12,551 research outputs found
A Quasilinear-Time Algorithm for Tiling the Plane Isohedrally with a Polyomino
A plane tiling consisting of congruent copies of a shape is isohedral provided that for any pair of copies, there exists a symmetry of the tiling mapping one copy to the other. We give a O(n log2 n)-time algorithm for deciding if a polyomino with n edges can tile the plane isohedrally. This improves on the O(n18)-time algorithm of Keating and Vince and generalizes recent work by Brlek, Provençal, Fédou, and the second author.SCOPUS: cp.pinfo:eu-repo/semantics/publishe
Motion magnification in coronal seismology
We introduce a new method for the investigation of low-amplitude transverse
oscillations of solar plasma non-uniformities, such as coronal loops,
individual strands in coronal arcades, jets, prominence fibrils, polar plumes,
and other contrast features, observed with imaging instruments. The method is
based on the two-dimensional dual tree complex wavelet transform
(DTWT). It allows us to magnify transverse, in the
plane-of-the-sky, quasi-periodic motions of contrast features in image
sequences. The tests performed on the artificial data cubes imitating
exponentially decaying, multi-periodic and frequency-modulated kink
oscillations of coronal loops showed the effectiveness, reliability and
robustness of this technique. The algorithm was found to give linear scaling of
the magnified amplitudes with the original amplitudes provided they are
sufficiently small. Also, the magnification is independent of the oscillation
period in a broad range of the periods. The application of this technique to
SDO/AIA EUV data cubes of a non-flaring active region allowed for the improved
detection of low-amplitude decay-less oscillations in the majority of loops.Comment: Accepted for publication in Solar Physic
Orbital configurations and dynamical stability of multi-planet systems around Sun-like stars HD 202206, 14 Her, HD 37124 and HD 108874
We perform a dynamical analysis of the recently published radial velocity
(RV) measurements of a few solar type stars which host multiple Jupiter-like
planets. In particular, we re-analyze the data for HD 202206, 14 Her, HD 37124
and HD 108874. We derive dynamically stable configurations which reproduce the
observed RV signals using our method called GAMP (an acronym of the Genetic
Algorithm with MEGNO Penalty). The GAMP relies on the N-body dynamics and makes
use of genetic algorithms merged with a stability criterion. For this purpose,
we use the maximal Lyapunov exponent computed with the dynamical fast indicator
MEGNO. Through a dynamical analysis of the phase-space in a neighborhood of the
obtained best-fit solutions, we derive meaningful limits on the parameters of
the planets. We demonstrate that GAMP is especially well suited for the
analysis of the RV data which only partially cover the longest orbital period
and/or correspond to multi-planet configurations involved in low-order mean
motion resonances (MMRs). In particular, our analysis reveals a presence of a
second Jupiter-like planet in the 14 Her system (14 Her c) involved in a 3:1 or
6:1 MMR with the known companion b. We also show that the dynamics of the HD
202206 system may be qualitatively different when coplanar and
mutually-inclined orbits of the companions are considered. We demonstrate that
the two outer planets in the HD 37124 system may reside in a close neighborhood
of the 5:2 MMR. Finally, we found a clear indication that the HD 108874 system
may be very close to, or locked in an exact 4:1 MMR.Comment: Contains 7 pages (text), 17 figures (some in low resolution suitable
for astro-ph), 1 table. Submitted to ApJ. The manuscript with high-resolution
figures is available from http://www.astri.uni.torun.pl/~chris/ms64109.ps.g
Closed-loop focal plane wavefront control with the SCExAO instrument
This article describes the implementation of a focal plane based wavefront
control loop on the high-contrast imaging instrument SCExAO (Subaru
Coronagraphic Extreme Adaptive Optics). The sensor relies on the Fourier
analysis of conventional focal-plane images acquired after an asymmetric mask
is introduced in the pupil of the instrument. This absolute sensor is used here
in a closed-loop to compensate the non-common path errors that normally affects
any imaging system relying on an upstream adaptive optics system.This specific
implementation was used to control low order modes corresponding to eight
zernike modes (from focus to spherical). This loop was successfully run on-sky
at the Subaru Telescope and is used to offset the SCExAO deformable mirror
shape used as a zero-point by the high-order wavefront sensor. The paper
precises the range of errors this wavefront sensing approach can operate within
and explores the impact of saturation of the data and how it can be bypassed,
at a cost in performance. Beyond this application, because of its low hardware
impact, APF-WFS can easily be ported in a wide variety of wavefront sensing
contexts, for ground- as well space-borne telescopes, and for telescope pupils
that can be continuous, segmented or even sparse. The technique is powerful
because it measures the wavefront where it really matters, at the level of the
science detector.Comment: 9 pages, 14 figures, accepted for publication by A&
Josephson Vortex Qubit based on a Confocal Annular Josephson Junction
We report theoretical and experimental work on the development of a Josephson
vortex qubit based on a confocal annular Josephson tunnel junction (CAJTJ). The
key ingredient of this geometrical configuration is a periodically variable
width that generates a spatial vortex potential with bistable states. This
intrinsic vortex potential can be tuned by an externally applied magnetic field
and tilted by a bias current. The two-state system is accurately modeled by a
one-dimensional sine-Gordon like equation by means of which one can numerically
calculate both the magnetic field needed to set the vortex in a given state as
well as the vortex depinning currents. Experimental data taken at 4.2K on
high-quality Nb/Al-AlOx/Nb CAJTJs with an individual trapped fluxon advocate
the presence of a robust and finely tunable double-well potential for which
reliable manipulation of the vortex state has been classically demonstrated.
The vortex is prepared in a given potential by means of an externally applied
magnetic field, while the state readout is accomplished by measuring the
vortex-depinning current in a small magnetic field. Our proof of principle
experiment convincingly demonstrates that the proposed vortex qubit based on
CAJTJs is robust and workable.Comment: 20 pages, 11 figure
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