93,943 research outputs found
Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets
Extrasolar planets found with radial velocity surveys have masses ranging
from several Earth to several Jupiter masses. While mass accretion onto
protoplanetary cores in weak-line T-Tauri disks may eventually be quenched by a
global depletion of gas, such a mechanism is unlikely to have stalled the
growth of some known planetary systems which contain relatively low-mass and
close-in planets along with more massive and longer period companions. Here, we
suggest a potential solution for this conundrum. In general, supersonic infall
of surrounding gas onto a protoplanet is only possible interior to both of its
Bondi and Roche radii. At a critical mass, a protoplanet's Bondi and Roche
radii are equal to the disk thickness. Above this mass, the protoplanets' tidal
perturbation induces the formation of a gap. Although the disk gas may continue
to diffuse into the gap, the azimuthal flux across the protoplanets' Roche lobe
is quenched. Using two different schemes, we present the results of numerical
simulations and analysis to show that the accretion rate increases rapidly with
the ratio of the protoplanet's Roche to Bondi radii or equivalently to the disk
thickness. In regions with low geometric aspect ratios, gas accretion is
quenched with relatively low protoplanetary masses. This effect is important
for determining the gas-giant planets' mass function, the distribution of their
masses within multiple planet systems around solar type stars, and for
suppressing the emergence of gas-giants around low mass stars
Statistical Modelling of Information Sharing: Community, Membership and Content
File-sharing systems, like many online and traditional information sharing
communities (e.g. newsgroups, BBS, forums, interest clubs), are dynamical
systems in nature. As peers get in and out of the system, the information
content made available by the prevailing membership varies continually in
amount as well as composition, which in turn affects all peers' join/leave
decisions. As a result, the dynamics of membership and information content are
strongly coupled, suggesting interesting issues about growth, sustenance and
stability.
In this paper, we propose to study such communities with a simple statistical
model of an information sharing club. Carrying their private payloads of
information goods as potential supply to the club, peers join or leave on the
basis of whether the information they demand is currently available.
Information goods are chunked and typed, as in a file sharing system where
peers contribute different files, or a forum where messages are grouped by
topics or threads. Peers' demand and supply are then characterized by
statistical distributions over the type domain.
This model reveals interesting critical behaviour with multiple equilibria. A
sharp growth threshold is derived: the club may grow towards a sustainable
equilibrium only if the value of an order parameter is above the threshold, or
shrink to emptiness otherwise. The order parameter is composite and comprises
the peer population size, the level of their contributed supply, the club's
efficiency in information search, the spread of supply and demand over the type
domain, as well as the goodness of match between them.Comment: accepted in International Symposium on Computer Performance,
Modeling, Measurements and Evaluation, Juan-les-Pins, France, October-200
The Economics of Testing for Biotech Grain: Application to StarLink Corn
StarLink corn, a biotech variety not approved for human food use, disrupted the marketing system in 2000 because of inadvertent commingling. Testing protocols have since been established for detection of StarLink in corn shipments to Japan. Domestic food manufacturers, anxious to avoid risks of contamination and product recalls, also test for StarLink kernels. This paper provides an overview of the economics of testing. What are the risks facing buyers and sellers, and how are these influenced by different testing protocols? How do market premiums and discounts, testing costs, and prior beliefs affect the incentives to test? A conceptual model is developed in which sellers can choose whether to pre-test grain prior to shipment. Simulation analysis is used to illustrate the impact of market premiums and other variables on testing incentives and buyer risk.Research and Development/Tech Change/Emerging Technologies,
The Economics of Testing for Biotech Grain: Application to StarLink Corn
StarLink corn, a variety not approved for human use, disrupted the marketing system in 2000 because of inadvertent commingling. This paper provides an overview of the economics of testing grain for biotech content. What are the risks facing buyers and sellers, and how are these influenced by testing protocols? How do market premiums and discounts, testing costs, and prior beliefs affect the incentives to test? A conceptual model is developed in which sellers choose whether to pre-test grain prior to shipment. Through simulation analysis, we illustrate the impact of market premiums and other variables on testing incentives and buyer risk.biotechnology, grain marketing, quality risk, StarLink, testing, Crop Production/Industries,
Superfluid shells for trapped fermions with mass and population imbalance
We map out the phase diagram of strongly interacting fermions in a potential
trap with mass and population imbalance between the two spin components. As a
unique feature distinctively different from the equal-mass case, we show that
the superfluid here forms a shell structure which is not simply connected in
space. Different types of normal states occupy the trap regions inside and
outside this superfluid shell. We calculate the atomic density profiles, which
provide an experimental signature for the superfluid shell structure.Comment: 4 pages, 3 figure
Atmospheric Dynamics of Short-period Extra Solar Gas Giant Planets I: Dependence of Night-Side Temperature on Opacity
More than two dozen short-period Jupiter-mass gas giant planets have been
discovered around nearby solar-type stars in recent years, several of which
undergo transits, making them ideal for the detection and characterization of
their atmospheres. Here we adopt a three-dimensional radiative hydrodynamical
numerical scheme to simulate atmospheric circulation on close-in gas giant
planets. In contrast to the conventional GCM and shallow water algorithms, this
method does not assume quasi hydrostatic equilibrium and it approximates
radiation transfer from optically thin to thick regions with flux-limited
diffusion. In the first paper of this series, we consider
synchronously-spinning gas giants. We show that a full three-dimensional
treatment, coupled with rotationally modified flows and an accurate treatment
of radiation, yields a clear temperature transition at the terminator. Based on
a series of numerical simulations with varying opacities, we show that the
night-side temperature is a strong indicator of the opacity of the planetary
atmosphere. Planetary atmospheres that maintain large, interstellar opacities
will exhibit large day-night temperature differences, while planets with
reduced atmospheric opacities due to extensive grain growth and sedimentation
will exhibit much more uniform temperatures throughout their photosphere's. In
addition to numerical results, we present a four-zone analytic approximation to
explain this dependence.Comment: 35 Pages, 13 Figure
Green's function for the Relativistic Coulomb System via Sum Over Perturbation Series
We evaluate the Green's function of the D-dimensional relativistic Coulomb
system via sum over perturbation series which is obtained by expanding the
exponential containing the potential term in the path integral
into a power series. The energy spectra and wave functions are extracted from
the resulting amplitude.Comment: 13 pages, ReVTeX, no figure
Remark on approximation in the calculation of the primordial spectrum generated during inflation
We re-examine approximations in the analytical calculation of the primordial
spectrum of cosmological perturbation produced during inflation. Taking two
inflation models (chaotic inflation and natural inflation) as examples, we
numerically verify the accuracy of these approximations.Comment: 10 pages, 6 figures, to appear in PR
Low-level gust gradient program and avialtion workshop effort
The Proceedings of the Workshop on Meteorological and Environmental Inputs to Aviation Systems, hosted by the University of Tennessee Space Institute, October 26-28, 1982, were prepared for publication. The Proceedings were submitted to FAA and will be distributed by August. Also, the proceedings of a one day workshop devoted specifically to wind shear and hosted during the same time frame were prepared and distributed. Plans for the 1983 workshop are proceeding extremely well. The workshop theme was established, the committee topics identified, and all ten committee chairmen contacted have agreed to accept their respective assignments. Additional logistics for the workshop are being carried out. The 1983 workshop is scheduled for October 26-28, 1983. Data gathered with the B-57B during the Joint Airport Weather Studies Project in Denver, Colorado, were analyzed. All runs for Flight 6 on July 16, 1982, were analyzed. Spectra, cross spectra and probability distributions were computed for each run. Also, Runs 10-14 of Flight 7 on July 15, 1982, were analyzed in similar detail
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