5,000 research outputs found
Massive planet migration: Theoretical predictions and comparison with observations
We quantify the utility of large radial velocity surveys for constraining
theoretical models of Type II migration and protoplanetary disk physics. We
describe a theoretical model for the expected radial distribution of extrasolar
planets that combines an analytic description of migration with an empirically
calibrated disk model. The disk model includes viscous evolution and mass loss
via photoevaporation. Comparing the predicted distribution to a uniformly
selected subsample of planets from the Lick / Keck / AAT planet search
programs, we find that a simple model in which planets form in the outer disk
at a uniform rate, migrate inward according to a standard Type II prescription,
and become stranded when the gas disk is dispersed, is consistent with the
radial distribution of planets for orbital radii 0.1 AU < a < 2.5 AU and planet
masses greater than 1.65 Jupiter masses. Some variant models are disfavored by
existing data, but the significance is limited (~95%) due to the small sample
of planets suitable for statistical analysis. We show that the favored model
predicts that the planetary mass function should be almost independent of
orbital radius at distances where migration dominates the massive planet
population. We also study how the radial distribution of planets depends upon
the adopted disk model. We find that the distribution can constrain not only
changes in the power-law index of the disk viscosity, but also sharp jumps in
the efficiency of angular momentum transport that might occur at small radii.Comment: ApJ, in press. References updated to match published versio
Reduction of Effective Terahertz Focal Spot Size By Means Of Nested Concentric Parabolic Reflectors
An ongoing limitation of terahertz spectroscopy is that the technique is
generally limited to the study of relatively large samples of order 4 mm across
due to the generally large size of the focal beam spot. We present a nested
concentric parabolic reflector design which can reduce the terahertz focal spot
size. This parabolic reflector design takes advantage of the feature that
reflected rays experience a relative time delay which is the same for all
paths. The increase in effective optical path for reflected light is equivalent
to the aperture diameter itself. We have shown that the light throughput of an
aperture of 2 mm can be increased by a factor 15 as compared to a regular
aperture of the same size at low frequencies. This technique can potentially be
used to reduce the focal spot size in terahertz spectroscopy and enable the
study of smaller samples
Evidence for universality in the initial planetesimal mass function
Planetesimals may form from the gravitational collapse of dense particle
clumps initiated by the streaming instability. We use simulations of
aerodynamically coupled gas-particle mixtures to investigate whether the
properties of planetesimals formed in this way depend upon the sizes of the
particles that participate in the instability. Based on three high resolution
simulations that span a range of dimensionless stopping time no statistically significant differences in the initial
planetesimal mass function are found. The mass functions are fit by a
power-law, , with and
errors of . Comparing the particle density fields prior
to collapse, we find that the high wavenumber power spectra are similarly
indistinguishable, though the large-scale geometry of structures induced via
the streaming instability is significantly different between all three cases.
We interpret the results as evidence for a near-universal slope to the mass
function, arising from the small-scale structure of streaming-induced
turbulence.Comment: 7 pages, 4 figures, accepted to ApJ Letters after minor
modifications, including two new figures and some new text that better
clarify our result
Quasi-Langmuir-Blodgett Thin Film Deposition of Carbon Nanotubes
The handling and manipulation of carbon nanotubes continues to be a challenge
to those interested in the application potential of these promising materials.
To this end, we have developed a method to deposit pure nanotube films over
large flat areas on substrates of arbitrary composition. The method bears some
resemblance to the Langmuir-Blodgett deposition method used to lay down thin
organic layers. We show that this redeposition technique causes no major
changes in the films' microstructure and that they retain the electronic
properties of as-deposited film laid down on an alumina membrane.Comment: 3 pages, 3 figures, submitted Journal of Applied Physic
Modelling bacterial behaviour close to a no-slip plane boundary: the influence of bacterial geometry
We describe a boundary-element method used to model the hydrodynamics of a bacterium propelled by a single helical flagellum. Using this model, we optimize the power efficiency of swimming with respect to cell body and flagellum geometrical parameters, and find that optima for swimming in unbounded fluid and near a no-slip plane boundary are nearly indistinguishable. We also consider the novel optimization objective of torque efficiency and find a very different optimal shape. Excluding effects such as Brownian motion and electrostatic interactions, it is demonstrated that hydrodynamic forces may trap the bacterium in a stable, circular orbit near the boundary, leading to the empirically observable surface accumulation of bacteria. Furthermore, the details and even the existence of this stable orbit depend on geometrical parameters of the bacterium, as described in this article. These results shed some light on the phenomenon of surface accumulation of micro-organisms and offer hydrodynamic explanations as to why some bacteria may accumulate more readily than others based on morphology
Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped BiSbTeS single crystals
Topological surface states have been extensively observed via optics in thin
films of topological insulators. However, in typical thick single crystals of
these materials, bulk states are dominant and it is difficult for optics to
verify the existence of topological surface states definitively. In this work,
we studied the charge dynamics of the newly formulated bulk-insulating Sn-doped
BiSbTeS crystal by using time-domain terahertz
spectroscopy. This compound shows much better insulating behavior than any
other bulk-insulating topological insulators reported previously. The
transmission can be enhanced an amount which is 5 of the zero-field
transmission by applying magnetic field to 7 T, an effect which we believe is
due to the suppression of topological surface states. This suppression is
essentially independent of the thicknesses of the samples, showing the
two-dimensional nature of the transport. The suppression of surface states in
field allows us to use the crystal slab itself as a reference sample to extract
the surface conductance, mobility, charge density and scattering rate. Our
measurements set the stage for the investigation of phenomena out of the
semi-classical regime, such as the topological magneto-electric effect.Comment: 5 pages, 3 figures, submitted in Augus
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