4,255 research outputs found
Refinement Type Inference via Horn Constraint Optimization
We propose a novel method for inferring refinement types of higher-order
functional programs. The main advantage of the proposed method is that it can
infer maximally preferred (i.e., Pareto optimal) refinement types with respect
to a user-specified preference order. The flexible optimization of refinement
types enabled by the proposed method paves the way for interesting
applications, such as inferring most-general characterization of inputs for
which a given program satisfies (or violates) a given safety (or termination)
property. Our method reduces such a type optimization problem to a Horn
constraint optimization problem by using a new refinement type system that can
flexibly reason about non-determinism in programs. Our method then solves the
constraint optimization problem by repeatedly improving a current solution
until convergence via template-based invariant generation. We have implemented
a prototype inference system based on our method, and obtained promising
results in preliminary experiments.Comment: 19 page
The chaotic behavior of the black hole system GRS 1915+105
A modified non-linear time series analysis technique, which computes the
correlation dimension , is used to analyze the X-ray light curves of the
black hole system GRS 1915+105 in all twelve temporal classes. For four of
these temporal classes saturates to which indicates that
the underlying dynamical mechanism is a low dimensional chaotic system. Of the
other eight classes, three show stochastic behavior while five show deviation
from randomness. The light curves for four classes which depict chaotic
behavior have the smallest ratio of the expected Poisson noise to the
variability () while those for the three classes which depict
stochastic behavior is the highest (). This suggests that the temporal
behavior of the black hole system is governed by a low dimensional chaotic
system, whose nature is detectable only when the Poisson fluctuations are much
smaller than the variability.Comment: Accepted for publication in Astrophysical Journa
Numerical simulations of the kappa-mechanism with convection
A strong coupling between convection and pulsations is known to play a major
role in the disappearance of unstable modes close to the red edge of the
classical Cepheid instability strip. As mean-field models of time-dependent
convection rely on weakly-constrained parameters, we tackle this problem by the
means of 2-D Direct Numerical Simulations (DNS) of kappa-mechanism with
convection.
Using a linear stability analysis, we first determine the physical conditions
favourable to the kappa-mechanism to occur inside a purely-radiative layer.
Both the instability strips and the nonlinear saturation of unstable modes are
then confirmed by the corresponding DNS. We next present the new simulations
with convection, where a convective zone and the driving region overlap. The
coupling between the convective motions and acoustic modes is then addressed by
using projections onto an acoustic subspace.Comment: 5 pages, 6 figures, accepted for publication in Astrophysics and
Space Science, HELAS workshop (Rome june 2009
A novel type of proximity focusing RICH counter with multiple refractive index aerogel radiator
A proximity focusing ring imaging Cherenkov detector, with the radiator
consisting of two or more aerogel layers of different refractive indices, has
been tested in 1-4 GeV/c pion beams at KEK. Essentially, a multiple refractive
index aerogel radiator allows for an increase in Cherenkov photon yield on
account of the increase in overall radiator thickness, while avoiding the
simultaneous degradation in single photon angular resolution associated with
the increased uncertainty of the emission point. With the refractive index of
consecutive layers suitably increasing in the downstream direction, one may
achieve overlapping of the Cherenkov rings from a single charged particle. In
the opposite case of decreasing refractive index, one may obtain well separated
rings. In the former combination an approximately 40% increase in photon yield
is accompanied with just a minor degradation in single photon angular
resolution. The impact of this improvement on the pion/kaon separation at the
upgraded Belle detector is discussed.Comment: submitted to Nucl. Instr. Meth.
Magnetic Connectivity between Active Regions 10987, 10988, and 10989 by Means of Nonlinear Force-Free Field Extrapolation
Extrapolation codes for modelling the magnetic field in the corona in
cartesian geometry do not take the curvature of the Sun's surface into account
and can only be applied to relatively small areas, \textit{e.g.}, a single
active region. We apply a method for nonlinear force-free coronal magnetic
field modelling of photospheric vector magnetograms in spherical geometry which
allows us to study the connectivity between multi-active regions. We use vector
magnetograph data from the Synoptic Optical Long-term Investigations of the Sun
survey (SOLIS)/Vector Spectromagnetograph(VSM) to model the coronal magnetic
field, where we study three neighbouring magnetically connected active regions
(ARs: 10987, 10988, 10989) observed on 28, 29, and 30 March 2008, respectively.
We compare the magnetic field topologies and the magnetic energy densities and
study the connectivities between the active regions(ARs). We have studied the
time evolution of magnetic field over the period of three days and found no
major changes in topologies as there was no major eruption event. From this
study we have concluded that active regions are much more connected
magnetically than the electric current.Comment: Solar Physic
A test of time-dependent theories of stellar convection
Context: In Cepheids close to the red edge of the classical instability
strip, a coupling occurs between the acoustic oscillations and the convective
motions close to the surface.The best topical models that account for this
coupling rely on 1-D time-dependent convection (TDC) formulations. However,
their intrinsic weakness comes from the large number of unconstrained free
parameters entering in the description of turbulent convection. Aims: We
compare two widely used TDC models with the first two-dimensional nonlinear
direct numerical simulations (DNS) of the convection-pulsation coupling in
which the acoustic oscillations are self-sustained by the kappa-mechanism.
Methods: The free parameters appearing in the Stellingwerf and Kuhfuss TDC
recipes are constrained using a chi2-test with the time-dependent convective
flux that evolves in nonlinear simulations of highly-compressible convection
with kappa-mechanism. Results: This work emphasises some inherent limits of TDC
models, that is, the temporal variability and non-universality of their free
parameters. More importantly, within these limits, Stellingwerf's formalism is
found to give better spatial and temporal agreements with the nonlinear
simulation than Kuhfuss's one. It may therefore be preferred in 1-D TDC
hydrocodes or stellar evolution codes.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in A&
Hemodynamic Assessment of Celiaco-mesenteric Anastomosis in Patients with Pancreaticoduodenal Artery Aneurysm Concomitant with Celiac Artery Occlusion using Flow-sensitive Four-dimensional Magnetic Resonance Imaging
ObjectivesMany pancreaticoduodenal artery (PDA) aneurysms are associated with celiac artery (CA) stenosis. The pathogenesis of PDA aneurysm may be associated with hemodynamic changes due to CA stenosis/occlusion. The aim of this study was to assess the hemodynamic changes of celiaco-mesenteric anastomosis in patients with PDA aneurysms concomitant with CA occlusion using four-dimensional flow-sensitive magnetic resonance imaging (4D-Flow).Methods4D-Flow was performed preoperatively on five patients. Seven age- and sex-matched individuals were used as controls. Hemodynamic parameters such as flow volume and maximum flow velocity in PDAs, gastroduodenal arteries, common hepatic arteries, and superior mesenteric arteries were compared between both groups. Wall shear stress (WSS) and oscillatory shear index (OSI) were mapped in both groups.ResultsIn the patient group, 4D-Flow identified retrograde flow of both gastroduodenal arteries and common hepatic arteries. Heterogeneous distribution patterns of both WSS and OSI were identified across the entire PDA in the patient group. OSI mapping showed multiple regions with extremely high OSI values (OSI > 0.3) in all patients. All PDA aneurysms, which were surgically resected, were atherosclerotic.Conclusions4D-Flow identified hemodynamic changes in celiaco-mesenteric arteries in patients with PDA aneurysms with concomitant CA occlusion. These hemodynamic changes may be associated with PDA aneurysm formation
Nonlinear force-free modelling: influence of inaccuracies in the measured magnetic vector
Context: Solar magnetic fields are regularly extrapolated into the corona
starting from photospheric magnetic measurements that can suffer from
significant uncertainties. Aims: Here we study how inaccuracies introduced into
the maps of the photospheric magnetic vector from the inversion of ideal and
noisy Stokes parameters influence the extrapolation of nonlinear force-free
magnetic fields. Methods: We compute nonlinear force-free magnetic fields based
on simulated vector magnetograms, which have been produced by the inversion of
Stokes profiles, computed froma 3-D radiation MHD simulation snapshot. These
extrapolations are compared with extrapolations starting directly from the
field in the MHD simulations, which is our reference. We investigate how line
formation and instrumental effects such as noise, limited spatial resolution
and the effect of employing a filter instrument influence the resulting
magnetic field structure. The comparison is done qualitatively by visual
inspection of the magnetic field distribution and quantitatively by different
metrics. Results: The reconstructed field is most accurate if ideal Stokes data
are inverted and becomes less accurate if instrumental effects and noise are
included. The results demonstrate that the non-linear force-free field
extrapolation method tested here is relatively insensitive to the effects of
noise in measured polarization spectra at levels consistent with present-day
instruments. Conclusions heading: Our results show that we can reconstruct the
coronal magnetic field as a nonlinear force-free field from realistic
photospheric measurements with an accuracy of a few percent, at least in the
absence of sunspots.Comment: A&A, accepted, 9 Pages, 4 Figure
Convective quenching of stellar pulsations
Context: we study the convection-pulsation coupling that occurs in cold
Cepheids close to the red edge of the classical instability strip. In these
stars, the surface convective zone is supposed to stabilise the radial
oscillations excited by the kappa-mechanism.
Aims: we study the influence of the convective motions onto the amplitude and
the nonlinear saturation of acoustic modes excited by kappa-mechanism. We are
interested in determining the physical conditions needed to lead to a quenching
of oscillations by convection.
Methods: we compute two-dimensional nonlinear simulations (DNS) of the
convection-pulsation coupling, in which the oscillations are sustained by a
continuous physical process: the kappa-mechanism. Thanks to both a frequential
analysis and a projection of the physical fields onto an acoustic subspace, we
study how the convective motions affect the unstable radial oscillations.
Results: depending on the initial physical conditions, two main behaviours
are obtained: (i) either the unstable fundamental acoustic mode has a large
amplitude, carries the bulk of the kinetic energy and shows a nonlinear
saturation similar to the purely radiative case; (ii) or the convective motions
affect significantly the mode amplitude that remains very weak. In this second
case, convection is quenching the acoustic oscillations. We interpret these
discrepancies in terms of the difference in density contrast: larger
stratification leads to smaller convective plumes that do not affect much the
purely radial modes, while large-scale vortices may quench the oscillations.Comment: 15 pages, 17 figures, 3 tables, accepted for publication in A&
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