6,507 research outputs found
Tidal interaction of a rotating 1 Msun star with a binary companion
We calculate the tidal torque on a uniformly rotating 1 Msun star at various
stages of core hydrogen burning by an orbiting companion. We apply the
`traditional approximation' and solve the radial part of the tidal
perturbations by matrix inversion of the set of finite difference equations on
a very fine grid. We have identified resonances with gravity- and
quasi-toroidal modes with up to 1000 radial nodes in the more evolved stellar
models. For low forcing frequencies we find significant tidal response due to
viscous damping of inertial modes in the convective envelope of the solar-type
star. We conclude that effects due to stellar rotation (including resonance
locking) may considerably enhance the speed of tidal evolution in solar-type
stars.Comment: accepted for publ. in A&A, 11 pages, 6 figure
Tidal evolution of eccentric orbits in massive binary systems; a study of resonance locking
We study the tidal evolution of a binary system consisting of a 1.4 Msun
compact object in elliptic orbit about a 10 Msun uniformly rotating main
sequence star for various values of the initial orbital parameters. We apply
our previously published results of 2D non-adiabatic calculations of the
non-radial g- and r-mode oscillations of the uniformly rotating MS star, and
include the effects of resonant excitation of these modes in the tidal
evolution calculations. A high orbital eccentricity enhances the effectiveness
of the tidal interaction because of the large number of harmonic components of
the tidal potential and the reduced orbital separation near periastron. By
including the evolution of the MS star, especially of its rotation rate, many
resonance crossings occur with enhanced tidal interaction. We analyse the
phenomenon of resonance locking whereby a particular tidal harmonic is kept
resonant with a stellar oscillation mode. Resonance locking of prograde g-modes
appears an effective mechanism for orbital circularization of eccentric orbits.
We consider the orbital evolution of the binary pulsar PSR J0045-7319 and
conclude that resonance locking could explain the observed short orbital decay
time of this system if the B-star spins in the direction counter to the orbital
motion.Comment: 21 pages, 11 figures; some at reduced resolution, accepted for
publication in A&
Evaluation of pressure and thermal data from a wind tunnel test of a large-scale, powered, STOL fighter model
A STOL fighter model employing the vectored-engine-over wing concept was tested at low speeds in the NASA/Ames 40 by 80-foot wind tunnel. The model, approximately 0.75 scale of an operational fighter, was powered by two General Electric J-97 turbojet engines. Limited pressure and thermal instrumentation were provided to measure power effects (chordwise and spanwise blowing) and control-surface-deflection effects. An indepth study of the pressure and temperature data revealed many flow field features - the foremost being wing and canard leading-edge vortices. These vortices delineated regions of attached and separated flow, and their movements were often keys to an understanding of flow field changes caused by power and control-surface variations. Chordwise blowing increased wing lift and caused a modest aft shift in the center of pressure. The induced effects of chordwise blowing extended forward to the canard and significantly increased the canard lift when the surface was stalled. Spanwise blowing effectively enhanced the wing leading-edge vortex, thereby increasing lift and causing a forward shift in the center of pressure
Spectrally resolved single-shot wavefront sensing of broadband high-harmonic sources
Wavefront sensors are an important tool to characterize coherent beams of
extreme ultraviolet radiation. However, conventional Hartmann-type sensors do
not allow for independent wavefront characterization of different spectral
components that may be present in a beam, which limits their applicability for
intrinsically broadband high-harmonic generation (HHG) sources. Here we
introduce a wavefront sensor that measures the wavefronts of all the harmonics
in a HHG beam in a single camera exposure. By replacing the mask apertures with
transmission gratings at different orientations, we simultaneously detect
harmonic wavefronts and spectra, and obtain sensitivity to spatiotemporal
structure such as pulse front tilt as well. We demonstrate the capabilities of
the sensor through a parallel measurement of the wavefronts of 9 harmonics in a
wavelength range between 25 and 49 nm, with up to lambda/32 precision.Comment: 12 pages, 6 figure
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18F-FAC PET Visualizes Brain-Infiltrating Leukocytes in a Mouse Model of Multiple Sclerosis.
Brain-infiltrating leukocytes contribute to multiple sclerosis (MS) and autoimmune encephalomyelitis and likely play a role in traumatic brain injury, seizure, and stroke. Brain-infiltrating leukocytes are also primary targets for MS disease-modifying therapies. However, no method exists for noninvasively visualizing these cells in a living organism. 1-(2'-deoxy-2'-18F-fluoroarabinofuranosyl) cytosine (18F-FAC) is a PET radiotracer that measures deoxyribonucleoside salvage and accumulates preferentially in immune cells. We hypothesized that 18F-FAC PET could noninvasively image brain-infiltrating leukocytes. Methods: Healthy mice were imaged with 18F-FAC PET to quantify if this radiotracer crosses the blood-brain barrier (BBB). Experimental autoimmune encephalomyelitis (EAE) is a mouse disease model with brain-infiltrating leukocytes. To determine whether 18F-FAC accumulates in brain-infiltrating leukocytes, EAE mice were analyzed with 18F-FAC PET, digital autoradiography, and immunohistochemistry, and deoxyribonucleoside salvage activity in brain-infiltrating leukocytes was analyzed ex vivo. Fingolimod-treated EAE mice were imaged with 18F-FAC PET to assess if this approach can monitor the effect of an immunomodulatory drug on brain-infiltrating leukocytes. PET scans of individuals injected with 2-chloro-2'-deoxy-2'-18F-fluoro-9-β-d-arabinofuranosyl-adenine (18F-CFA), a PET radiotracer that measures deoxyribonucleoside salvage in humans, were analyzed to evaluate whether 18F-CFA crosses the human BBB. Results: 18F-FAC accumulates in the healthy mouse brain at levels similar to 18F-FAC in the blood (2.54 ± 0.2 and 3.04 ± 0.3 percentage injected dose per gram, respectively) indicating that 18F-FAC crosses the BBB. EAE mice accumulate 18F-FAC in the brain at 180% of the levels of control mice. Brain 18F-FAC accumulation localizes to periventricular regions with significant leukocyte infiltration, and deoxyribonucleoside salvage activity is present at similar levels in brain-infiltrating T and innate immune cells. These data suggest that 18F-FAC accumulates in brain-infiltrating leukocytes in this model. Fingolimod-treated EAE mice accumulate 18F-FAC in the brain at 37% lower levels than control-treated EAE mice, demonstrating that 18F-FAC PET can monitor therapeutic interventions in this mouse model. 18F-CFA accumulates in the human brain at 15% of blood levels (0.08 ± 0.01 and 0.54 ± 0.07 SUV, respectively), indicating that 18F-CFA does not cross the BBB in humans. Conclusion: 18F-FAC PET can visualize brain-infiltrating leukocytes in a mouse MS model and can monitor the response of these cells to an immunomodulatory drug. Translating this strategy into humans will require exploring additional radiotracers
On clocks and clouds
Cumulus clouds exhibit a life cycle that consists of (a) the growth phase
(increasing size, most notably in the vertical direction); (b) the mature
phase (growth ceases; any precipitation that develops is strongest during
this period); and (c) the dissipation phase (cloud dissipates because of
precipitation and/or entrainment; no more dynamical support). Although radar
can track clouds over time and give some sense of the age of a cloud, most
aircraft in situ measurements lack temporal context. We use large eddy
simulations of trade wind cumulus cloud fields from cases during the Barbados
Oceanographic and Meteorological Experiment (BOMEX) and Rain In Cumulus over
the Ocean (RICO) campaigns to demonstrate a potential cumulus cloud
"clock." We find that the volume-averaged total water mixing ratio rt
is a useful cloud clock for the 12 clouds studied. A cloud's initial rt
is set by the subcloud mixed-layer mean rt and decreases monotonically
from the initial value due primarily to entrainment. The clock is insensitive
to aerosol loading, environmental sounding and extrinsic cloud properties
such as lifetime and volume. In some cases (more commonly for larger clouds),
multiple pulses of buoyancy occur, which complicate the cumulus clock by
replenishing rt. The clock is most effectively used to classify clouds
by life phase
Gamification techniques for raising cyber security awareness
Due to the prevalence of online services in modern society, such as internet banking and social media, it is important for users to have an understanding of basic security measures in order to keep themselves safe online. However, users often do not know how to make their online interactions secure, which demonstrates an educational need in this area. Gamification has grown in popularity in recent years and has been used to teach people about a range of subjects. This paper presents an exploratory study investigating the use of gamification techniques to educate average users about password security, with the aim of raising overall security awareness. To explore the impact of such techniques, a role-playing quiz application (RPG) was developed for the Android platform to educate users about password security. Results gained from the work highlightedthat users enjoyed learning via the use of the password application, and felt they benefitted from the inclusion of gamification techniques. Future work seeks to expand the prototype into a full solution, covering a range of security awareness issues
Isomonodromic deformation theory and the next-to-diagonal correlations of the anisotropic square lattice Ising model
In 1980 Jimbo and Miwa evaluated the diagonal two-point correlation function
of the square lattice Ising model as a -function of the sixth Painlev\'e
system by constructing an associated isomonodromic system within their theory
of holonomic quantum fields. More recently an alternative isomonodromy theory
was constructed based on bi-orthogonal polynomials on the unit circle with
regular semi-classical weights, for which the diagonal Ising correlations arise
as the leading coefficient of the polynomials specialised appropriately. Here
we demonstrate that the next-to-diagonal correlations of the anisotropic Ising
model are evaluated as one of the elements of this isomonodromic system or
essentially as the Cauchy-Hilbert transform of one of the bi-orthogonal
polynomials.Comment: 11 pages, 1 figur
A Robust Measure of Tidal Circularization in Coeval Binary Populations: The solar-type spectroscopic Binary Population in The Open Cluster M35
We present a new homogeneous sample of 32 spectroscopic binary orbits in the
young (~ 150 Myr) main-sequence open cluster M35. The distribution of orbital
eccentricity vs. orbital period (e-log(P)) displays a distinct transition from
eccentric to circular orbits at an orbital period of ~ 10 days. The transition
is due to tidal circularization of the closest binaries. The population of
binary orbits in M35 provide a significantly improved constraint on the rate of
tidal circularization at an age of 150 Myr. We propose a new and more robust
diagnostic of the degree of tidal circularization in a binary population based
on a functional fit to the e-log(P) distribution. We call this new measure the
tidal circularization period. The tidal circularization period of a binary
population represents the orbital period at which a binary orbit with the most
frequent initial orbital eccentricity circularizes (defined as e = 0.01) at the
age of the population. We determine the tidal circularizationperiod for M35 as
well as for 7 additional binary populations spanning ages from the pre
main-sequence (~ 3 Myr) to late main-sequence (~ 10 Gyr), and use Monte Carlo
error analysis to determine the uncertainties on the derived circularization
periods. We conclude that current theories of tidal circularization cannot
account for the distribution of tidal circularization periods with population
age.Comment: 37 pages, 9 figures, to be published in The Astrophysical Journal,
February 200
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