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Fast Image-Based Model of Mitral Valve Closure for Surgical Planning
Surgical repair of the mitral valve results in better
outcomes than valve replacement, yet diseased valves
are often replaced due to the technical difficulty of the
repair process. A surgical planning system based on
patient-specific medical images that allows surgeons to
simulate and compare potential repair strategies could
greatly improve surgical outcomes. The system must
simulate valve closure quickly and handle the complex
boundary conditions imposed by the chords that tether
the valve leaflets. We have developed a process for
generating a triangulated mesh of the valve surface
from volumetric image data of the opened valve. The
closed position of the mesh is then computed using a
mass-spring model of dynamics. In the mass-spring
model, triangle sides are treated as linear springs
supporting only tension. Chords are also treated as
linear springs, and self-collisions are detected and
handled inelastically. The equations of motion are
solved using implicit numerical integration. The
simulated closed state is compared with an image of the
same valve taken in the closed state to assess accuracy
of the model. The model exhibits rapid valve closure
and is able to predict the closed state of the valve with
reasonable accuracy.Engineering and Applied Science
The Solar-System-Scale Disk Around AB Aurigae
The young star AB Aurigae is surrounded by a complex combination of gas-rich
and dust dominated structures. The inner disk which has not been studied
previously at sufficient resolution and imaging dynamic range seems to contain
very little gas inside a radius of least 130 astronomical units (AU) from the
star. Using adaptive-optics coronagraphy and polarimetry we have imaged the
dust in an annulus between 43 and 302 AU from the star, a region never seen
before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along
with a clearing at closer radii inside this annulus, suggests the formation of
at least one small body at an orbital distance of about 100 AU. This structure
seems consistent with crude models of mean motion resonances, or accumulation
of material at two of the Lagrange points relative to the putative object and
the star. We also report a low significance detection of a point source in this
outer annulus of dust. This source may be an overdensity in the disk due to
dust accreting onto an unseen companion. An alternate interpretation suggests
that the object's mass is between 5 and 37 times the mass of Jupiter. The
results have implications for circumstellar disk dynamics and planet formation.Comment: 11 pages, 5 figures, accepted for publication in Astrophysical
Journal, V. 680, June 10, 200
The Lyot Project Direct Imaging Survey of Substellar Companions: Statistical Analysis and Information from Nondetections
The Lyot project used an optimized Lyot coronagraph with Extreme Adaptive
Optics at the 3.63m Advanced Electro-Optical System telescope (AEOS) to observe
86 stars from 2004 to 2007. In this paper we give an overview of the survey
results and a statistical analysis of the observed nondetections around 58 of
our targets to place constraints on the population of substellar companions to
nearby stars. The observations did not detect any companion in the substellar
regime. Since null results can be as important as detections, we analyzed each
observation to determine the characteristics of the companions that can be
ruled out. For this purpose we use a Monte Carlo approach to produce artificial
companions, and determine their detectability by comparison with the
sensitivity curve for each star. All the non-detection results are combined
using a Bayesian approach and we provide upper limits on the population of
giant exoplanets and brown dwarfs for this sample of stars. Our nondetections
confirm the rarity of brown dwarfs around solar-like stars and we constrain the
frequency of massive substellar companions (M>40Mjup) at orbital separation
between and 10 and 50 AU to be <20%.Comment: 32 pages, 11 figures, 2 tables. Published in the Astrophysical
Journa
Silicon Nanoparticles: Source of Extended Red Emission?
We have reviewed the characteristics of the extended red emission (ERE) as
observed in many dusty astronomical environments, in particular, the diffuse
interstellar medium of the Galaxy. The spectral nature and the photon
conversion efficiency of the ERE identify the underlying process as highly
efficient photoluminescence by an abundant component of interstellar dust. We
have compared the photoluminescence properties of a variety of carbon- and
silicon-based materials proposed as sources for the ERE with the
observationally established constraints. We found that silicon nanoparticles
provide the best match to the spectrum and the efficiency requirement of the
ERE. If present in interstellar space with an abundance sufficient to explain
the intensity of the ERE, silicon nanoparticles will also contribute to the
interstellar 9.7 micron Si-O stretch feature in absorption, to the near- and
mid-IR nonequilibrium thermal background radiation, and to the continuum
extinction in the near- and far-UV. About 36% of the interstellar silicon
depleted into the dust phase would be needed in the form of silicon
nanoparticles, amounting to less than 5% of the interstellar dust mass. We
propose that silicon nanoparticles form through the nucleation of SiO in
oxygen-rich stellar mass outflows and that they represent an important
small-grain component of the interstellar dust spectrum.Comment: 5 pages; 1 included figure; accepted 1998 May 1, ApJ
The Lyot project: toward exoplanet imaging and spectroscopy
Among the adaptive optics systems available to astronomers, the US Air Force Advanced Electro-Optical System (AEOS) is unique because it delivers very high order wave front correction. The Lyot Project includes the construction and installation of the worldâs first diffraction-limited, optimized coronagraph that exploits the full astronomical potential of AEOS and represents a critical step toward the long-term goal of directly imaging and studying extrasolar planets (a.k.a. âexoplanetsâ). We provide an update on the Project, whose coronagraph saw first light in March 2004. The coronagraph is operating at least as well as predicted by simulations, and a survey of nearby stars has begun
Adsorption dynamics of hydrophobically modified polymers at an air-water interface
The adsorption dynamics of a series of hydrophobically modified polymers, PAAαCn, at the air-water interface is studied by measuring the dynamic surface tension. The PAAαCn are composed of a poly(acrylic acid) backbone grafted with a percentage α of C8 or C12 alkyl moieties, at pH conditions where the PAA backbone is not charged. The observed adsorption dynamics is very slow and follows a logarithmic behavior at long times indicating the building of an energy barrier which grows over time. After comparison of our experimental results to models from the literature, a new model which accounts for both the deformation of the incoming polymer coils as well as the deformation of the adsorbed pseudo-brush is described. This model enables to fit very well the experimental data. The two fitting parameters give expected values for the monomer size and for the area per adsorbed polymer chain.This article is uploaded in "arXiv.org"
https://arxiv.org/abs/1706.0710
Speckle Suppression Through Dual Imaging Polarimetry, and a Ground-Based Image of the HR 4796A Circumstellar Disk
We demonstrate the versatility of a dual imaging polarimeter working in
tandem with a Lyot coronagraph and Adaptive Optics to suppress the highly
static speckle noise pattern--the greatest hindrance to ground-based direct
imaging of planets and disks around nearby stars. Using a double difference
technique with the polarimetric data, we quantify the level of speckle
suppression, and hence improved sensitivity, by placing an ensemble of
artificial faint companions into real data, with given total brightness and
polarization. For highly polarized sources within 0.5 arcsec, we show that we
achieve 3 to 4 magnitudes greater sensitivity through polarimetric speckle
suppression than simply using a coronagraph coupled to a high-order Adaptive
Optics system. Using such a polarimeter with a classical Lyot coronagraph at
the 3.63m AEOS telescope, we have obtained a 6.5 sigma detection in the H-band
of the 76 AU diameter circumstellar debris disk around the star HR 4796A. Our
data represent the first definitive, ground-based, near-IR polarimetric image
of the HR 4796A debris disk and clearly show the two outer ansae of the disk,
evident in Hubble Space Telescope NICMOS/STIS imaging. We derive a lower limit
to the fractional linear polarization of 29% caused by dust grains in the disk.
In addition, we fit simple morphological models of optically thin disks to our
data allowing us to constrain the dust disk scale height to 2.5{+5.0}_{-1.3} AU
and scattering asymmetry parameter (g=0.20^{+.07}_{-.10}). These values are
consistent with several lines of evidence suggesting that the HR 4796A disk is
dominated by a micron-sized dust population, and are indeed typical of disks in
transition between those surrounding the Herbig Ae stars to those associated
with Vega-like stars.Comment: Accepted to ApJ, 8 pages, 4 figures, minor typos fixed, one reference
adde
Discovery and Characterization of a Faint Stellar Companion to the A3V Star Zeta Virginis
Through the combination of high-order Adaptive Optics and coronagraphy, we
report the discovery of a faint stellar companion to the A3V star zeta
Virginis. This companion is ~7 magnitudes fainter than its host star in the
H-band, and infrared imaging spanning 4.75 years over five epochs indicates
this companion has common proper motion with its host star. Using evolutionary
models, we estimate its mass to be 0.168+/-.016 solar masses, giving a mass
ratio for this system q = 0.082. Assuming the two objects are coeval, this mass
suggests a M4V-M7V spectral type for the companion, which is confirmed through
integral field spectroscopic measurements. We see clear evidence for orbital
motion from this companion and are able to constrain the semi-major axis to be
greater than 24.9 AU, the period > 124$ yrs, and eccentricity > 0.16.
Multiplicity studies of higher mass stars are relatively rare, and binary
companions such as this one at the extreme low end of the mass ratio
distribution are useful additions to surveys incomplete at such a low mass
ratio. Moreover, the frequency of binary companions can help to discriminate
between binary formation scenarios that predict an abundance of low-mass
companions forming from the early fragmentation of a massive circumstellar
disk. A system such as this may provide insight into the anomalous X-ray
emission from A stars, hypothesized to be from unseen late-type stellar
companions. Indeed, we calculate that the presence of this M-dwarf companion
easily accounts for the X-ray emission from this star detected by ROSAT.Comment: 9 pages, 6 figures, Accepted to Ap
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