350 research outputs found
On detecting the large separation in the autocorrelation of stellar oscillation times series
The observations carried out by the space missions CoRoT and Kepler provide a
large set of asteroseismic data. Their analysis requires an efficient procedure
first to determine if the star is reliably showing solar-like oscillations,
second to measure the so-called large separation, third to estimate the
asteroseismic information that can be retrieved from the Fourier spectrum. We
develop in this paper a procedure, based on the autocorrelation of the seismic
Fourier spectrum. We have searched for criteria able to predict the output that
one can expect from the analysis by autocorrelation of a seismic time series.
First, the autocorrelation is properly scaled for taking into account the
contribution of white noise. Then, we use the null hypothesis H0 test to assess
the reliability of the autocorrelation analysis. Calculations based on solar
and CoRoT times series are performed in order to quantify the performance as a
function of the amplitude of the autocorrelation signal. We propose an
automated determination of the large separation, whose reliability is
quantified by the H0 test. We apply this method to analyze a large set of red
giants observed by CoRoT. We estimate the expected performance for photometric
time series of the Kepler mission. Finally, we demonstrate that the method
makes it possible to distinguish l=0 from l=1 modes. The envelope
autocorrelation function has proven to be very powerful for the determination
of the large separation in noisy asteroseismic data, since it enables us to
quantify the precision of the performance of different measurements: mean large
separation, variation of the large separation with frequency, small separation
and degree identification.Comment: A&A, in pres
On the detection of Lorentzian profiles in a power spectrum: A Bayesian approach using ignorance priors
Aims. Deriving accurate frequencies, amplitudes, and mode lifetimes from
stochastically driven pulsation is challenging, more so, if one demands that
realistic error estimates be given for all model fitting parameters. As has
been shown by other authors, the traditional method of fitting Lorentzian
profiles to the power spectrum of time-resolved photometric or spectroscopic
data via the Maximum Likelihood Estimation (MLE) procedure delivers good
approximations for these quantities. We, however, show that a conservative
Bayesian approach allows one to treat the detection of modes with minimal
assumptions (i.e., about the existence and identity of the modes).
Methods. We derive a conservative Bayesian treatment for the probability of
Lorentzian profiles being present in a power spectrum and describe an efficient
implementation that evaluates the probability density distribution of
parameters by using a Markov-Chain Monte Carlo (MCMC) technique.
Results. Potentially superior to "best-fit" procedure like MLE, which only
provides formal uncertainties, our method samples and approximates the actual
probability distributions for all parameters involved. Moreover, it avoids
shortcomings that make the MLE treatment susceptible to the built-in
assumptions of a model that is fitted to the data. This is especially relevant
when analyzing solar-type pulsation in stars other than the Sun where the
observations are of lower quality and can be over-interpreted. As an example,
we apply our technique to CoRoT observations of the solar-type pulsator HD
49933.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Mode width fitting with a simple bayesian approach. Application to CoRoT targets HD 181420 and HD 49933
We investigate the asteroseismology of two solar-like targets as observed
with the CoRoT satellite, with particular attention paid to the mode fitting.
HD 181420 and HD 49933 are typical CoRoT solar-like targets (156 and 60-day
runs). The low signal-to-noise ratio (SNR) of about 3-10 prevents us from
unambiguously identifying the individual oscillation modes. In particular,
convergence problems appear at the edges of the oscillation spectrum. HD 181420
and HD 49933 are typical CoRoT solar-like targets (156 and 60-day runs). The
low signal-to-noise ratio (SNR) of about 3-10 prevents us from unambiguously
identifying the individual oscillation modes. In particular, convergence
problems appear at the edges of the oscillation spectrum. We apply a Bayesian
approach to the analysis of these data. We compare the global fitting of the
power spectra of this time series, obtained by the classical maximum likelihood
(MLE) and the maximum a posteriori (MAP) estimators. We examine the impact of
the choice of the priors upon the fitted parameters. We also propose to reduce
the number of free parameters in the fitting, by replacing the individual
estimate of mode height associated with each overtone by a continuous function
of frequency (Gaussian profile). The MAP appears as a powerful tool to
constrain the global fits, but it must be used carefully and only with reliable
priors. The mode width of the stars increases with the frequency over all the
oscillation spectrum.Comment: 10 pages, 9 figures, 2 table
The art of fitting p-mode spectra: Part II. Leakage and noise covariance matrices
In Part I we have developed a theory for fitting p-mode Fourier spectra
assuming that these spectra have a multi-normal distribution. We showed, using
Monte-Carlo simulations, how one can obtain p-mode parameters using 'Maximum
Likelihood Estimators'. In this article, hereafter Part II, we show how to use
the theory developed in Part I for fitting real data. We introduce 4 new
diagnostics in helioseismology: the echelle diagramme, the cross
echelle diagramme, the inter echelle diagramme, and the ratio cross spectrum.
These diagnostics are extremely powerful to visualize and understand the
covariance matrices of the Fourier spectra, and also to find bugs in the data
analysis code. These diagrammes can also be used to derive quantitative
information on the mode leakage and noise covariance matrices. Numerous
examples using the LOI/SOHO and GONG data are given.Comment: 17 pages with tex and ps files, submitted to A&A,
[email protected]
The art of fitting p-mode spectra: Part I. Maximum Likelihood Estimation
In this article we present our state of the art of fitting helioseismic
p-mode spectra. We give a step by step recipe for fitting the spectra:
statistics of the spectra both for spatially unresolved and resolved data, the
use of Maximum Likelihood estimates, the statistics of the p-mode parameters,
the use of Monte-Carlo simulation and the significance of fitted parameters.
The recipe is applied to synthetic low-resolution data, similar to those of the
LOI, using Monte-Carlo simulations. For such spatially resolved data, the
statistics of the Fourier spectrum is assumed to be a multi-normal
distribution; the statistics of the power spectrum is \emph{not} a
with 2 degrees of freedom. Results for shows that all parameters
describing the p modes can be obtained without bias and with minimum variance
provided that the leakage matrix is known. Systematic errors due to an
imperfect knowledge of the leakage matrix are derived for all the p-mode
parameters.Comment: 13 pages, ps file gzipped. Submitted to A&
On deriving p-mode parameters for inclined solar-like stars
Thanks to their high quality, new and upcoming asteroseismic observations -
with CoRoT, Kepler, and from the ground... - can benefit from the experience
gained with helioseismology. We focus in this paper on solar-like oscillations,
for which the inclination of the rotation axis is unknown. We present a
theoretical study of the errors of p-mode parameters determined by means of a
maximum-likelihood estimator, and we also analyze correlations and biases. We
have used different, complementary approaches: we have performed either
semi-analytical computation of the Hessian matrix, fitting of single mean
profiles, or Monte Carlo simulations. We give first analytical approximations
for the errors of frequency, inclination and rotational splitting. The
determination of the inclination is very challenging for the common case of
slow rotators (like the Sun), making difficult the determination of a reliable
rotational splitting. Moreover, due to the numerous correlations, biases - more
or less significant - can appear in the determination of various parameters in
the case of bad inclination fittings, especially when a locking at 90 degrees
occurs. This issue concerning inclination locking is also discussed.
Nevertheless, the central frequency and some derived parameters such as the
total power of the mode are free of such biases.Comment: 9 pages, 6 figures, to appear in A&
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