38,509 research outputs found
Magnetic pressure support and accretion disk spectra
Stellar atmosphere models of ionized accretion disks have generally neglected
the contribution of magnetic fields to the vertical hydrostatic support,
although magnetic fields are widely believed to play a critical role in the
transport of angular momentum. Simulations of magnetorotational turbulence in a
vertically stratified shearing box geometry show that magnetic pressure support
can be dominant in the upper layers of the disk. We present calculations of
accretion disk spectra that include this magnetic pressure support, as well as
a vertical dissipation profile based on simulation. Magnetic pressure support
generically produces a more vertically extended disk atmosphere with a larger
density scale height. This acts to harden the spectrum compared to models that
neglect magnetic pressure support. We estimate the significance of this effect
on disk-integrated spectra by calculating an illustrative disk model for a
stellar mass black hole, assuming that similar magnetic pressure support exists
at all radii.Comment: submitted to Ap
Recommendations for approaches to meticillin-resistant staphylococcal infections of small animals: diagnosis, therapeutic considerations and preventative measures
Low-cost, aerial photographic inventory of tidal wetlands
There are no author-identified significant results in this report
Poynting Vector Flow in a Circular Circuit
A circuit is considered in the shape of a ring, with a battery of negligible
size and a wire of uniform resistance. A linear charge distribution along the
wire maintains an electrostatic field and a steady current, which produces a
constant magnetic field. Earlier studies of the Poynting vector and the rate of
flow of energy considered only idealized geometries in which the Poynting
vector was confined to the space within the circuit. But in more realistic
cases the Poynting vector is nonzero outside as well as inside the circuit. An
expression is obtained for the Poynting vector in terms of products of
integrals, which are evaluated numerically to show the energy flow. Limiting
expressions are obtained analytically. It is shown that the total power
generated by the battery equals the energy flowing into the wire per unit time.Comment: 19 pages, 8 figure
Earth feature identification for onboard multispectral data editing: Computational experiments
A computational model of the processes involved in multispectral remote sensing and data classification is developed as a tool for designing smart sensors which can process, edit, and classify the data that they acquire. An evaluation of sensor system performance and design tradeoffs involves classification rates and errors as a function of number and location of spectral channels, radiometric sensitivity and calibration accuracy, target discrimination assignments, and accuracy and frequency of compensation for imaging conditions. This model provides a link between the radiometric and statistical properties of the signals to be classified and the performance characteristics of electro-optical sensors and data processing devices. Preliminary computational results are presented which illustrate the editing performance of several remote sensing approaches
Tele-operated high speed anthropomorphic dextrous hands with object shape and texture identification
This paper reports on the development of two number of robotic hands have been developed which focus on tele-operated high speed anthropomorphic dextrous robotic hands. The aim of developing these hands was to achieve a system that seamlessly interfaced between humans and robots. To provide sensory feedback, to a remote operator tactile sensors were developed to be mounted on the robotic hands. Two systems were developed, the first, being a skin sensor capable of shape reconstruction placed on the palm of the hand to feed back the shape of objects grasped and the second is a highly sensitive tactile array for surface texture identification
Fringe spacing and phase of interfering matter waves
We experimentally investigate the outcoupling of atoms from Bose-Einstein
condensates using two radio-frequency (rf) fields in the presence of gravity.
We show that the fringe separation in the resulting interference pattern
derives entirely from the energy difference between the two rf fields and not
the gravitational potential difference. We subsequently demonstrate how the
phase and polarisation of the rf radiation directly control the phase of the
matter wave interference and provide a semi-classical interpretation of the
results.Comment: 4 pages, 3 figure
Long Strings, Anomaly Cancellation, Phase Transitions, T-duality and Locality in the 2d Heterotic String
We study the noncritical two-dimensional heterotic string. Long fundamental
strings play a crucial role in the dynamics. They cancel anomalies and lead to
phase transitions when the system is compactified on a Euclidean circle. A
careful analysis of the gauge symmetries of the system uncovers new subtleties
leading to modifications of the worldsheet results. The compactification on a
Euclidean thermal circle is particularly interesting. It leads us to an
incompatibility between T-duality (and its corresponding gauge symmetry) and
locality.Comment: 36 pages, 2 figure
The stringy nature of the 2d type-0A black hole
We investigate the thermodynamics of the RR charged two-dimensional type-0A
black hole background at finite temperature, and compare with known 0A matrix
model results. It has been claimed that there is a disagreement for the free
energy between the spacetime and the dual matrix model. Here we find that this
discrepancy is sensitive to how the cutoff is implemented on the spacetime
side. In particular, the disagreement is resolved once we put the cutoff at a
fixed distance away from the horizon, as opposed to a fixed position in space.
Furthermore, the mass and the entropy of the black hole itself add up to an
analytic contribution to the free energy, which is precisely reproduced by the
0A matrix model. We also use results from the 0A matrix model to predict the
next to leading order contribution to the entropy of the black hole. Finally,
we note that the black hole is characterized by a Hagedorn growth in its
density of states below the Hagedorn temperature. This, together with other
results, suggests there is a phase transition at this temperature.Comment: 1+21 pages; v2: Substantial changes in the body of the paper, main
results the same. Clarified discussion on the thermodynamics, added section
on a phase transition, references added. v3: Typos corrected. v4: Final
version, to appear in JHE
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