17,189 research outputs found
Disk-planets interactions and the diversity of period ratios in Kepler's multi-planetary systems
The Kepler mission is dramatically increasing the number of planets known in
multi-planetary systems. Many adjacent planets have orbital period ratios near
resonant values, with a tendency to be larger than required for exact
first-order mean-motion resonances. This intriguing feature has been shown to
be a natural outcome of orbital circularization of resonant planetary pairs due
to star-planet tidal interactions. However, this feature holds in
multi-planetary systems with periods longer than ten days, for which tidal
circularization is unlikely to provide efficient divergent evolution of the
planets orbits. Gravitational interactions between planets and their parent
protoplanetary disk may instead provide efficient divergent evolution. For a
planet pair embedded in a disk, we show that interactions between a planet and
the wake of its companion can reverse convergent migration, and significantly
increase the period ratio from a near-resonant value. Divergent evolution due
to wake-planet interactions is particularly efficient when at least one of the
planets opens a partial gap around its orbit. This mechanism could help account
for the diversity of period ratios in Kepler's multiple systems comprising
super-Earth to sub-jovian planets with periods greater than about ten days.
Diversity is also expected for planet pairs massive enough to merge their gap.
The efficiency of wake-planet interactions is then much reduced, but convergent
migration may stall with a variety of period ratios depending on the density
structure in the common gap. This is illustrated for the Kepler-46 system, for
which we reproduce the period ratio of Kepler-46b and c.Comment: 15 pages, 11 figures, accepted for publication in Ap
Scaling properties of velocity and temperature spectra above the surface friction layer in a convective atmospheric boundary layer
International audienceWe report velocity and temperature spectra measured at nine levels from 1.42 meters up to 25.7 m over a smooth playa in Western Utah. Data are from highly convective conditions when the magnitude of the Obukhov length (our proxy for the depth of the surface friction layer) was less than 2 m. Our results are somewhat similar to the results reported from the Minnesota experiment of Kaimal et al. (1976), but show significant differences in detail. Our velocity spectra show no evidence of buoyant production of kinetic energy at at the scale of the thermal structures. We interpret our velocity spectra to be the result of outer eddies interacting with the ground, not "local free convection". We observe that velocity spectra represent the spectral distribution of the kinetic energy of the turbulence, so we use energy scales based on total turbulence energy in the convective boundary layer (CBL) to collapse our spectra. For the horizontal velocity spectra this scale is (zi ?o)2/3, where zi is inversion height and ?o is the dissipation rate in the bulk CBL. This scale functionally replaces the Deardorff convective velocity scale. Vertical motions are blocked by the ground, so the outer eddies most effective in creating vertical motions come from the inertial subrange of the outer turbulence. We deduce that the appropriate scale for the peak region of the vertical velocity spectra is (z ?o)2/3 where z is height above ground. Deviations from perfect spectral collapse under these scalings at large and small wavenumbers are explained in terms of the energy transport and the eddy structures of the flow. We find that the peaks of the temperature spectra collapse when wavenumbers are scaled using (z1/2 zi1/2). That is, the lengths of the thermal structures depend on both the lengths of the transporting eddies, ~9z, and the progressive aggregation of the plumes with height into the larger-scale structures of the CBL. This aggregation depends, in top-down fashion, on zi. The whole system is therefore highly organized, with even the smallest structures conforming to the overall requirements of the whole flow
Relationships among Prices across Alternative Marketing Arrangements for Fed Cattle and Hogs
Reduced reliance on cash market prices for fed cattle and hogs raise questions about the role of cash prices in price discovery. We use seven years of weekly data from mandatory price reports to determine whether or not cash market prices are cointegrated with other procurement prices and then test for causality among the price series. Cash prices were cointegrated with all but one procurement price series. Cash market prices Granger cause all other procurement prices. Bidirectional causality was found in some but not all cases. Thus, cash market prices remain of central importance in price discovery for fed cattle and hogs.cattle, cointegration, causality, hogs, Johansen, marketing, prices, Stock-Watson, vector error correction, Livestock Production/Industries, Marketing, Q13(of Q18),
FACTORS INFLUENCING CONSUMER DECISIONS RELATED TO "NATURAL" BEEF IN THE SOUTHERN PLAINS
Consumer/Household Economics,
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