2,286 research outputs found
Ground-based adaptive optics coronagraphic performance under closed-loop predictive control
The discovery of the exoplanet Proxima b highlights the potential for the
coming generation of giant segmented mirror telescopes (GSMTs) to characterize
terrestrial --- potentially habitable --- planets orbiting nearby stars with
direct imaging. This will require continued development and implementation of
optimized adaptive optics systems feeding coronagraphs on the GSMTs. Such
development should proceed with an understanding of the fundamental limits
imposed by atmospheric turbulence. Here we seek to address this question with a
semi-analytic framework for calculating the post-coronagraph contrast in a
closed-loop AO system. We do this starting with the temporal power spectra of
the Fourier basis calculated assuming frozen flow turbulence, and then apply
closed-loop transfer functions. We include the benefits of a simple predictive
controller, which we show could provide over a factor of 1400 gain in raw PSF
contrast at 1 on bright stars, and more than a factor of 30 gain on
an I = 7.5 mag star such as Proxima. More sophisticated predictive control can
be expected to improve this even further. Assuming a photon noise limited
observing technique such as High Dispersion Coronagraphy, these gains in raw
contrast will decrease integration times by the same large factors. Predictive
control of atmospheric turbulence should therefore be seen as one of the key
technologies which will enable ground-based telescopes to characterize
terrrestrial planets.Comment: Accepted to JATI
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Multi-epoch Direct Imaging and Time-variable Scattered Light Morphology of the HD 163296 Protoplanetary Disk
We present H-band polarized scattered light imagery and JHK high-contrast spectroscopy of the protoplanetary disk around HD 163296 observed with the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) and Subaru Coronagraphic Extreme Adaptive Optics (SCExAO)/Coronagraphic High Angular Resolution Imaging Spectrograph (CHARTS) instruments at Subaru Observatory. The polarimetric imagery resolve a broken ring structure surrounding HD 163296 that peaks at a distance along the major axis of 0 ''.65 (66 au) and extends out to 0 ''.98 (100 au) along the major axis. Our 2011 H-band data exhibit clear axisymmetry, with the NW and SE side of the disk exhibiting similar intensities. Our data are clearly different from 2016 epoch H-band observations of the Very Large Telescope (VLT)/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE), which found a strong 2.7 x asymmetry between the NW and SE side of the disk. Collectively, these results indicate the presence of time-variable, non-azimuthally symmetric illumination of the outer disk. While our SCExAO/CHARIS data are sensitive enough to recover the planet candidate identified from NIRC2 in the thermal infrared (IR), we fail to detect an object with JHK brightness nominally consistent with this object. This suggests that the candidate is either fainter in JHK bands than model predictions, possibly due to extinction from the disk or atmospheric dust/clouds, or that it is an artifact of the data set/data processing, such as a residual speckle or partially subtracted disk feature. Assuming standard hot-start evolutionary models and a system age of 5 Myr, we set new, direct mass limits for the inner (outer) Atacama Large Millimeter/submillimeter Array (ALMA)-predicted protoplanet candidate along the major (minor) disk axis of of 1.5 (2) M-J.NASA XRP program [NNX-17AF88G]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
Adaptive optics in high-contrast imaging
The development of adaptive optics (AO) played a major role in modern
astronomy over the last three decades. By compensating for the atmospheric
turbulence, these systems enable to reach the diffraction limit on large
telescopes. In this review, we will focus on high contrast applications of
adaptive optics, namely, imaging the close vicinity of bright stellar objects
and revealing regions otherwise hidden within the turbulent halo of the
atmosphere to look for objects with a contrast ratio lower than 10^-4 with
respect to the central star. Such high-contrast AO-corrected observations have
led to fundamental results in our current understanding of planetary formation
and evolution as well as stellar evolution. AO systems equipped three
generations of instruments, from the first pioneering experiments in the
nineties, to the first wave of instruments on 8m-class telescopes in the years
2000, and finally to the extreme AO systems that have recently started
operations. Along with high-contrast techniques, AO enables to reveal the
circumstellar environment: massive protoplanetary disks featuring spiral arms,
gaps or other asymmetries hinting at on-going planet formation, young giant
planets shining in thermal emission, or tenuous debris disks and micron-sized
dust leftover from collisions in massive asteroid-belt analogs. After
introducing the science case and technical requirements, we will review the
architecture of standard and extreme AO systems, before presenting a few
selected science highlights obtained with recent AO instruments.Comment: 24 pages, 14 figure
Laser Guide Star for Large Segmented-Aperture Space Telescopes, Part I: Implications for Terrestrial Exoplanet Detection and Observatory Stability
Precision wavefront control on future segmented-aperture space telescopes
presents significant challenges, particularly in the context of high-contrast
exoplanet direct imaging. We present a new wavefront control architecture that
translates the ground-based artificial guide star concept to space with a laser
source aboard a second spacecraft, formation flying within the telescope
field-of-view. We describe the motivating problem of mirror segment motion and
develop wavefront sensing requirements as a function of guide star magnitude
and segment motion power spectrum. Several sample cases with different values
for transmitter power, pointing jitter, and wavelength are presented to
illustrate the advantages and challenges of having a non-stellar-magnitude
noise limited wavefront sensor for space telescopes. These notional designs
allow increased control authority, potentially relaxing spacecraft stability
requirements by two orders of magnitude, and increasing terrestrial exoplanet
discovery space by allowing high-contrast observations of stars of arbitrary
brightness.Comment: Submitted to A
Space-time estimation of a particle system model
13 pagesLet X be a discrete time contact process (CP) on the discrete bidimensional lattice as define by Durett - Levin (1994) . We study estimation of the model based on space-time evolution on a finite subset of sites. For this, we make use of a marginal pseudo-likelihood. The estimator obtained is consistent and asymptoticaly normal for non-vanishing supercritical CP. Numerical studies confirm these results
Three-dimensional foam flow resolved by fast X-ray tomographic microscopy
Thanks to ultra fast and high resolution X-ray tomography, we managed to
capture the evolution of the local structure of the bubble network of a 3D foam
flowing around a sphere. As for the 2D foam flow around a circular obstacle, we
observed an axisymmetric velocity field with a recirculation zone, and
indications of a negative wake downstream the obstacle. The bubble
deformations, quantified by a shape tensor, are smaller than in 2D, due to a
purely 3D feature: the azimuthal bubble shape variation. Moreover, we were able
to detect plastic rearrangements, characterized by the neighbor-swapping of
four bubbles. Their spatial structure suggest that rearrangements are triggered
when films faces get smaller than a characteristic area.Comment: 5 pages, 5 figure
Diffraction-limited polarimetric imaging of protoplanetary disks and mass-loss shells with VAMPIRES
Both the birth and death of a stellar system are areas of key scientific importance. Whether it's understanding the process of planetary formation in a star's early years, or uncovering the cause of the enormous mass-loss that takes place during a star's dying moments, a key to scientific understanding lies in the inner few AU of the circumstellar environment. Corresponding to scales of 10s of milli-arcseconds, these observations pose a huge technical challenge due to the high angular-resolutions and contrasts required. A major stumbling block is the problem of the Earth's own atmospheric turbulence. The other difficulty is that precise calibration is required to combat the extremely high contrast ratios and high resolutions faced. By taking advantage of the fact that starlight scattered by dust in the circumstellar region is polarized, differential polarimetry can help achieve this calibration. Spectral features can also be utilized
Is This a Joke? Detecting Humor in Spanish Tweets
While humor has been historically studied from a psychological, cognitive and
linguistic standpoint, its study from a computational perspective is an area
yet to be explored in Computational Linguistics. There exist some previous
works, but a characterization of humor that allows its automatic recognition
and generation is far from being specified. In this work we build a
crowdsourced corpus of labeled tweets, annotated according to its humor value,
letting the annotators subjectively decide which are humorous. A humor
classifier for Spanish tweets is assembled based on supervised learning,
reaching a precision of 84% and a recall of 69%.Comment: Preprint version, without referra
Digging into acceptor splice site prediction : an iterative feature selection approach
Feature selection techniques are often used to reduce data dimensionality, increase classification performance, and gain insight into the processes that generated the data. In this paper, we describe an iterative procedure of feature selection and feature construction steps, improving the classification of acceptor splice sites, an important subtask of gene prediction.
We show that acceptor prediction can benefit from feature selection, and describe how feature selection techniques can be used to gain new insights in the classification of acceptor sites. This is illustrated by the identification of a new, biologically motivated feature: the AG-scanning feature.
The results described in this paper contribute both to the domain of gene prediction, and to research in feature selection techniques, describing a new wrapper based feature weighting method that aids in knowledge discovery when dealing with complex datasets
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