28,789 research outputs found
On the formation time scale and core masses of gas giant planets
Numerical simulations show that the migration of growing planetary cores may
be dominated by turbulent fluctuations in the protoplanetary disk, rather than
by any mean property of the flow. We quantify the impact of this stochastic
core migration on the formation time scale and core mass of giant planets at
the onset of runaway gas accretion. For standard Solar Nebula conditions, the
formation of Jupiter can be accelerated by almost an order of magnitude if the
growing core executes a random walk with an amplitude of a few tenths of an au.
A modestly reduced surface density of planetesimals allows Jupiter to form
within 10 Myr, with an initial core mass below 10 Earth masses, in better
agreement with observational constraints. For extrasolar planetary systems, the
results suggest that core accretion could form massive planets in disks with
lower metallicities, and shorter lifetimes, than the Solar Nebula.Comment: ApJL, in pres
Control of flow separation and mixing by aerodynamic excitation
The recent research in the control of shear flows using unsteady aerodynamic excitation conducted at the NASA Lewis Research Center is reviewed. The program is of a fundamental nature, concentrating on the physics of the unsteady aerodynamic processes. This field of research is a fairly new development with great promise in the areas of enhanced mixing and flow separation control. Enhanced mixing research includes influence of core turbulence, forced pairing of coherent structures, and saturation of mixing enhancement. Separation flow control studies included are for a two-dimensional diffuser, conical diffusers, and single airfoils. Ultimate applications include aircraft engine inlet flow control at high angle of attack, wide angle diffusers, highly loaded airfoils as in turbomachinery, and ejector/suppressor nozzles for the supersonic transport. An argument involving the Coanda Effect is made that all of the above mentioned application areas really only involve forms of shear layer mixing enhancement. The program also includes the development of practical excitation devices which might be used in aircraft applications
Measured acoustic properties of variable and low density bulk absorbers
Experimental data were taken to determine the acoustic absorbing properties of uniform low density and layered variable density samples using a bulk absober with a perforated plate facing to hold the material in place. In the layered variable density case, the bulk absorber was packed such that the lowest density layer began at the surface of the sample and progressed to higher density layers deeper inside. The samples were placed in a rectangular duct and measurements were taken using the two microphone method. The data were used to calculate specific acoustic impedances and normal incidence absorption coefficients. Results showed that for uniform density samples the absorption coefficient at low frequencies decreased with increasing density and resonances occurred in the absorption coefficient curve at lower densities. These results were confirmed by a model for uniform density bulk absorbers. Results from layered variable density samples showed that low frequency absorption was the highest when the lowest density possible was packed in the first layer near the exposed surface. The layers of increasing density within the sample had the effect of damping the resonances
Magnetic Response in the Underdoped Cuprates
We examine the dynamical magnetic response of the underdoped cuprates by
employing a phenomenological theory of a doped resonant valence bond state
where the Fermi surface is truncated into four pockets. This theory predicts a
resonant spin response which with increasing energy (0 to 100meV) appears as an
hourglass. The very low energy spin response is found at (pi,pi +- delta) and
(pi +- delta,pi) and is determined by scattering from the pockets' frontside to
the tips of opposite pockets where a van Hove singularity resides. At energies
beyond 100 meV, strong scattering is seen from (pi,0) to (pi,pi). This theory
thus provides a semi-quantitative description of the spin response seen in both
INS and RIXS experiments at all relevant energy scales
Towards a Comprehensive Fueling-Controlled Theory on the Growth of Massive Black Holes and Host Spheroids
We study the relation between nuclear massive black holes and their host
spheroid gravitational potential. Using AMR numerical simulations, we analyze
how gas is transported in the nuclear (central kpc) regions of galaxies. We
study the gas fueling onto the inner accretion disk (sub-pc scale) and the star
formation in a massive nuclear disk like those generally found in
proto-spheroids (ULIRGs, SCUBA Galaxies). These sub-pc resolution simulation of
gas fueling that is mainly depleted by star formation naturally satisfy the
`M_BH - $M_virial' relation, with a scatter considerably less than the observed
one. We found a generalized version of Kennicutt-Schmidt Law for starbursts is
satisfied, in which the total gas depletion rate (dot{M}_gas = dot{M}_BH +
dot{M}_SF) is the one that scales as M_gas/t_orbital. We also found that the
`M_BH - sigma' relation is a byproduct of the `M_BH - M_virial' relation in the
fueling controlled scenario.Comment: 12 pages, figures, submited to ApJ, email: [email protected]
Herpetofaunal Inventories of the National Parks of South Florida and the Caribbean: Volume III. Big Cypress National Preserve
Amphibian declines and extinctions have been documented around the world, often in protected
natural areas. Concern for this trend has prompted the U.S. Geological Survey and the National Park
Service to document all species of amphibians that occur within U.S. National Parks and to search for any
signs that amphibians may be declining. This study, an inventory of amphibian species in Big Cypress
National Preserve, was conducted from 2002 to 2003. The goals of the project were to create a
georeferenced inventory of amphibian species, use new analytical techniques to estimate proportion of
sites occupied by each species, look for any signs of amphibian decline (missing species, disease, die-offs,
and so forth.), and to establish a protocol that could be used for future monitoring efforts.
Several sampling methods were used to accomplish these goals. Visual encounter surveys and
anuran vocalization surveys were conducted in all habitats throughout the park to estimate the proportion
of sites or proportion of area occupied (PAO) by each amphibian species in each habitat. Opportunistic
collections, as well as limited drift fence data, were used to augment the visual encounter methods for
highly aquatic or cryptic species. A total of 545 visits to 104 sites were conducted for standard sampling
alone, and 2,358 individual amphibians and 374 reptiles were encountered. Data analysis was conducted
in program PRESENCE to provide PAO estimates for each of the anuran species.
All of the amphibian species historically found in Big Cypress National Preserve were detected
during this project. At least one individual of each of the four salamander species was captured during
sampling. Each of the anuran species in the preserve was adequately sampled using standard
herpetological sampling methods, and PAO estimates were produced for each species of anuran by habitat.
This information serves as an indicator of habitat associations of the species and relative abundance of
sites occupied, but it will also be useful as a comparative baseline for future monitoring efforts.
In addition to sampling for amphibians, all encounters with reptiles were documented. The
sampling methods used for detecting amphibians are also appropriate for many reptile species. These
reptile locations are included in this report, but the number of reptile observations was not sufficient to
estimate PAO for reptile species. We encountered 35 of the 46 species of reptiles believed to be present in
Big Cypress National Preserve during this study, and evidence exists of the presence of four other reptile
species in the Preserve.
This study found no evidence of amphibian decline in Big Cypress National Preserve. Although no
evidence of decline was observed, several threats to amphibians were identified. Introduced species,
especially the Cuban treefrog (Osteopilus septentrionalis), are predators and competitors with several
native frog species. The recreational use of off-road vehicles has the potential to affect some amphibian
populations, and a study on those potential impacts is currently underway. Also, interference by humans
with the natural hydrologic cycle of south Florida has the potential to alter the amphibian community.
Continued monitoring of the amphibian species in Big Cypress National Preserve is recommended.
The methods used in this study were adequate to produce reliable estimates of the proportion of sites
occupied by most anuran species, and are a cost-effective means of determining the status of their
populations
Umklapp scattering as the origin of -linear resistivity in the normal state of high- cuprate superconductors
The high-temperature normal state of the unconventional cuprate
superconductors has resistivity linear in temperature , which persists to
values well beyond the Mott-Ioffe-Regel upper bound. At low-temperature, within
the pseudogap phase, the resistivity is instead quadratic in , as would be
expected from Fermi liquid theory. Developing an understanding of these normal
phases of the cuprates is crucial to explain the unconventional
superconductivity. We present a simple explanation for this behavior, in terms
of umklapp scattering of electrons. This fits within the general picture
emerging from functional renormalization group calculations that spurred the
Yang-Rice-Zhang ansatz: umklapp scattering is at the heart of the behavior in
the normal phase.Comment: v1 6+1 pages, 4 figures; v2 6+2 pages, 4 figures; v3 6 + 2.5 pages, 5
figure
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