6,645 research outputs found
An alternative to mode fitting
The space mission CoRoT provides us with a large amount of high-duty cycle
long-duration observations. Mode fitting has proven to be efficient for the
complete and detailed analysis of the oscillation pattern, but remains time
consuming. Furthermore, the photometric background due to granulation severely
complicates the analysis. Therefore, we attempt to provide an alternative to
mode fitting, for the determination of large separations. With the envelope
autocorrelation function and a dedicated filter, it is possible to measure the
variation of the large separation independently for the ridges with even and
odd degrees. The method appears to be as accurate as the mode fitting. It can
be very easily implemented and is very rapid.Comment: Proceedings of the 4th HELAS International Conference held in
Lanzarote, 201
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
Overview
Overview of Special Issue: Federal Reserve Policy Responses to the Financial Crisis.Financial crises ; Federal Reserve System ; Bank liquidity
Period spacings in red giants II. Automated measurement
The space missions CoRoT and Kepler have provided photometric data of
unprecedented quality for asteroseismology. A very rich oscillation pattern has
been discovered for red giants, including mixed modes that are used to decipher
the red giants interiors. They carry information on the radiative core of red
giant stars and bring strong constraints on stellar evolution. Since more than
15,000 red giant light curves have been observed by Kepler, we have developed a
simple and efficient method for automatically characterizing the mixed-mode
pattern and measuring the asymptotic period spacing. With the asymptotic
expansion of the mixed modes, we have revealed the regularity of the
gravity-mode pattern. The stretched periods were used to study the evenly space
periods with a Fourier analysis and to measure the gravity period spacing, even
when rotation severely complicates the oscillation spectra. We automatically
measured gravity period spacing for more than 6,100 Kepler red giants. The
results confirm and extend previous measurements made by semi-automated
methods. We also unveil the mass and metallicity dependence of the relation
between the frequency spacings and the period spacings for stars on the red
giant branch. The delivery of thousands of period spacings combined with all
other seismic and non-seismic information provides a new basis for detailed
ensemble asteroseismology.Comment: 13 pages, 13 figure
Sounding stellar cores with mixed modes
The space-borne missions CoRoT and Kepler have opened a new era in stellar
physics, especially for evolved stars, with precise asteroseismic measurements
that help determine precise stellar parameters and perform ensemble astero
seismology. This paper deals with the quality of the information that we can
retrieve from the oscillations. It focusses on the conditions for obtaining the
most accurate measurement of the radial and non-radial oscillation patterns.
This accuracy is a prerequisite for making the best with asteroseismic data.
From radial modes, we derive proxies of the stellar mass and radii with an
unprecedented accuracy for field stars. For dozens of subgiants and thousands
of red giants, the identification of mixed modes (corresponding to gravity
waves propagating in the core coupled to pressure waves propagating in the
envelope) indicates unambiguously their evolutionary status. As probes of the
stellar core, these mixed modes also reveal the internal differential rotation
and show the spinning down of the core rotation of stars ascending the red
giant branch. A toy model of the coupling of waves constructing mixed modes is
exposed, for illustrating many of their features.Comment: Meeting: New advances in stellar physics: from microscopic to
macroscopic processes Roscoff, 27-31 May 201
Measuring the core rotation of red giant stars
Red giant stars present mixed modes, which behave as pressure modes in the
convective envelope and as gravity modes in the radiative interior. This mixed
character allows to probe the physical conditions in their core. With the
advent of long-duration time series from space-borne missions such as CoRoT and
Kepler, it becomes possible to study the red giant core rotation. As more than
15 000 red giant light curves have been recorded, it is crucial to develop a
robust and efficient method to measure this rotation. Such measurements of
thousands of mean core rotation would open the way to a deeper understanding of
the physical mechanisms that are able to transport angular momentum from the
core to the envelope in red giants. In this work, we detail the principle of
the method we developed to obtain automatic measurements of the red giant mean
core rotation. This method is based on the stretching of the oscillation
spectra and on the use of the so-called Hough transform. We finally validate
this method for stars on the red giant branch, where overlapping rotational
splittings and mixed-mode spacings produce complicated frequency spectra.Comment: 8 pages, 3 figures, 1 tabl
Rapidly rotating red giants
Stellar oscillations give seismic information on the internal properties of
stars. Red giants are targets of interest since they present mixed modes, which
behave as pressure modes in the convective envelope and as gravity modes in the
radiative core. Mixed modes thus directly probe red giant cores, and allow in
particular the study of their mean core rotation. The high-quality data
obtained by CoRoT and Kepler satellites represent an unprecedented perspective
to obtain thousands of measurements of red giant core rotation, in order to
improve our understanding of stellar physics in deep stellar interiors. We
developed an automated method to obtain such core rotation measurements and
validated it for stars on the red giant branch. In this work, we particularly
focus on the specific application of this method to red giants having a rapid
core rotation. They show complex spectra where it is tricky to disentangle
rotational splittings from mixed-mode period spacings. We demonstrate that the
method based on the identification of mode crossings is precise and efficient.
The determination of the mean core rotation directly derives from the precise
measurement of the asymptotic period spacing {\Delta}{\Pi}1 and of the
frequency at which the crossing of the rotational components is observed.Comment: 4 pages, 2 figures, 2 tables, to be published in the Astro Fluid 2016
Conference Proceedings, editor EAS Publications Serie
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