiper Belt. For
models with lower optical depths (10^-6 and 10^-7), synthetic infrared images
show that the ring widens and a gap opens in the ring at the location of of
Neptune; this feature is caused by trapping of dust grains in Neptune's mean
motion resonances. At low optical depths, a secondary ring also appears
associated with the hole cleared in the center of the disk by Saturn. Our
simulations, which incorporate 25 different grain sizes, illustrate that
grain-grain collisions are important in sculpting today's Kuiper Belt dust, and
probably other aspects of the Solar System dust complex; collisions erase all
signs of azimuthal asymmetry from the submillimeter image of the disk at every
dust level we considered. The model images switch from being dominated by
resonantly-trapped small grains ("transport dominated") to being dominated by
the birth ring ("collision dominated") when the optical depth reaches a
critical value of tau ~ v/c, where v is the local Keplerian speed.Comment: 31 pages, including 9 figure
Seasonal Price Patterns for Arkansas Soybeans
Seasonality is generally regarded as a major feature in soybean market price variations. Recent years in Arkansas have seen considerable construction of on-farm storage, a move that could mitigate the seasonality effect on price variations. A study comparing cash price indices from the past ten years with results from a 1986 Arkansas study and a recent national-level study found that Arkansas soybean prices appear to have followed a consistent and logical pattern around their national average in spite of increased variability and uncertainty.Seasonality, soybean, cash price index, Demand and Price Analysis, Farm Management, Marketing,
Gated rotation mechanism of site-specific recombination by Ď•C31 integrase
Integrases, such as that of the Streptomyces temperate bacteriophage ϕC31, promote site-specific recombination between DNA sequences in the bacteriophage and bacterial genomes to integrate or excise the phage DNA. ϕC31 integrase belongs to the serine recombinase family, a large group of structurally related enzymes with diverse biological functions. It has been proposed that serine integrases use a “subunit rotation” mechanism to exchange DNA strands after double-strand DNA cleavage at the two recombining att sites, and that many rounds of subunit rotation can occur before the strands are religated. We have analyzed the mechanism of ϕC31 integrase-mediated recombination in a topologically constrained experimental system using hybrid “phes” recombination sites, each of which comprises a ϕC31 att site positioned adjacent to a regulatory sequence recognized by Tn3 resolvase. The topologies of reaction products from circular substrates containing two phes sites support a right-handed subunit rotation mechanism for catalysis of both integrative and excisive recombination. Strand exchange usually terminates after a single round of 180° rotation. However, multiple processive “360° rotation” rounds of strand exchange can be observed, if the recombining sites have nonidentical base pairs at their centers. We propose that a regulatory “gating” mechanism normally blocks multiple rounds of strand exchange and triggers product release after a single round
Study of zero-gravity, vapor/liquid separators
Heat exchange, mechanical separation, surface tension, and dielectrophoretic methods of separating vapor from liquid at zero gravity for vapor ventin
The Detectability of Exo-Earths and Super-Earths Via Resonant Signatures in Exozodiacal Clouds
Directly imaging extrasolar terrestrial planets necessarily means contending
with the astrophysical noise of exozodiacal dust and the resonant structures
created by these planets in exozodiacal clouds. Using a custom tailored hybrid
symplectic integrator we have constructed 120 models of resonant structures
created by exo-Earths and super-Earths on circular orbits interacting with
collisionless steady-state dust clouds around a Sun-like star. Our models
include enough particles to overcome the limitations of previous simulations
that were often dominated by a handful of long-lived particles, allowing us to
quantitatively study the contrast of the resulting ring structures. We found
that in the case of a planet on a circular orbit, for a given star and dust
source distribution, the morphology and contrast of the resonant structures
depend on only two parameters: planet mass and , where
is the planet's semi-major axis and is the ratio of
radiation pressure force to gravitational force on a grain. We constructed
multiple-grain-size models of 25,000 particles each and showed that in a
collisionless cloud, a Dohnanyi crushing law yields a resonant ring whose
optical depth is dominated by the largest grains in the distribution, not the
smallest. We used these models to estimate the mass of the lowest-mass planet
that can be detected through observations of a resonant ring for a variety of
assumptions about the dust cloud and the planet's orbit. Our simulations
suggest that planets with mass as small as a few times Mar's mass may produce
detectable signatures in debris disks for semi-major axes greater than 10 AU.Comment: 33 pages, 12 figure
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