217 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
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
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
Melting of magnetic correlations in charge-orbital ordered La(0.5)Sr(1.5)MnO(4) : competition of ferro and antiferromagnetic states
The magnetic correlations in the charge- and orbital-ordered manganite
La(0.5)Sr(1.5)MnO(4) have been studied by elastic and inelastic neutron
scattering techniques. Out of the well-defined CE-type magnetic structure with
the corresponding magnons a competition between CE-type and ferromagnetic
fluctuations develops. Whereas ferromagnetic correlations are fully suppressed
by the static CE-type order at low temperature, elastic and inelastic CE-type
correlations disappear with the melting of the charge-orbital order at high
temperature. In its charge-orbital disordered phase, La(0.5)Sr(1.5)MnO(4)
exhibits a dispersion of ferromagnetic correlations which remarkably resembles
the magnon dispersion in ferromagnetically ordered metallic perovskite
manganites.Comment: 14 pages, 11 figure
Magnetic correlations in La2-xSrxCoO4 studied by neutron scattering : possible evidence for stripe phases
Spin correlations in LSCO have been studied by neutron scattering. The
commensurate antiferromagnetic order of the parent compound persists on a very
short range up to a Sr content of x=0.3, whereas small amounts of Sr suppress
commensurate antiferromagnetism in cuprates and in nickelates. For x>0.3 we
find incommensurate spin ordering with the modulation closely following the
amount of doping. These incommensurate phases strongly resemble the stripe
phases observed in cuprates and nickelates, but incommensurate magnetic
ordering appears only at larger Sr content in the cobaltates due to a reduced
charge mobility
Modelling the Autocovariance of the Power Spectrum of a Solar-Type Oscillator
Asteroseismology is able to conduct studies on the interiors of solar-type
stars from the analysis of stellar acoustic spectra. However, such an analysis
process often has to rely upon subjective choices made throughout. A recurring
problem is to determine whether a signal in the acoustic spectrum originates
from a radial or a dipolar oscillation mode. In order to overcome this problem,
we present a procedure for modelling and fitting the autocovariance of the
power spectrum which can be used to obtain global seismic parameters of
solar-type stars, doing so in an automated fashion without the need to make
subjective choices. From the set of retrievable global seismic parameters we
emphasize the mean small frequency separation and, depending on the intrinsic
characteristics of the power spectrum, the mean rotational frequency splitting.
Since this procedure is automated, it can serve as a useful tool in the
analysis of the more than one thousand solar-type stars expected to be observed
as part of the Kepler Asteroseismic Investigation (KAI). We apply the
aforementioned procedure to simulations of the Sun. Assuming different apparent
magnitudes, we address the issues of how accurately and how precisely we can
retrieve the several global seismic parameters were the Sun to be observed as
part of the KAI.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Seismic evidence for a rapidly rotating core in a lower-giant-branch star observed with Kepler
Rotation is expected to have an important influence on the structure and the
evolution of stars. However, the mechanisms of angular momentum transport in
stars remain theoretically uncertain and very complex to take into account in
stellar models. To achieve a better understanding of these processes, we
desperately need observational constraints on the internal rotation of stars,
which until very recently were restricted to the Sun. In this paper, we report
the detection of mixed modes - i.e. modes that behave both as g modes in the
core and as p modes in the envelope - in the spectrum of the early red giant
KIC7341231, which was observed during one year with the Kepler spacecraft. By
performing an analysis of the oscillation spectrum of the star, we show that
its non-radial modes are clearly split by stellar rotation and we are able to
determine precisely the rotational splittings of 18 modes. We then find a
stellar model that reproduces very well the observed atmospheric and seismic
properties of the star. We use this model to perform inversions of the internal
rotation profile of the star, which enables us to show that the core of the
star is rotating at least five times faster than the envelope. This will shed
new light on the processes of transport of angular momentum in stars. In
particular, this result can be used to place constraints on the angular
momentum coupling between the core and the envelope of early red giants, which
could help us discriminate between the theories that have been proposed over
the last decades.Comment: Accepted in ApJ, 39 pages, 16 figure
Solar-like oscillations in KIC11395018 and KIC11234888 from 8 months of Kepler data
We analyze the photometric short-cadence data obtained with the Kepler
Mission during the first eight months of observations of two solar-type stars
of spectral types G and F: KIC 11395018 and KIC 11234888 respectively, the
latter having a lower signal-to-noise ratio compared to the former. We estimate
global parameters of the acoustic (p) modes such as the average large and small
frequency separations, the frequency of the maximum of the p-mode envelope and
the average linewidth of the acoustic modes. We were able to identify and to
measure 22 p-mode frequencies for the first star and 16 for the second one even
though the signal-to-noise ratios of these stars are rather low. We also derive
some information about the stellar rotation periods from the analyses of the
low-frequency parts of the power spectral densities. A model-independent
estimation of the mean density, mass and radius are obtained using the scaling
laws. We emphasize the importance of continued observations for the stars with
low signal-to-noise ratio for an improved characterization of the oscillation
modes. Our results offer a preview of what will be possible for many stars with
the long data sets obtained during the remainder of the mission.Comment: 39 pages, 9 figures. Accepted for publication in Ap
Asteroseismology from multi-month Kepler photometry: the evolved Sun-like stars KIC 10273246 and KIC 10920273
The evolved main-sequence Sun-like stars KIC 10273246 (F-type) and KIC
10920273 (G-type) were observed with the NASA Kepler satellite for
approximately ten months with a duty cycle in excess of 90%. Such continuous
and long observations are unprecedented for solar-type stars other than the
Sun.
We aimed mainly at extracting estimates of p-mode frequencies - as well as of
other individual mode parameters - from the power spectra of the light curves
of both stars, thus providing scope for a full seismic characterization.
The light curves were corrected for instrumental effects in a manner
independent of the Kepler Science Pipeline. Estimation of individual mode
parameters was based both on the maximization of the likelihood of a model
describing the power spectrum and on a classic prewhitening method. Finally, we
employed a procedure for selecting frequency lists to be used in stellar
modeling.
A total of 30 and 21 modes of degree l=0,1,2 - spanning at least eight radial
orders - have been identified for KIC 10273246 and KIC 10920273, respectively.
Two avoided crossings (l=1 ridge) have been identified for KIC 10273246,
whereas one avoided crossing plus another likely one have been identified for
KIC 10920273. Good agreement is found between observed and predicted mode
amplitudes for the F-type star KIC 10273246, based on a revised scaling
relation. Estimates are given of the rotational periods, the parameters
describing stellar granulation and the global asteroseismic parameters
and .Comment: 15 pages, 15 figures, to be published in Astronomy & Astrophysic
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