581 research outputs found
Fourier phase analysis in radio-interferometry
Most statistical tools used to characterize the complex structures of the
interstellar medium can be related to the power spectrum, and therefore to the
Fourier amplitudes of the observed fields. To tap into the vast amount of
information contained in the Fourier phases, one may consider the probability
distribution function (PDF) of phase increments, and the related concepts of
phase entropy and phase structure quantity. We use these ideas here with the
purpose of assessing the ability of radio-interferometers to detect and recover
this information. By comparing current arrays such as the VLA and Plateau de
Bure to the future ALMA instrument, we show that the latter is definitely
needed to achieve significant detection of phase structure, and that it will do
so even in the presence of a fair amount of atmospheric phase fluctuations. We
also show that ALMA will be able to recover the actual "amount'' of phase
structure in the noise-free case, if multiple configurations are used.Comment: Accepted for publication in "Astronomy & Astrophysics
A novel estimator of the polarization amplitude from normally distributed Stokes parameters
We propose a novel estimator of the polarization amplitude from a single
measurement of its normally distributed Stokes components. Based on the
properties of the Rice distribution and dubbed 'MAS' (Modified ASymptotic), it
meets several desirable criteria:(i) its values lie in the whole positive
region; (ii) its distribution is continuous; (iii) it transforms smoothly with
the signal-to-noise ratio (SNR) from a Rayleigh-like shape to a Gaussian one;
(iv) it is unbiased and reaches its components' variance as soon as the SNR
exceeds 2; (v) it is analytic and can therefore be used on large data-sets. We
also revisit the construction of its associated confidence intervals and show
how the Feldman-Cousins prescription efficiently solves the issue of classical
intervals lying entirely in the unphysical negative domain. Such intervals can
be used to identify statistically significant polarized regions and conversely
build masks for polarization data. We then consider the case of a general
covariance matrix and perform a generalization of the estimator that
preserves its asymptotic properties. We show that its bias does not depend on
the true polarization angle, and provide an analytic estimate of its variance.
The estimator value, together with its variance, provide a powerful
point-estimate of the true polarization amplitude that follows an unbiased
Gaussian distribution for a SNR as low as 2. These results can be applied to
the much more general case of transforming any normally distributed random
variable from Cartesian to polar coordinates.Comment: Accepted by MNRA
Synthetic observations of first hydrostatic cores in collapsing low-mass dense cores II. Simulated ALMA dust emission maps
First hydrostatic cores are predicted by theories of star formation, but
their existence has never been demonstrated convincingly by (sub)millimeter
observations. Furthermore, the multiplicity at the early phases of the star
formation process is poorly constrained. The purpose of this paper is twofold.
First, we seek to provide predictions of ALMA dust continuum emission maps from
early Class 0 objects. Second, we show to what extent ALMA will be able to
probe the fragmentation scale in these objects. Following our previous paper
(Commer\c{c}on et al. 2012, hereafter paper I), we post-process three
state-of-the-art radiation-magneto-hydrodynamic 3D adaptive mesh refinement
calculations to compute the emanating dust emission maps. We then produce
synthetic ALMA observations of the dust thermal continuum from first
hydrostatic cores. We present the first synthetic ALMA observations of dust
continuum emission from first hydrostatic cores. We analyze the results given
by the different bands and configurations and we discuss for which combinations
of the two the first hydrostatic cores would most likely be observed. We also
show that observing dust continuum emission with ALMA will help in identifying
the physical processes occurring within collapsing dense cores. If the magnetic
field is playing a role, the emission pattern will show evidence of a
pseudo-disk and even of a magnetically driven outflow, which pure
hydrodynamical calculations cannot reproduce. The capabilities of ALMA will
enable us to make significant progress towards understanding fragmentation at
the early Class 0 stage and discovering first hydrostatic cores.Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Polarized thermal emission from Galactic dust as seen by Planck
The Planck maps of polarized thermal emission from Galactic dust reveal a wealth of fascinating data, greatly superseding previous observations in both sensitivity and statistical significance. I will present the results from a number of studies which have used this data to investigate properties of the Galactic magnetic field (GMF), focusing on the statistics of the polarization fraction and angle. I will also present a set of models showing, through a comparison with synthetic observations, that the turbulent structure of the GMF is most likely responsible for the observed statistical properties of this polarized emission
SKA HI end2end simulation
The current status of the HI simulation efforts is presented, in which a self
consistent simulation path is described and basic equations to calculate array
sensitivities are given. There is a summary of the SKA Design Study (SKADS) sky
simulation and a method for implementing it into the array simulator is
presented. A short overview of HI sensitivity requirements is discussed and
expected results for a simulated HI survey are presented.Comment: 7 pages, 6 figues, need skads2009.cls file to late
Simulated CII observations for SPICA/SAFARI
We investigate the case of CII 158 micron observations for SPICA/SAFARI using
a three-dimensional magnetohydrodynamical (MHD) simulation of the diffuse
interstellar medium (ISM) and the Meudon PDR code. The MHD simulation consists
of two converging flows of warm gas (10,000 K) within a cubic box 50 pc in
length. The interplay of thermal instability, magnetic field and self-gravity
leads to the formation of cold, dense clumps within a warm, turbulent
interclump medium. We sample several clumps along a line of sight through the
simulated cube and use them as input density profiles in the Meudon PDR code.
This allows us to derive intensity predictions for the CII 158 micron line and
provide time estimates for the mapping of a given sky area.Comment: 4 pages, 5 figures, to appear in the proceedings of the workshop "The
Space Infrared Telescope for Cosmology & Astrophysics: Revealing the Origins
of Planets and Galaxies" (July 2009, Oxford, United Kingdom
Polarization measurements analysis II. Best estimators of polarization fraction and angle
With the forthcoming release of high precision polarization measurements,
such as from the Planck satellite, it becomes critical to evaluate the
performance of estimators for the polarization fraction and angle. These two
physical quantities suffer from a well-known bias in the presence of
measurement noise, as has been described in part I of this series. In this
paper, part II of the series, we explore the extent to which various estimators
may correct the bias. Traditional frequentist estimators of the polarization
fraction are compared with two recent estimators: one inspired by a Bayesian
analysis and a second following an asymptotic method. We investigate the
sensitivity of these estimators to the asymmetry of the covariance matrix which
may vary over large datasets. We present for the first time a comparison among
polarization angle estimators, and evaluate the statistical bias on the angle
that appears when the covariance matrix exhibits effective ellipticity. We also
address the question of the accuracy of the polarization fraction and angle
uncertainty estimators. The methods linked to the credible intervals and to the
variance estimates are tested against the robust confidence interval method.
From this pool of estimators, we build recipes adapted to different use-cases:
build a mask, compute large maps, and deal with low S/N data. More generally,
we show that the traditional estimators suffer from discontinuous distributions
at low S/N, while the asymptotic and Bayesian methods do not. Attention is
given to the shape of the output distribution of the estimators, and is
compared with a Gaussian. In this regard, the new asymptotic method presents
the best performance, while the Bayesian output distribution is shown to be
strongly asymmetric with a sharp cut at low S/N.Finally, we present an
optimization of the estimator derived from the Bayesian analysis using adapted
priors
Alexithymie et appauvrissement onirique chez des populations cliniques souffrant de troubles du sommeil
Le premier objectif de cette étude était d’évaluer la relation entre l’alexithymie et différents troubles du sommeil chez des patients diagnostiqués (N= 580) selon la polysomnographie et la classification de l’American Academy of Sleep Medicine (AASM) et chez des sujets contrôle (N= 145) en utilisant l’Échelle d’Alexithymie de Toronto à 20 items (TAS-20). Le deuxième objectif était d’estimer le lien entre l’alexithymie et des caractéristiques de rêves suivant un Questionnaire sur les Rêves de 14 items.
Les résultats confirment un lien entre l’alexithymie et les troubles du sommeil. Sa prévalence était supérieure dans le groupe clinique comparativement au groupe contrôle, et était différente selon les troubles. Les hommes cotaient plus haut que les femmes à l’Échelle d’Alexithymie de Toronto à 20 items (TAS-20) et sur ses sous-échelles DDF (difficulty describing feeling) et EOT (externally oriented thinking). L’EOT pourrait être impliquée dans les troubles de sommeil en étant l’unique sous-échelle, où un effet principal des diagnostics était significatif dans le groupe clinique. Pour les rêves, le score du TAS-20 corrélait positivement avec le facteur « détresse des cauchemars »; et négativement avec « rappel de rêves » et « signification des rêves ». Les sous-échelles du TAS-20 avaient des corrélations différentes: positive entre DIF et « détresse des cauchemars », négative entre DDF et « rappel de rêves » et EOT avec « signification des rêves ». À part quelques exceptions, ces modèles sont obtenus pour les groupes cliniques et non-cliniques, et pour les hommes et les femmes dans ces deux groupes. Ces résultats suggèrent un modèle consistant, et reproductible, de relations entre l’alexithymie et les composantes des rêves.Using a large clinical group of patients suffering from sleep disorders (N= 580) and non-clinical comparison subjects (N= 145), the first aim of this study was to evaluate the relationship between alexithymia and several different sleep disorders diagnosed from polysomnography following the American Academy of Sleep Medicine classification system and using a Canadian French translation of the 20-item Toronto Alexithymia scale (TAS-20). The second objective was to evaluate the link between alexithymia and dream characteristics using a 14-item Dreaming Questionnaire.
Results confirm a relationship between alexithymia and sleep disorders. Its prevalence was higher in the clinical than the non-clinical group and differed according to sleep disorder diagnoses. Men scored higher than women on the TAS-20 total score and on the DDF (difficulty describing feeling) and EOT (externally oriented thinking) subscales. EOT could be implicated in sleep disorders pathology as it was the only subscale for which the sleep diagnosis effect was significant in the clinical group. Concerning the second objective, TAS-20 total score correlated positively with nightmare distress and negatively with dream recall; and correlated negatively with dream meaning. TAS-20 subscales were differentially correlated with the 3 dream factors of the Dreaming Questionnaire: DIF with increased nightmare distress, DDF with decreased dream recall and EOT with decreased dream meaning. With some exceptions, these patterns were obtained for clinical and non-clinical groups and for men and women. These results suggest a consistent and replicable pattern of relationships between alexithymia and dreaming components
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