281 research outputs found
Asteroseismic classification of stellar populations among 13000 red giants observed by Kepler
Of the more than 150000 targets followed by the Kepler Mission, about 10%
were selected as red giants. Due to their high scientific value, in particular
for Galaxy population studies and stellar structure and evolution, their Kepler
light curves were made public in late 2011. More than 13000 (over 85%) of these
stars show intrinsic flux variability caused by solar-like oscillations making
them ideal for large scale asteroseismic investigations. We automatically
extracted individual frequencies and measured the period spacings of the dipole
modes in nearly every red giant. These measurements naturally classify the
stars into various populations, such as the red giant branch, the low-mass
(M/Msol
1.8) secondary clump. The period spacings also reveal that a large fraction of
the stars show rotationally induced frequency splittings. This sample of stars
will undoubtedly provide an extremely valuable source for studying the stellar
population in the direction of the Kepler field, in particular when combined
with complementary spectroscopic surveys.Comment: 6 page, 5 figures, accepted by ApJ
Butterfly diagram of a Sun-like star observed using asteroseismology
Stellar magnetic fields are poorly understood but are known to be important
for stellar evolution and exoplanet habitability. They drive stellar activity,
which is the main observational constraint on theoretical models for magnetic
field generation and evolution. Starspots are the main manifestation of the
magnetic fields at the stellar surface. In this study we measure the variation
of their latitude with time, called a butterfly diagram in the solar case, for
the solar analogue HD 173701 (KIC 8006161). To that effect, we use Kepler data,
to combine starspot rotation rates at different epochs and the
asteroseismically determined latitudinal variation of the stellar rotation
rates. We observe a clear variation of the latitude of the starspots. It is the
first time such a diagram is constructed using asteroseismic data.Comment: 8 pages, 4 figures, accepted in A&A Letter
Asteroseismic detection of latitudinal differential rotation in 13 Sun-like stars
The differentially rotating outer layers of stars are thought to play a role
in driving their magnetic activity, but the underlying mechanisms that generate
and sustain differential rotation are poorly understood. We report the
measurement of latitudinal differential rotation in the convection zones of 40
Sun-like stars using asteroseismology. For the most significant detections, the
stars' equators rotate approximately twice as fast as their mid-latitudes. The
latitudinal shear inferred from asteroseismology is much larger than
predictions from numerical simulations.Comment: 45 pages, 11 figures, 4 tables, published in Scienc
Oscillation mode frequencies of 61 main sequence and subgiant stars observed by Kepler
Solar-like oscillations have been observed by Kepler and CoRoT in several
solar-type stars, thereby providing a way to probe the stars using
asteroseismology.
We provide the mode frequencies of the oscillations of various stars required
to perform a comparison with those obtained from stellar modelling.
We used a time series of nine months of data for each star. The 61 stars
observed were categorised in three groups: simple, F-like and mixed-mode. The
simple group includes stars for which the identification of the mode degree is
obvious. The F-like group includes stars for which the identification of the
degree is ambiguous. The mixed-mode group includes evolved stars for which the
modes do not follow the asymptotic relation of low-degree frequencies.
Following this categorisation, the power spectra of the 61 main sequence and
subgiant stars were analysed using both maximum likelihood estimators and
Bayesian estimators, providing individual mode characteristics such as
frequencies, linewidths, and mode heights. We developed and describe a
methodology for extracting a single set of mode frequencies from multiple sets
derived by different methods and individual scientists. We report on how one
can assess the quality of the fitted parameters using the likelihood ratio test
and the posterior probabilities.
We provide the mode frequencies of 61 stars (with their 1-sigma error bars),
as well as their associated echelle diagrams.Comment: 83 pages, 17 figures, 61 tables, paper accepted by Astronomy and
Astrophysic
Open issues in probing interiors of solar-like oscillating main sequence stars: 2. Diversity in the HR diagram
We review some major open issues in the current modelling of low and
intermediate mass, main sequence stars based on seismological studies. The
solar case was discussed in a companion paper, here several issues specific to
other stars than the Sun are illustrated with a few stars observed with CoRoT
and expectations from Kepler data.Comment: GONG 2010 - SoHO 24, A new era of seismology of the Sun and
solar-like stars, To be published in the Journal of Physics: Conference
Series (JPCS
Accident vasculaire cerebral ischemique post-chimiotherapie pour cancer du sein
IntroductionLa chimiothérapie anticancéreuse est pourvoyeuse d’un large spectre de complications neurologiques. Parmi ces complications les accidents vasculaires cérébraux ischémiques (AVCI) restent un évènement inhabituel dont l’incidence est de 0,14%.ObjectifRapporter et commenter l’observation d’une patiente ayant présenté un AVCI dans les suites d’une chimiothérapie pour cancer du sein.ObservationPatiente âgée de 46ans, traitée pour une épilepsie généralisée, et sans facteur de risque cardiovasculaire, est suivie pour un cancer du sein. Elle a présenté au décours de la chimiothérapie un accident vasculaire cérébral ischémique sylvien droit confirmé au scanner cérébral. Les explorations cardiaques sont normales, ainsi que le bilan biologique étiologique.DiscussionLe diagnostic d’AVC post-chimiothérapie a été retenu chez notre patiente sur la base de l’absence de facteur de risque vasculaire et la négativité du bilan étiologique, ainsi que l’usage du 5 Fluorouracile qui a été incriminé dans la survenue d’AVC ischémiques. La survenue de cette complication dépend du type de drogue utilisé, et de certaines associations thérapeutiques, et relèverait de plusieurs mécanismes pouvant être intriqués. Le bilan doit être le plus complet possible afin d’affirmer l’imputabilité de la chimiothérapie.ConclusionNotre observation souligne la sévérité des complications neurologiques de la chimiothérapie, ainsi que le dilemme thérapeutique posé, en l’absence de protocole de prise en charge codifié.Mots clés : Accident vasculaire cérébral, cancer du sein, chimiothérapie, toxicité, 5-Fluorouracil
Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler
Solar-like oscillations have been observed by Kepler and CoRoT in many
solar-type stars, thereby providing a way to probe the stars using
asteroseismology. We provide the mode linewidths and mode heights of the
oscillations of various stars as a function of frequency and of effective
temperature. We used a time series of nearly two years of data for each star.
The 23 stars observed belong to the simple or F-like category. The power
spectra of the 23 main-sequence stars were analysed using both maximum
likelihood estimators and Bayesian estimators, providing individual mode
characteristics such as frequencies, linewidths, and mode heights. We study the
source of systematic errors in the mode linewidths and mode heights, and we
present a way to correct these errors with respect to a common reference fit.
Using the correction, we could explain all sources of systematic errors, which
could be reduced to less than 15% for mode linewidths and heights, and
less than 5% for amplitude, when compared to the reference fit. The effect
of a different estimated stellar background and a different estimated splitting
will provide frequency-dependent systematic errors that might affect the
comparison with theoretical mode linewidth and mode height, therefore affecting
the understanding of the physical nature of these parameters. All other sources
of relative systematic errors are less dependent upon frequency. We also
provide the dependence of the so-called linewidth dip, in the middle of the
observed frequency range, as a function of effective temperature. We show that
the depth of the dip decreases with increasing effective temperature. The
dependence of the dip on effective temperature may imply that the mixing length
parameter or the convective flux may increase with effective
temperature.Comment: Accepted by A&A, 38 pages, 35 figures, 26 table
Bayesian peak-bagging of solar-like oscillators using MCMC: A comprehensive guide
Context: Asteroseismology has entered a new era with the advent of the NASA
Kepler mission. Long and continuous photometric observations of unprecedented
quality are now available which have stimulated the development of a number of
suites of innovative analysis tools.
Aims: The power spectra of solar-like oscillations are an inexhaustible
source of information on stellar structure and evolution. Robust methods are
hence needed in order to infer both individual oscillation mode parameters and
parameters describing non-resonant features, thus making a seismic
interpretation possible.
Methods: We present a comprehensive guide to the implementation of a Bayesian
peak-bagging tool that employs a Markov chain Monte Carlo (MCMC). Besides
making it possible to incorporate relevant prior information through Bayes'
theorem, this tool also allows one to obtain the marginal probability density
function for each of the fitted parameters. We apply this tool to a couple of
recent asteroseismic data sets, namely, to CoRoT observations of HD 49933 and
to ground-based observations made during a campaign devoted to Procyon.
Results: The developed method performs remarkably well at constraining not
only in the traditional case of extracting oscillation frequencies, but also
when pushing the limit where traditional methods have difficulties. Moreover it
provides an rigorous way of comparing competing models, such as the ridge
identifications, against the asteroseismic data.Comment: Accepted for publication in A&
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