492 research outputs found
Differential interferometric phases at high spectral resolution as a sensitive physical diagnostic of circumstellar disks
Context. The circumstellar disks ejected by many rapidly rotating B stars
(so-called Be stars) offer the rare opportunity of studying the structure and
dynamics of gaseous disks at high spectral as well as angular resolution. Aims.
This paper explores a newly identified effect in spectro-interferometric phase
that can be used for probing the inner regions of gaseous edge-on disks on a
scale of a few stellar radii. Methods. The origin of this effect (dubbed
central quasi-emission phase signature, CQE-PS) lies in the velocity-dependent
line absorption of photospheric radiation by the circumstellar disk. At high
spectral and marginal interferometric resolution, photocenter displacements
between star and isovelocity regions in the Keplerian disk reveal themselves
through small interferometric phase shifts. To investigate the diagnostic
potential of this effect, a series of models are presented, based on detailed
radiative transfer calculations in a viscous decretion disk. Results. Amplitude
and detailed shape of the CQE-PS depend sensitively on disk density and size
and on the radial distribution of the material with characteristic shapes in
differential phase diagrams. In addition, useful lower limits to the angular
size of the central stars can be derived even when the system is almost
unresolved. Conclusions. The full power of this diagnostic tool can be expected
if it can be applied to observations over a full life-cycle of a disk from
first ejection through final dispersal, over a full cycle of disk oscillations,
or over a full orbital period in a binary system
Estimation of confidence limits for descriptive indexes derived from autoregressive analysis of time series: Methods and application to heart rate variability
The growing interest in personalized medicine requires making inferences from descriptive indexes estimated from individual recordings of physiological signals, with statistical analyses focused on individual differences between/within subjects, rather than comparing supposedly homogeneous cohorts. To this end, methods to compute confidence limits of individual estimates of descriptive indexes are needed. This study introduces numerical methods to compute such confidence limits and perform statistical comparisons between indexes derived from autoregressive (AR) modeling of individual time series. Analytical approaches are generally not viable, because the indexes are usually nonlinear functions of the AR parameters. We exploit Monte Carlo (MC) and Bootstrap (BS) methods to reproduce the sampling distribution of the AR parameters and indexes computed from them. Here, these methods are implemented for spectral and information-theoretic indexes of heart-rate variability (HRV) estimated from AR models of heart-period time series. First, the MS and BC methods are tested in a wide range of synthetic HRV time series, showing good agreement with a gold-standard approach (i.e. multiple realizations of the "true" process driving the simulation). Then, real HRV time series measured from volunteers performing cognitive tasks are considered, documenting (i) the strong variability of confidence limits\ue2\u80\u99 width across recordings, (ii) the diversity of individual responses to the same task, and (iii) frequent disagreement between the cohort-average response and that of many individuals. We conclude that MC and BS methods are robust in estimating confidence limits of these AR-based indexes and thus recommended for short-term HRV analysis. Moreover, the strong inter-individual differences in the response to tasks shown by AR-based indexes evidence the need of individual-by-individual assessments of HRV features. Given their generality, MC and BS methods are promising for applications in biomedical signal processing and beyond, providing a powerful new tool for assessing the confidence limits of indexes estimated from individual recordings
SAMplus: adaptive optics at optical wavelengths for SOAR
Adaptive Optics (AO) is an innovative technique that substantially improves
the optical performance of ground-based telescopes. The SOAR Adaptive Module
(SAM) is a laser-assisted AO instrument, designed to compensate ground-layer
atmospheric turbulence in near-IR and visible wavelengths over a large Field of
View. Here we detail our proposal to upgrade SAM, dubbed SAMplus, that is
focused on enhancing its performance in visible wavelengths and increasing the
instrument reliability. As an illustration, for a seeing of 0.62 arcsec at 500
nm and a typical turbulence profile, current SAM improves the PSF FWHM to 0.40
arcsec, and with the upgrade we expect to deliver images with a FWHM of
arcsec -- up to 0.23 arcsec FWHM PSF under good seeing
conditions. Such capabilities will be fully integrated with the latest SAM
instruments, putting SOAR in an unique position as observatory facility.Comment: To appear in Proc. SPIE 10703 (Ground-based and Airborne
Instrumentation for Astronomy VII; SPIEastro18
Gradients of O-information highlight synergy and redundancy in physiological applications
The study of high order dependencies in complex systems has recently led to the introduction of statistical synergy, a novel quantity corresponding to a form of emergence in which patterns at large scales are not traceable from lower scales. As a consequence, several works in the last years dealt with the synergy and its counterpart, the redundancy. In particular, the O-information is a signed metric that measures the balance between redundant and synergistic statistical dependencies. In spite of its growing use, this metric does not provide insight about the role played by low-order scales in the formation of high order effects. To fill this gap, the framework for the computation of the O-information has been recently expanded introducing the so-called gradients of this metric, which measure the irreducible contribution of a variable (or a group of variables) to the high order informational circuits of a system. Here, we review the theory behind the O-information and its gradients and present the potential of these concepts in the field of network physiology, showing two new applications relevant to brain functional connectivity probed via functional resonance imaging and physiological interactions among the variability of heart rate, arterial pressure, respiration and cerebral blood flow
VLTI/PIONIER images the Achernar disk swell
Context. The mechanism of disk formation around fast-rotating Be stars is not
well understood. In particular, it is not clear which mechanisms operate, in
addition to fast rotation, to produce the observed variable ejection of matter.
The star Achernar is a privileged laboratory to probe these additional
mechanisms because it is close, presents B-Be phase variations on timescales
ranging from 6 yr to 15 yr, a companion star was discovered around it, and
probably presents a polar wind or jet. Aims. Despite all these previous
studies, the disk around Achernar was never directly imaged. Therefore we seek
to produce an image of the photosphere and close environment of the star.
Methods. We used infrared long-baseline interferometry with the PIONIER/VLTI
instrument to produce reconstructed images of the photosphere and close
environment of the star over four years of observations. To study the disk
formation, we compared the observations and reconstructed images to previously
computed models of both the stellar photosphere alone (normal B phase) and the
star presenting a circumstellar disk (Be phase). Results. The observations
taken in 2011 and 2012, during the quiescent phase of Achernar, do not exhibit
a disk at the detection limit of the instrument. In 2014, on the other hand, a
disk was already formed and our reconstructed image reveals an extended H-band
continuum excess flux. Our results from interferometric imaging are also
supported by several H-alpha line profiles showing that Achernar started an
emission-line phase sometime in the beginning of 2013. The analysis of our
reconstructed images shows that the 2014 near-IR flux extends to 1.7 - 2.3
equatorial radii. Our model-independent size estimation of the H-band continuum
contribution is compatible with the presence of a circumstellar disk, which is
in good agreement with predictions from Be-disk models
Multitechnique testing of the viscous decretion disk model I. The stable and tenuous disk of the late-type Be star CMi
The viscous decretion disk (VDD) model is able to explain most of the
currently observable properties of the circumstellar disks of Be stars.
However, more stringent tests, focusing on reproducing multitechnique
observations of individual targets via physical modeling, are needed to study
the predictions of the VDD model under specific circumstances. In the case of
nearby, bright Be star CMi, these circumstances are a very stable
low-density disk and a late-type (B8Ve) central star. The aim is to test the
VDD model thoroughly, exploiting the full diagnostic potential of individual
types of observations, in particular, to constrain the poorly known structure
of the outer disk if possible, and to test truncation effects caused by a
possible binary companion using radio observations. We use the Monte Carlo
radiative transfer code HDUST to produce model observables, which we compare
with a very large set of multitechnique and multiwavelength observations that
include ultraviolet and optical spectra, photometry covering the interval
between optical and radio wavelengths, optical polarimetry, and optical and
near-IR (spectro)interferometry. Due to the absence of large scale variability,
data from different epochs can be combined into a single dataset. A parametric
VDD model with radial density exponent of = 3.5, which is the canonical
value for isothermal flaring disks, is found to explain observables typically
formed in the inner disk, while observables originating in the more extended
parts favor a shallower, = 3.0, density falloff. Modeling of radio
observations allowed for the first determination of the physical extent of a Be
disk (35 stellar radii), which might be caused by a binary
companion. Finally, polarization data allowed for an indirect measurement of
the rotation rate of the star, which was found to be , i.e.,
very close to critical.Comment: 19 pages (35 including online material), 17 figures, 2 online
figures, 2 online tables with dat
Revealing the structure of the outer disks of Be stars
Context. The structure of the inner parts of Be star disks (20 stellar radii)
is well explained by the viscous decretion disk (VDD) model, which is able to
reproduce the observable properties of most of the objects studied so far. The
outer parts, on the ther hand, are not observationally well-explored, as they
are observable only at radio wavelengths. A steepening of the spectral slope
somewhere between infrared and radio wavelengths was reported for several Be
stars that were previously detected in the radio, but a convincing physical
explanation for this trend has not yet been provided. Aims. We test the VDD
model predictions for the extended parts of a sample of six Be disks that have
been observed in the radio to address the question of whether the observed
turndown in the spectral energy distribution (SED) can be explained in the
framework of the VDD model, including recent theoretical development for
truncated Be disks in binary systems. Methods. We combine new multi-wavelength
radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama
Pathfinder Experiment (APEX) with previously published radio data and archival
SED measurements at ultraviolet, visual, and infrared wavelengths. The density
structure of the disks, including their outer parts, is constrained by
radiative transfer modeling of the observed spectrum using VDD model
predictions. In the VDD model we include the presumed effects of possible tidal
influence from faint binary companions. Results. For 5 out of 6 studied stars,
the observed SED shows strong signs of SED turndown between far-IR and radio
wavelengths. A VDD model that extends to large distances closely reproduces the
observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED.
... (abstract continues but did not fit here)Comment: 20 pages, 8 figure
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