250 research outputs found
Asteroseismology of eclipsing binary stars using Kepler and the HERMES spectrograph
We introduce our PhD project in which we focus on pulsating stars in
eclipsing binaries. The combination of high-precision Kepler photometry with
high-resolution HERMES spectroscopy allows for detailed descriptions of our
sample of target stars. We report here the detection of three false positives
by radial velocity measurements.Comment: Proceedings paper, 2 pages, 2 figures, to appear in "Setting a New
Standard in the Analysis of Binary Stars", Eds K. Pavlovski, A. Tkachenko,
and G. Torres, EAS Publications Serie
Global stellar variability study in the field-of-view of the Kepler satellite
We present the results of an automated variability analysis of the Kepler
public data measured in the first quarter (Q1) of the mission. In total, about
150 000 light curves have been analysed to detect stellar variability, and to
identify new members of known variability classes. We also focus on the
detection of variables present in eclipsing binary systems, given the important
constraints on stellar fundamental parameters they can provide. The methodology
we use here is based on the automated variability classification pipeline which
was previously developed for and applied successfully to the CoRoT exofield
database and to the limited subset of a few thousand Kepler asteroseismology
light curves. We use a Fourier decomposition of the light curves to describe
their variability behaviour and use the resulting parameters to perform a
supervised classification. Several improvements have been made, including a
separate extractor method to detect the presence of eclipses when other
variability is present in the light curves. We also included two new
variability classes compared to previous work: variables showing signs of
rotational modulation and of activity. Statistics are given on the number of
variables and the number of good candidates per class. A comparison is made
with results obtained for the CoRoT exoplanet data. We present some special
discoveries, including variable stars in eclipsing binary systems. Many new
candidate non-radial pulsators are found, mainly Delta Sct and Gamma Dor stars.
We have studied those samples in more detail by using 2MASS colours. The full
classification results are made available as an online catalogue.Comment: 15 pages, 5 figures, Accepted for publication in Astronomy and
Astrophysics on 09/02/201
Automated supervised classification of variable stars I. Methodology
The fast classification of new variable stars is an important step in making
them available for further research. Selection of science targets from large
databases is much more efficient if they have been classified first. Defining
the classes in terms of physical parameters is also important to get an
unbiased statistical view on the variability mechanisms and the borders of
instability strips. Our goal is twofold: provide an overview of the stellar
variability classes that are presently known, in terms of some relevant stellar
parameters; use the class descriptions obtained as the basis for an automated
`supervised classification' of large databases. Such automated classification
will compare and assign new objects to a set of pre-defined variability
training classes. For every variability class, a literature search was
performed to find as many well-known member stars as possible, or a
considerable subset if too many were present. Next, we searched on-line and
private databases for their light curves in the visible band and performed
period analysis and harmonic fitting. The derived light curve parameters are
used to describe the classes and define the training classifiers. We compared
the performance of different classifiers in terms of percentage of correct
identification, of confusion among classes and of computation time. We describe
how well the classes can be separated using the proposed set of parameters and
how future improvements can be made, based on new large databases such as the
light curves to be assembled by the CoRoT and Kepler space missions.Comment: This paper has been accepted for publication in Astronomy and
Astrophysics (reference AA/2007/7638) Number of pages: 27 Number of figures:
1
Automated classification of periodic variable stars{Improved methodology for the automated classification of periodic variable stars}
We present a novel automated methodology to detect and classify periodic
variable stars in a large database of photometric time series. The methods are
based on multivariate Bayesian statistics and use a multi-stage approach. We
applied our method to the ground-based data of the TrES Lyr1 field, which is
also observed by the Kepler satellite, covering ~26000 stars. We found many
eclipsing binaries as well as classical non-radial pulsators, such as slowly
pulsating B stars, Gamma Doradus, Beta Cephei and Delta Scuti stars. Also a few
classical radial pulsators were found.Comment: 11 pages, 6 figures Accepted for publication in MNRA
IRAS 19135+3937: An SRd variable as interacting binary surrounded by a circumbinary disc
Semi-regular (SR) variables are not a homogeneous class and their variability
is often explained due to pulsations and/or binarity. This study focuses on
IRAS 19135+3937, an SRd variable with an infra-red excess indicative of a dusty
disc. A time-series of high-resolution spectra, UBV photometry as well as a
very accurate light curve obtained by the Kepler satellite, allowed us to study
the object in unprecedented detail. We discovered it to be a binary with a
period of 127 days. The primary has a low surface gravity and an atmosphere
depleted in refractory elements. This combination of properties unambiguously
places IRAS 19135+3937 in the subclass of post-Asymptotic Giant Branch stars
with dusty discs.
We show that the light variations in this object can not be due to
pulsations, but are likely caused by the obscuration of the primary by the
circumbinary disc during orbital motion. Furthermore, we argue that the
double-peaked Fe emission lines provide evidence for the existence of a gaseous
circumbinary Keplerian disc inside the dusty disc. A secondary set of
absorption lines has been detected near light minimum, which we attribute to
the reflected spectrum of the primary on the disc wall, which segregates due to
the different Doppler shift. This corroborates the recent finding that
reflection in the optical by this type of discs is very efficient. The system
also shows a variable Halpha profile indicating a collimated outflow
originating around the companion. IRAS 19135+3937 thus encompasses all the
major emergent trends about evolved disc systems, that will eventually help to
place these objects in the evolutionary context.Comment: Accepted to MNRA
"Обратная" переходность среди частей речи как актуализация внутренней формы слова
В статье рассматривается специфика внутренней формы синтаксических дериватов, которую предлагается обозначить термином "грамматикализованная". На примере существительных адъективного склонения, наречий/наречных выражений и отглагольных прилагательных анализируется механизм речевой "обратной" морфолого-синтаксической транспозиции, в основе которого лежит инициируемая внутренней формой транспозитов и квазитранспозитов грамматическая рефлексия говорящих.У статті розглядається специфіка внутрішньої форми синтаксичних деріватов, яку пропонується позначити терміном "граматикалізована". На прикладі іменників ад'єктивного відміни, наречий/наречных виразів і віддієслівних прикметників аналізується механізм мовної "зворотної" морфолого-синтаксичної транспозиції, в основі якого лежить ініційована внутрішньою формою транспозітів і квазітранспозітов граматична рефлексія говорючих.The article examines the specificity of internal form of syntactic derivatives, which is to be named as "grammaticalized" one. By the example of substantives of adjective declension model, adverbs/adverb expressions and verbal adjectives, the author analyzes the mechanism of speech "reverse" morphological-syntactic transposition, and it is speaker grammatical reflexion initiated by internal form of transposites and "quasi-transposites" what underlies this mechanism
A mid-IR interferometric survey with MIDI/VLTI: resolving the second-generation protoplanetary disks around post-AGB binaries
We present a mid-IR interferometric survey of the circumstellar environment
of a specific class of post-Asymptotic Giant Branch (post-AGB) binaries. For
this class the presence of a compact dusty disk has been postulated on the
basis of various spatially unresolved measurements. Our interferometric survey
was performed with the MIDI instrument on the VLTI. In total 19 different
systems were observed using variable baseline configurations. Combining all the
visibilities at a single wavelength at 10.7 micron, we fitted two parametric
models to the data: a uniform disk (UD) and a ring model mimicking a
temperature gradient. We compared our observables of the whole sample, with
synthetic data computed from a grid of radiative transfer models of passively
irradiated disks in hydrostatic equilibrium. These models are computed with a
Monte Carlo code that has been widely applied to describe the structure of
protoplanetary disks around young stellar objects (YSO). The spatially resolved
observations show that the majority of our targets cluster closely together in
the distance-independent size-colour diagram, and have extremely compact N-band
emission regions. The typical uniform disk diameter of the N-band emission
region is about 40 mass which corresponds to a typical brightness temperature
of 400-600~K. The resolved objects display very similar characteristics in the
interferometric observables and in the spectral energy distributions.
Therefore, the physical properties of the disks around our targets must be
similar. The grid of protoplanetary disk models covers very well the observed
objects. Much like for young stars, the spatially resolved N-band emission
region is determined by the hot inner rim of the disk. Continued comparisons
between post-AGB and protoplanetary disks will help to understand grain growth
and disk evolution processes,Comment: 30 pages, 21 figures, in press in Astronomy and Astrophysic
On reducing the Heun equation to the hypergeometric equation
The reductions of the Heun equation to the hypergeometric equation by
polynomial transformations of its independent variable are enumerated and
classified. Heun-to-hypergeometric reductions are similar to classical
hypergeometric identities, but the conditions for the existence of a reduction
involve features of the Heun equation that the hypergeometric equation does not
possess; namely, its cross-ratio and accessory parameters. The reductions
include quadratic and cubic transformations, which may be performed only if the
singular points of the Heun equation form a harmonic or an equianharmonic
quadruple, respectively; and several higher-degree transformations. This result
corrects and extends a theorem in a previous paper, which found only the
quadratic transformations. [See K. Kuiken, "Heun's equation and the
hypergeometric equation", SIAM Journal on Mathematical Analysis 10:3 (1979),
655-657.]Comment: 36 pages, a few additional misprints correcte
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