423 research outputs found
Why did European Radiology reject my radiomic biomarker paper? How to correctly evaluate imaging biomarkers in a clinical setting
This review explains in simple terms, accessible to the non-statistician, general principles regarding the correct research methods to develop and then evaluate imaging biomarkers in a clinical setting, including radiomic biomarkers. The distinction between diagnostic and prognostic biomarkers is made and emphasis placed on the need to assess clinical utility within the context of a multivariable model. Such models should not be restricted to imaging biomarkers and must include relevant disease and patient characteristics likely to be clinically useful. Biomarker utility is based on whether its addition to the basic clinical model improves diagnosis or prediction. Approaches to both model development and evaluation are explained and the need for adequate amounts of representative data stressed so as to avoid underpowering and overfitting. Advice is provided regarding how to report the research correctly
An interferometric study of the post-AGB binary 89 Herculis. II Radiative transfer models of the circumbinary disk
The presence of disks and outflows is widespread among post-AGB binaries. In
the first paper of this series, a surprisingly large fraction of optical light
was found to be resolved in the 89 Her post-AGB system. The data showed this
flux to arise from close to the central binary. Scattering off the inner rim of
the circumbinary disk, or in a dusty outflow were suggested as two possible
origins. With detailed dust radiative transfer models of the disk we aim to
discriminate between these two configurations. By including Herschel/SPIRE
photometry, we extend the SED such that it now fully covers UV to sub-mm
wavelengths. The MCMax radiative transfer code is used to create a large grid
of disk models. Our models include a self-consistent treatment of dust settling
as well as of scattering. A Si-rich composition with two additional opacity
sources, metallic Fe or amorphous C, are tested. The SED is fit together with
mid-IR (MIDI) visibilities as well as the optical and near-IR visibilities of
Paper I, to constrain the structure of the disk and in particular of its inner
rim. The near-IR visibility data require a smooth inner rim, here obtained with
a two-power-law parameterization of the radial surface density distribution. A
model can be found that fits all the IR photometric and interferometric data
well, with either of the two continuum opacity sources. Our best-fit passive
models are characterized by a significant amount of mm-sized grains, which are
settled to the midplane of the disk. Not a single disk model fits our data at
optical wavelengths though, the reason being the opposing constraints imposed
by the optical and near-IR interferometric data. A geometry in which a passive,
dusty, and puffed-up circumbinary disk is present, can reproduce all the IR but
not the optical observations of 89 Her. Another dusty, outflow or halo,
component therefore needs to be added to the system.Comment: 15 pages, in pres
Kalman-filter control schemes for fringe tracking. Development and application to VLTI/GRAVITY
The implementation of fringe tracking for optical interferometers is
inevitable when optimal exploitation of the instrumental capacities is desired.
Fringe tracking allows continuous fringe observation, considerably increasing
the sensitivity of the interferometric system. In addition to the correction of
atmospheric path-length differences, a decent control algorithm should correct
for disturbances introduced by instrumental vibrations, and deal with other
errors propagating in the optical trains. We attempt to construct control
schemes based on Kalman filters. Kalman filtering is an optimal data processing
algorithm for tracking and correcting a system on which observations are
performed. As a direct application, control schemes are designed for GRAVITY, a
future four-telescope near-infrared beam combiner for the Very Large Telescope
Interferometer (VLTI). We base our study on recent work in adaptive-optics
control. The technique is to describe perturbations of fringe phases in terms
of an a priori model. The model allows us to optimize the tracking of fringes,
in that it is adapted to the prevailing perturbations. Since the model is of a
parametric nature, a parameter identification needs to be included. Different
possibilities exist to generalize to the four-telescope fringe tracking that is
useful for GRAVITY. On the basis of a two-telescope Kalman-filtering control
algorithm, a set of two properly working control algorithms for four-telescope
fringe tracking is constructed. The control schemes are designed to take into
account flux problems and low-signal baselines. First simulations of the
fringe-tracking process indicate that the defined schemes meet the requirements
for GRAVITY and allow us to distinguish in performance. In a future paper, we
will compare the performances of classical fringe tracking to our Kalman-filter
control.Comment: 17 pages, 8 figures, accepted for publication in A&
Constraining the cosmic ray spectrum in the vicinity of the supernova remnant W28:from sub-GeV to multi-TeV energies
Supernova remnants interacting with molecular clouds are ideal laboratories to study the acceleration of particles at shock waves and their transport and interactions in the surrounding interstellar medium. In this paper, we focus on the supernova remnant W28, which over the years has been observed in all energy domains from radio waves to very-high-energy gamma rays. The bright gamma-ray emission detected from molecular clouds located in its vicinity revealed the presence of accelerated GeV and TeV particles in the region. An enhanced ionization rate has also been measured by means of millimetre observations, but such observations alone cannot tell us whether the enhancement is due to low energy (MeV) cosmic rays (either protons or electrons) or the X-ray photons emitted by the shocked gas. The goal of this study is to determine the origin of the enhanced ionization rate and to infer from multiwavelength observations the spectrum of cosmic rays accelerated at the supernova remnant shock in the unprecedented range spanning from MeV to multi-TeV particle energies. We developed a model to describe the transport of X-ray photons into the molecular cloud, and we fitted the radio, millimeter, and gamma-ray data to derive the spectrum of the radiating particles. The contribution from X-ray photons to the enhanced ionization rate is negligible, and therefore the ionization must be due to cosmic rays. Even though we cannot exclude a contribution to the ionization rate coming from cosmic ray electrons, we show that a scenario where cosmic ray protons explain both the gamma-ray flux and the enhanced ionization rate provides the most natural fit to multiwavelength data. This strongly suggests that the intensity of CR protons is enhanced in the region for particle energies in a very broad range covering almost 6 orders of magnitude: from MeV up to several tens of TeV
On the structure of the transition disk around TW Hya
For over a decade, the structure of the inner cavity in the transition disk
of TW Hydrae has been a subject of debate. Modeling the disk with data obtained
at different wavelengths has led to a variety of proposed disk structures.
Rather than being inconsistent, the individual models might point to the
different faces of physical processes going on in disks, such as dust growth
and planet formation. Our aim is to investigate the structure of the transition
disk again and to find to what extent we can reconcile apparent model
differences. A large set of high-angular-resolution data was collected from
near-infrared to centimeter wavelengths. We investigated the existing disk
models and established a new self-consistent radiative-transfer model. A
genetic fitting algorithm was used to automatize the parameter fitting. Simple
disk models with a vertical inner rim and a radially homogeneous dust
composition from small to large grains cannot reproduce the combined data set.
Two modifications are applied to this simple disk model: (1) the inner rim is
smoothed by exponentially decreasing the surface density in the inner ~3 AU,
and (2) the largest grains (>100 um) are concentrated towards the inner disk
region. Both properties can be linked to fundamental processes that determine
the evolution of protoplanetary disks: the shaping by a possible companion and
the different regimes of dust-grain growth, respectively. The full
interferometric data set from near-infrared to centimeter wavelengths requires
a revision of existing models for the TW Hya disk. We present a new model that
incorporates the characteristic structures of previous models but deviates in
two key aspects: it does not have a sharp edge at 4 AU, and the surface density
of large grains differs from that of smaller grains. This is the first
successful radiative-transfer-based model for a full set of interferometric
data.Comment: 22 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
The orbits of subdwarf B + main-sequence binaries. I: The sdB+G0 system PG 1104+243
The predicted orbital period histogram of an sdB population is bimodal with a
peak at short ( 250 days) periods. Observationally, there
are many short-period sdB systems known, but only very few long-period sdB
binaries are identified. As these predictions are based on poorly understood
binary interaction processes, it is of prime importance to confront the
predictions to observational data. In this contribution we aim to determine the
absolute dimensions of the long-period sdB+MS binary system PG1104+243.
High-resolution spectroscopy time-series were obtained with HERMES at the
Mercator telescope at La Palma, and analyzed to obtain radial velocities of
both components. Photometry from the literature was used to construct the
spectral energy distribution (SED) of the binary. Atmosphere models were used
to fit this SED and determine the surface gravity and temperature of both
components. The gravitational redshift provided an independent confirmation of
the surface gravity of the sdB component. An orbital period of 753 +- 3 d and a
mass ratio of q = 0.637 +- 0.015 were found from the RV-curves. The sdB
component has an effective temperature of Teff = 33500 +- 1200 K and a surface
gravity of logg = 5.84 +- 0.08 dex, while the cool companion is found to be a
G-type star with Teff = 5930 +- 160 K and logg = 4.29 +- 0.05 dex. Assuming a
canonical mass of Msdb = 0.47 Msun, the MS component has a mass of 0.74 +- 0.07
Msun, and its Teff corresponds to what is expected for a terminal age
main-sequence star with sub-solar metalicity. PG1104+243 is the first
long-period sdB binary in which accurate physical parameters of both components
could be determined, and the first sdB binary in which the gravitational
redshift is measured. Furthermore, PG1104+243 is the first sdB+MS system that
shows consistent evidence for being formed through stable Roche-lobe overflow.Comment: Accepted by A&A on 05-10-201
Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae
We report on the discovery of an infrared emission band in the Spitzer
spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in
the circumstellar environment. We modelled the observed emission to derive the
temperature of the TiO molecules (\approx 600 K), an upper limit on the column
density (\approx 10^17.25 cm^{-2}) and a lower limit on the spatial extent of
the layer that contains these molecules. (\approx 4.6 stellar radii). This is
the first time that this TiO emission band is observed. A search for similar
emission features in the sample of S-type stars yielded two additional
candidates. However, owing to the additional dust emission, the identification
is less stringent. By comparing the stellar characteristics of NP Aur to those
of the other stars in our sample, we find that all stars with TiO emission show
large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These
characteristics might be necessary requirements for a star to show TiO in
emission, but they are not sufficient.Comment: 4 pages, 4 figures, letter to the edito
A Tale of Two Stars: Interferometric Studies of Post-AGB Binaries
Binaries with circumbinary disks are commonly found among optically bright
post-AGB stars. Although clearly linked to binary interaction processes, the
formation, evolution and fate of these disks are still badly understood. Due to
their compactness, interferometric techniques are required to resolve them.
Here, we discuss our high-quality multiwavelength interferometric data of two
prototypical yet very different post-AGB binaries, AC and 89 Herculis, as well
as the modeling thereof with radiative transfer models. A detailed account of
the data and models of both objects is published in three separate papers
elsewhere; here we focus on comparing the modeling results for the two objects.
In particular we discuss the successes and limitations of the models which were
developed for protoplanetary disks around young stars. We conclude that
multiwavelength high-angular-resolution observations and radiative transfer
disk models are indispensible to understand these complex interacting objects
and their place in the grand scheme of the (binary) evolution of low and
intermediate mass stars.Comment: 5 pages, 1 figure, Conference proceedings for contributed talk at
"Why Galaxies care about AGB stars III
Structural and Luminescence Properties of Silica-Based Hybrids Containing New Silylated-Diketonato Europium(III) Complex
A new betadiketonate ligand displaying a trimethoxysilyl group as grafting function and a diketone moiety as complexing site (TTA-Si = 4,4,4-trifluoro-2-(3-trimethoxysilyl)propyl)-1-3-butanedione (C4H3S)COCH[(CH2)3Si(OCH3)3]COCF3) and its highly luminescent europium(III) complex [Eu(TTA-Si)3] have been synthesized and fully characterized. Luminescent silica-based hybrids have been prepared as well with this new complex grafted on the surface of dense silica nanoparticles (28 (+/-3 nm) or on mesoporous
silica particles. The covalent bonding of Eu(TTA-Si)3 inside the core of uniform silica
nanoparticles (40 (+/- 5 nm) was also achieved. Luminescence properties are discussed in relation to the europium chemical environment involved in each of the three hybrids. The general methodology proposed allowed high grafting ratios and overcame chelate release and tendency to agglomeration, and it could be applied to any silica matrix (in the core or at the surface, nanosized or not, dense or mesoporous) and therefore numerous applications such as luminescent markers and luminophors could be foreseen
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