6,141 research outputs found
Image Ellipticity from Atmospheric Aberrations
We investigate the ellipticity of the point-spread function (PSF) produced by
imaging an unresolved source with a telescope, subject to the effects of
atmospheric turbulence. It is important to quantify these effects in order to
understand the errors in shape measurements of astronomical objects, such as
those used to study weak gravitational lensing of field galaxies. The PSF
modeling involves either a Fourier transform of the phase information in the
pupil plane or a ray-tracing approach, which has the advantage of requiring
fewer computations than the Fourier transform. Using a standard method,
involving the Gaussian weighted second moments of intensity, we then calculate
the ellipticity of the PSF patterns. We find significant ellipticity for the
instantaneous patterns (up to more than 10%). Longer exposures, which we
approximate by combining multiple (N) images from uncorrelated atmospheric
realizations, yield progressively lower ellipticity (as 1 / sqrt(N)). We also
verify that the measured ellipticity does not depend on the sampling interval
in the pupil plane using the Fourier method. However, we find that the results
using the ray-tracing technique do depend on the pupil sampling interval,
representing a gradual breakdown of the geometric approximation at high spatial
frequencies. Therefore, ray tracing is generally not an accurate method of
modeling PSF ellipticity induced by atmospheric turbulence unless some
additional procedure is implemented to correctly account for the effects of
high spatial frequency aberrations. The Fourier method, however, can be used
directly to accurately model PSF ellipticity, which can give insights into
errors in the statistics of field galaxy shapes used in studies of weak
gravitational lensing.Comment: 9 pages, 5 color figures (some reduced in size). Accepted for
publication in the Astrophysical Journa
The RCB star V854 Cen is surrounded by a hot dusty shell
Aims : The hydrogen-deficient supergiants known as R Coronae Borealis (RCB)
stars might be the result of a double-degenerate merger of two white dwarfs
(WDs), or a final helium shell flash in a planetary nebula central star. In
this context, any information on the geometry of their circumstellar
environment and, in particular, the potential detection of elongated
structures, is of great importance. Methods : We obtained near-IR observations
of V854 Cen with the AMBER recombiner located at the Very Large Telescope
Interferometer (VLTI) array with the compact array (B35m) in 2013 and the
long array (B140m) in 2014. At each time, V854 Cen was at maximum light.
The - and -band continua were investigated by means of spectrally
dependant geometric models. These data were supplemented with mid-IR VISIR/VLT
images. Results : A dusty slightly elongated over density is discovered both in
the - and -band images. With the compact array, the central star is
unresolved (\,mas), but a flattened dusty environment of mas is discovered whose flux increases from about 20% in the
band to reach about 50% at 2.3\micron, which indicates hot
(T1500\,K) dust in the close vicinity of the star. The major axis is
oriented at a position angle (P.A.) of 12629. Adding the long-array
configuration dataset provides tighter constraints on the star diameter
( mas), a slight increase of the overdensity to
mas and a consistent P.A. of 13349. The closure phases, sensitive to
asymmetries, are null and compatible with a centro-symmetric, unperturbed
environment excluding point sources at the level of 3% of the total flux in
2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20%
level at larger distances (1\arcsec) with a position angle of
9219, marginally consistent with the interferometric observations.
Conclusions : This is the first time that a moderately elongated structure has
been observed around an RCB star. These observations confirm the numerous
suggestions for a bipolar structure proposed for this star in the literature,
which were mainly based on polarimetric and spectroscopic observations.Comment: Accepted by A\&A, new version after language editing, Astronomy and
Astrophysics (2014
Delivery outcomes of patients with pre-eclampsia at 27 - 34 weeksâ gestation at Tygerberg Hospital, Cape Town, South Africa: A retrospective observational study
Background. Obstetricians are cognisant of the serious nature of hypertensive disorders in pregnancy. Despite a 17% overall reduction in maternal deaths in South Africa between 2011 and 2016, there was a 14% increase in deaths due to hypertension. Delivery is the only known cure for pre-eclampsia, but the question regarding the safest route of delivery remains difficult to answer.Objectives. To determine the success rate of induction of labour (IoL) in patients with early-onset pre-eclampsia with severe features (EOPES) before 34 weeksâ gestation. Furthermore, the data from the induction group were compared with those of the caesarean delivery (CD) groups where patients were not eligible for IoL. Additional objectives were to identify variables that could influence the success rate, to determine whether any delivery method was associated with increased morbidity, to assess the short-term maternal and neonatal outcomes, and to make recommendations for future decision-making regarding delivery for women with EOPES.Methods. In this single-institution retrospective observational study, all cases in which a decision for delivery was made before 34 weeks 0 days of gestation (or the infantâs birthweight was â€2 000 g with uncertain gestation) at Tygerberg Hospital, Cape Town, between 1 January and 30 June 2017 were identified from the electronic birth register. The cohort fitting the inclusion criteria was subdivided into IoL and CD groups.Results. From a total of 3 938 deliveries, 168 patients met the inclusion criteria. IoL was indicated in 55 cases, resulting in 20 vaginal deliveries (VDs) (36%) and 35 CDs (64%). The remaining 113 patients were not candidates for IoL; of these, 89 required emergency CDs and 24 had semi-elective CDs. In the IoL group with abnormal umbilical artery Dopplers (UADs) there was 1 VD, and 4 CDs were performed for fetal compromise. Of cases with an estimated fetal weight (EFW) â€3rd centile, emergency CD was required in 24 (65%), and 8 (22%) were considered for IoL, in 6 of which CD was required.Conclusions. Of the EOPES population, 36% had successful IoL that culminated in VD. VD was more likely to occur with fetal growth appropriate for gestational age. The likelihood of CD increased if the UAD was abnormal, if the EFW was â€3rd centile or if eclampsia was present. The decision to induce should be considered carefully in these circumstances.
Reconstruction of three-dimensional porous media using generative adversarial neural networks
To evaluate the variability of multi-phase flow properties of porous media at
the pore scale, it is necessary to acquire a number of representative samples
of the void-solid structure. While modern x-ray computer tomography has made it
possible to extract three-dimensional images of the pore space, assessment of
the variability in the inherent material properties is often experimentally not
feasible. We present a novel method to reconstruct the solid-void structure of
porous media by applying a generative neural network that allows an implicit
description of the probability distribution represented by three-dimensional
image datasets. We show, by using an adversarial learning approach for neural
networks, that this method of unsupervised learning is able to generate
representative samples of porous media that honor their statistics. We
successfully compare measures of pore morphology, such as the Euler
characteristic, two-point statistics and directional single-phase permeability
of synthetic realizations with the calculated properties of a bead pack, Berea
sandstone, and Ketton limestone. Results show that GANs can be used to
reconstruct high-resolution three-dimensional images of porous media at
different scales that are representative of the morphology of the images used
to train the neural network. The fully convolutional nature of the trained
neural network allows the generation of large samples while maintaining
computational efficiency. Compared to classical stochastic methods of image
reconstruction, the implicit representation of the learned data distribution
can be stored and reused to generate multiple realizations of the pore
structure very rapidly.Comment: 21 pages, 20 figure
Perinatal selective serotonin reuptake inhibitor exposure and behavioral outcomes : A systematic review and meta-analyses of animal studies
Funding Information: This work was supported by a ZonMw grant (MKMD project; ID 114024127 ) awarded to JO & AR, and a grant awarded to JH from the Donders Institute for Brain, Cognition and Behavior . Funding bodies had no role in study design, data collection, data analysis, data interpretation, or the writing of the report.Peer reviewedPublisher PD
Resonance-assisted tunneling in near-integrable systems
Dynamical tunneling between symmetry related invariant tori is studied in the
near-integrable regime. Using the kicked Harper model as an illustration, we
show that the exponential decay of the wave functions in the classically
forbidden region is modified due to coupling processes that are mediated by
classical resonances. This mechanism leads to a substantial deviation of the
splitting between quasi-degenerate eigenvalues from the purely exponential
decrease with 1 / hbar obtained for the integrable system. A simple
semiclassical framework, which takes into account the effect of the resonance
substructure on the KAM tori, allows to quantitatively reproduce the behavior
of the eigenvalue splittings.Comment: 4 pages, 2 figures, gzipped tar file, to appear in Phys. Rev. Lett,
text slightly condensed compared to first versio
Perinatal fluoxetine exposure disrupts the circadian response to a phase-shifting challenge in female rats
Funding Information: This research was funded by the Dobberke foundation, grant number UPS/BP/4151 2016-27.Peer reviewedPublisher PD
Exponential Stellar Disks in Low Surface Brightness Galaxies: A Critical Test of Viscous Evolution
Viscous redistribution of mass in Milky Way-type galactic disks is an appealing way of generating an exponential stellar profile over many scale lengths, almost independent of initial conditions, requiring only that the viscous timescale and star formation timescale are approximately equal. However, galaxies with solid-body rotation curves cannot undergo viscous evolution. Low surface brightness (LSB) galaxies have exponential surface brightness profiles, yet have slowly rising, nearly solid-body, rotation curves. Because of this, viscous evolution may be inefficient in LSB galaxies: the exponential profiles, instead would give important insight into initial conditions for galaxy disk formation. Using star formation laws from the literature, and tuning the efficiency of viscous processes to reproduce an exponential stellar profile in Milky Way-type galaxies, I test the role of viscous evolution in LSB galaxies. Under the conservative and not unreasonable condition that LSB galaxies are gravitationally unstable for at least a part of their lives, I find that it is impossible to rule out a significant role for viscous evolution. This type of model still offers an attractive way of producing exponential disks, even in LSB galaxies with slowly-rising rotation curves
Magnetic phases of the quasi-two-dimensional compounds FexCo1 - xTa2O6
International audienceWe report new results on the magnetic properties of the FexCo1 - xTa2O6 series of compounds. Essentially using neutron-diffraction and magnetic measurements we study, in more detail, the low-x limit of the temperature versus x phase diagram, where a new bicritical point is observed. The complete phase diagram shows three different magnetic phases at low temperature, for a high, intermediate and very low iron content. These phases consist of distinct antiferromagnetic orderings, characterized by different pairs of propagation vectors. We obtain information about the intraplane exchange interactions by fitting a high-temperature series of the magnetic susceptibility. Here we improve on a previously employed model, showing that two non-equivalent next-nearest-neighbor interactions must be taken into account in order to allow for in-plane magnetic orderings that are consistent with the neutron-diffraction results
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