29,116 research outputs found
Planetary ring dynamics and morphology
Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Prometheus is discussed. It is argued that little-known observations of magnetospheric electron density by Pioneer 11 imply substantial, ongoing injections of mass into the 2000 km region which surrounds the F ring. A hypothesis is presented that these events result naturally from interparticle collisions between the smaller members of an optically thin belt of moonlets. Also discussed is work on Uranus ring structure and photometry, image processing and analysis of the Jonian ring strucure, photometric and structural studies of the A ring of Saturn, and improvements to an image processing system for ring studies
Clustering documents with active learning using Wikipedia
Wikipedia has been applied as a background knowledge base to various text mining problems, but very few attempts have been made to utilize it for document clustering. In this paper we propose to exploit the semantic knowledge in Wikipedia for clustering, enabling the automatic grouping of documents with similar themes. Although clustering is intrinsically unsupervised, recent research has shown that incorporating supervision improves clustering performance, even when limited supervision is provided. The approach presented in this paper applies supervision using active learning. We first utilize Wikipedia to create a concept-based representation of a text document, with each concept associated to a Wikipedia article. We then exploit the semantic relatedness between Wikipedia concepts to find pair-wise instance-level constraints for supervised clustering, guiding clustering towards the direction indicated by the constraints. We test our approach on three standard text document datasets. Empirical results show that our basic document representation strategy yields comparable performance to previous attempts; and adding constraints improves clustering performance further by up to 20%
Charge separation instability in an unmagnetized disk plasma around a Kerr black hole
In almost all of plasma theories for astrophysical objects, we have assumed
the charge quasi-neutrality of unmagnetized plasmas in global scales. This
assumption has been justified because if there is a charged plasma, it induces
electric field which attracts the opposite charge, and this opposite charge
reduces the charge separation. Here, we report a newly discovered instability
which causes a charge separation in a rotating plasma inside of an innermost
stable circular orbit (ISCO) around a black hole. The growth rate of the
instability is smaller than that of the disk instability even in the unstable
disk region and is forbidden in the stable disk region outside of the ISCO.
However, this growth rate becomes comparable to that of the disk instability
when the plasma density is much lower than a critical density inside of the
ISCO. In such case, the charge separation instability would become apparent and
cause the charged accretion into the black hole, thus charge the hole up.Comment: 15pages, 1 figur
ENLIVE: An Efficient Nonlinear Method for Calibrationless and Robust Parallel Imaging
Robustness against data inconsistencies, imaging artifacts and acquisition
speed are crucial factors limiting the possible range of applications for
magnetic resonance imaging (MRI). Therefore, we report a novel calibrationless
parallel imaging technique which simultaneously estimates coil profiles and
image content in a relaxed forward model. Our method is robust against a wide
class of data inconsistencies, minimizes imaging artifacts and is comparably
fast combining important advantages of many conceptually different
state-of-the-art parallel imaging approaches. Depending on the experimental
setting, data can be undersampled well below the Nyquist limit. Here, even high
acceleration factors yield excellent imaging results while being robust to
noise and the occurrence of phase singularities in the image domain, as we show
on different data. Moreover, our method successfully reconstructs acquisitions
with insufficient field-of-view. We further compare our approach to ESPIRiT and
SAKE using spin-echo and gradient echo MRI data from the human head and knee.
In addition, we show its applicability to non-Cartesian imaging on radial FLASH
cardiac MRI data. Using theoretical considerations, we show that ENLIVE can be
related to a low-rank formulation of blind multi-channel deconvolution,
explaining why it inherently promotes low-rank solutions.Comment: 17 pages, 10 figure
Final spins from the merger of precessing binary black holes
The inspiral of binary black holes is governed by gravitational radiation
reaction at binary separations r < 1000 M, yet it is too computationally
expensive to begin numerical-relativity simulations with initial separations r
> 10 M. Fortunately, binary evolution between these separations is well
described by post-Newtonian equations of motion. We examine how this
post-Newtonian evolution affects the distribution of spin orientations at
separations r ~ 10 M where numerical-relativity simulations typically begin.
Although isotropic spin distributions at r ~ 1000 M remain isotropic at r ~ 10
M, distributions that are initially partially aligned with the orbital angular
momentum can be significantly distorted during the post-Newtonian inspiral.
Spin precession tends to align (anti-align) the binary black hole spins with
each other if the spin of the more massive black hole is initially partially
aligned (anti-aligned) with the orbital angular momentum, thus increasing
(decreasing) the average final spin. Spin precession is stronger for
comparable-mass binaries, and could produce significant spin alignment before
merger for both supermassive and stellar-mass black hole binaries. We also
point out that precession induces an intrinsic accuracy limitation (< 0.03 in
the dimensionless spin magnitude, < 20 degrees in the direction) in predicting
the final spin resulting from the merger of widely separated binaries.Comment: 20 pages, 16 figures, new PN terms, submitted to PR
Bends in the plane with variable curvature
Explicit formulae for planar variable curvature bends are constructed using Euler’s method of natural equations. The bend paths are expressed in terms of special functions. It is shown that the length of the different bend types varies linearly with increasing radius and that the curvature of variable curvature bends can be expressed as a multiple of the curvature of a circle
Where's the Doughnut? LBV bubbles and Aspherical Fast Winds
In this paper we address the issue of the origin of LBV bipolar bubbles.
Previous studies have explained the shapes of LBV nebulae, such as Car,
by invoking the interaction of an isotropic fast wind with a previously
deposited, slow aspherical wind (a ``slow torus''). In this paper we focus on
the opposite scenario where an aspherical fast wind expands into a previously
deposited isotropic slow wind. Using high resolution hydrodynamic simulations,
which include the effects of radiative cooling, we have completed a series of
numerical experiments to test if and how aspherical fast winds effect wind
blown bubble morphologies. Our experiments explore a variety of models for the
latitudinal variations of fast wind flow parameters. The simulations
demonstrate that aspherical fast winds can produce strongly bipolar outflows.
In addition the properties of outflows recover some important aspects of LBV
bubbles which the previous "slow torus" models can not.Comment: 23 pages, 6 figures, to appear the Astrophysical Journa
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