69 research outputs found
Gravito-inertial waves in a differentially rotating spherical shell
The gravito-inertial waves propagating over a shellular baroclinic flow
inside a rotating spherical shell are analysed using the Boussinesq
approximation. The wave properties are examined by computing paths of
characteristics in the non-dissipative limit, and by solving the full
dissipative eigenvalue problem using a high-resolution spectral method.
Gravito-inertial waves are found to obey a mixed-type second-order operator and
to be often focused around short-period attractors of characteristics or
trapped in a wedge formed by turning surfaces and boundaries. We also find
eigenmodes that show a weak dependence with respect to viscosity and heat
diffusion just like truly regular modes. Some axisymmetric modes are found
unstable and likely destabilized by baroclinic instabilities. Similarly, some
non-axisymmetric modes that meet a critical layer (or corotation resonance) can
turn unstable at sufficiently low diffusivities. In all cases, the instability
is driven by the differential rotation. For many modes of the spectrum, neat
power laws are found for the dependence of the damping rates with diffusion
coefficients, but the theoretical explanation for the exponent values remains
elusive in general. The eigenvalue spectrum turns out to be very rich and
complex, which lets us suppose an even richer and more complex spectrum for
rotating stars or planets that own a differential rotation driven by
baroclinicity.Comment: 33 pages, 14 figures, accepted for publication in Journal of Fluid
Mechanic
Non-adiabatic oscillations of fast-rotating stars: the example of Rasalhague
Early-type stars generally tend to be fast rotators. In these stars, mode
identification is very challenging as the effects of rotation are not well
known. We consider here the example of Ophiuchi, for which dozens of
oscillation frequencies have been measured. We model the star using the
two-dimensional structure code ESTER, and we compute both adiabatic and
non-adiabatic oscillations using the TOP code. Both calculations yield very
complex spectra, and we used various diagnostic tools to try and identify the
observed pulsations. While we have not reached a satisfactory mode-to-mode
identification, this paper presents promising early results.Comment: 4 pages, 3 figures. SF2A 2017 proceeding
Seismic diagnosis from gravity modes strongly affected by rotation
Most of the information we have about the internal rotation of stars comes
from modes that are weakly affected by rotation, for example by using
rotational splittings. In contrast, we present here a method, based on the
asymptotic theory of Prat et al. (2016), which allows us to analyse the
signature of rotation where its effect is the most important, that is in
low-frequency gravity modes that are strongly affected by rotation. For such
modes, we predict two spectral patterns that could be confronted to observed
spectra and those computed using fully two-dimensional oscillation codes.Comment: 3 pages, 1 figure, to appear in the proceedings of the Joint TASC2 &
KASC9 Workshop SPACEINN & HELAS8 Conference "Seismology of the Sun and the
Distant Stars 2016
Internal magnetic fields in 13 red giants detected by asteroseismology
While surface fields have been measured for stars across the HR diagram,
internal magnetic fields remain largely unknown. The recent seismic detection
of magnetic fields in the cores of several Kepler red giants has opened a new
avenue to understand better the origin of magnetic fields and their impact on
stellar structure and evolution. We aim to use asteroseismology to
systematically search for internal magnetic fields in red giant stars and to
determine the strengths and geometries of these fields. Magnetic fields are
known to break the symmetry of rotational multiplets. In red giants,
oscillation modes are mixed, behaving as pressure modes in the envelope and as
gravity modes in the core. Magnetism-induced asymmetries are expected to be
stronger for g-dominated modes than for p-dominated modes and to decrease with
frequency. After collecting a sample of 2500 Kepler red giant stars with clear
mixed-mode patterns, we specifically searched for targets among 1200 stars with
dipole triplets. We identified 13 stars exhibiting clear asymmetric multiplets
and measured their parameters, especially the asymmetry parameter and the
magnetic frequency shift. By combining these estimates with best-fitting
stellar models, we measured average core magnetic fields ranging from 20 to
150kG, corresponding to 5% to 30% of the critical field strengths. We showed
that the detected core fields have various horizontal geometries, some of which
significantly differ from a dipolar configuration. We found that the field
strengths decrease with stellar evolution, despite the fact that the cores of
these stars are contracting. Even though these stars have strong internal
magnetic fields, they display normal core rotation rates, suggesting no
significantly different histories of angular momentum transport compared to
other red giant stars. We also discuss the possible origin of the detected
fields.Comment: Accepted for publication in A&A. Long appendi
Gap interpolation by inpainting methods : Application to Ground and Space-based Asteroseismic data
In asteroseismology, the observed time series often suffers from incomplete
time coverage due to gaps. The presence of periodic gaps may generate spurious
peaks in the power spectrum that limit the analysis of the data. Various
methods have been developed to deal with gaps in time series data. However, it
is still important to improve these methods to be able to extract all the
possible information contained in the data. In this paper, we propose a new
approach to handle the problem, the so-called inpainting method. This
technique, based on a sparsity prior, enables to judiciously fill-in the gaps
in the data, preserving the asteroseismic signal, as far as possible. The
impact of the observational window function is reduced and the interpretation
of the power spectrum is simplified. This method is applied both on ground and
space-based data. It appears that the inpainting technique improves the
oscillation modes detection and estimation. Additionally, it can be used to
study very long time series of many stars because its computation is very fast.
For a time series of 50 days of CoRoT-like data, it allows a speed-up factor of
1000, if compared to methods of the same accuracy.Comment: 29 pages, 7 figures, A&A pending final acceptance from edito
Investigating the properties of granulation in the red giants observed by Kepler
More than 1000 red giants have been observed by NASA/Kepler mission during a
nearly continuous period of ~ 13 months. The resulting high-frequency
resolution (< 0.03 muHz) allows us to study the granulation parameters of these
stars. The granulation pattern results from the convection motions leading to
upward flows of hot plasma and downward flows of cooler plasma. We fitted
Harvey-like functions to the power spectra, to retrieve the timescale and
amplitude of granulation. We show that there is an anti-correlation between
both of these parameters and the position of maximum power of acoustic modes,
while we also find a correlation with the radius, which agrees with the theory.
We finally compare our results with 3D models of the convection.Comment: 4 pages, 1 figure. To appear in the ASP proceedings of "The 61st
Fujihara seminar: Progress in solar/stellar physics with helio- and
asteroseismology", 13th-17th March 2011, Hakone, Japa
Calibrating Convective properties of Solar-like Stars in the Kepler Field of View
Stellar models generally use simple parametrizations to treat convection. The
most widely used parametrization is the so-called "Mixing Length Theory" where
the convective eddy sizes are described using a single number, \alpha, the
mixing-length parameter. This is a free parameter, and the general practice is
to calibrate \alpha using the known properties of the Sun and apply that to all
stars. Using data from NASA's Kepler mission we show that using the
solar-calibrated \alpha is not always appropriate, and that in many cases it
would lead to estimates of initial helium abundances that are lower than the
primordial helium abundance. Kepler data allow us to calibrate \alpha for many
other stars and we show that for the sample of stars we have studied, the
mixing-length parameter is generally lower than the solar value. We studied the
correlation between \alpha and stellar properties, and we find that \alpha
increases with metallicity. We therefore conclude that results obtained by
fitting stellar models or by using population-synthesis models constructed with
solar values of \alpha are likely to have large systematic errors. Our results
also confirm theoretical expectations that the mixing-length parameter should
vary with stellar properties.Comment: 16 pages, 4 figures, accepted for publication in ApJ
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