rieutord

explanation blue bibcodes open ADS page with paths to full text

Author name code: rieutord
ADS astronomy entries on 2022-09-14
author:"Rieutord, Michel"

---------------------------------------------------------
Title: Predictions for Gravity-mode Periods and Surface Abundances
in Intermediate-mass Dwarfs from Shear Mixing and Radiative Levitation
Authors: Mombarg, Joey S. G.; Dotter, Aaron; Rieutord, Michel;
Michielsen, Mathias; Van Reeth, Timothy; Aerts, Conny
2022ApJ...925..154M Altcode: 2021arXiv211114853M
The treatment of chemical mixing in the radiative envelopes of
intermediate-mass stars has hardly been calibrated so far. Recent
asteroseismic studies demonstrated that a constant diffusion coefficient
in the radiative envelope is not able to explain the periods of trapped
gravity modes in the oscillation spectra of γ Doradus pulsators. We
present a new generation of MESA stellar models with two major
improvements. First, we present a new implementation for computing
radiative accelerations and Rosseland mean opacities that requires
significantly less CPU time. Second, the inclusion of shear mixing based
on rotation profiles computed with the 2D stellar structure code ESTER
is considered. We show predictions for the mode periods of these models
covering stellar masses from 1.4 to 3.0 M ⊙ across the
main sequence, computed for different metallicities. The morphology of
the chemical mixing profile resulting from shear mixing in combination
with atomic diffusion and radiative levitation does allow for mode
trapping, while the diffusion coefficient in the outer envelope is
large (&gt;106 cm2 s-1). Furthermore,
we make predictions for the evolution of surface abundances for which
radiative accelerations can be computed. We find that the N/C and
C/O abundance ratios correlate with stellar age. We predict that these
correlations are observable with precisions ≲ 0.1 dex on these ratios,
given that a precise age estimate can be made.

---------------------------------------------------------
Title: Seismology of Altair with MOST
Authors: Le Dizès, Cécile; Rieutord, Michel; Charpinet, Stéphane
2021A&A...653A..26L Altcode: 2021arXiv210710515L
Context. Altair is the fastest rotating star at less than 10 parsecs
from the Sun. Its precise modelling is a landmark for our understanding
of stellar evolution with fast rotation, and all observational
constraints are most welcome to better determine the fundamental
parameters of this star. Aims: We wish to improve the seismic
spectrum of Altair and confirm the δ-Scuti nature of this star. 
Methods: We used the photometric data collected by the Microvariability
and Oscillations of STars (MOST) satellite in the form of a series
of Fabry images to derive Altair light curves at four epochs, namely
in 2007, 2011, 2012, and 2013. Results: We first confirm the
presence of δ-Scuti oscillations in the light curves of Altair. We
extend the precision of some eigenfrequencies and add new ones to the
spectrum of Altair, which now has 15 detected eigenmodes. The rotation
period, which is expected at ∼7h46min from models reproducing
interferometric data, seems to appear in the 2012 data set, but it
still needs confirmation. Finally, Altair modal oscillations show
noticeable amplitude variations on a timescale of 10-15 days, which
may be the signature of a coupling between oscillations and thermal
convection in the layer where the kappa-mechanism is operating. 
Conclusions: The Altair oscillation spectrum does not contain a large
number of excited eigenmodes, which is similar to the fast rotating
star HD220811. This supports the idea that fast rotation hinders
the excitation of eigenmodes as already pointed out by theoretical
investigations. This work is based on data from the MOST satellite,
a Canadian Space Agency mission, jointly operated by Dynacon Inc., the
University of Toronto Institute for Aerospace Studies and the University
of British Columbia, with the assistance of the University of Vienna.

---------------------------------------------------------
Title: Oscillations of 2D ESTER models. I. The adiabatic case
Authors: Reese, D. R.; Mirouh, G. M.; Espinosa Lara, F.; Rieutord,
M.; Putigny, B.
2021A&A...645A..46R Altcode: 2020arXiv201011312R
Context. Recent numerical and theoretical considerations have shown
that low-degree acoustic modes in rapidly rotating stars follow an
asymptotic formula. In parallel, recent studies have revealed the
presence of regular pulsation frequency patterns in rapidly rotating
δ Scuti stars that seem to match theoretical expectations. 
Aims: In this context, a key question is whether strong gradients
or discontinuities can adversely affect the asymptotic frequency
pattern to the point of hindering its identification. Other important
questions are how rotational splittings are affected by the 2D
rotation profiles expected from baroclinic effects and whether it is
possible to probe the rotation profile using these splittings. 
Methods: In order to address these questions, we numerically calculate
stellar pulsation modes in continuous and discontinuous rapidly
rotating models produced by the 2D Evolution STEllaire en Rotation
(ESTER) code. This code self-consistently calculates the rotation
profile based on baroclinic effects and uses a spectral multi-domain
approach, thus making it possible to introduce discontinuities
at the domain interfaces without loss of numerical accuracy. The
pulsation calculations are carried out using an adiabatic version of
the Two-dimensional Oscillation Program (TOP) code. The variational
principle is then used to confirm the high numerical accuracy of
the pulsation frequencies and to derive an integral formula for the
generalised rotational splittings. Acoustic glitch theory, combined
with ray dynamics, is applied to the discontinuous models in order to
interpret their pulsation spectra. Results: Our results show that
the generalised rotational splittings are very well approximated by the
integral formula, except for modes involved in avoided crossings. This
potentially allows the application of inverse theory for probing the
rotation profile. We also show that glitch theory applied along the
island mode orbit can correctly predict the periodicity of the glitch
frequency pattern produced by the discontinuity or Γ1 dip
related to the He II ionisation zone in some of the models. Furthermore,
the asymptotic frequency pattern remains sufficiently well preserved
to potentially allow its detection in observed stars.

---------------------------------------------------------
Title: Stress-driven spin-down of a viscous fluid within a spherical
shell
Authors: Gagnier, D.; Rieutord, M.
2020JFM...904A..35G Altcode: 2020arXiv200310403G
We investigate the linear properties of the steady and axisymmetric
stress-driven spin-down flow of a viscous fluid inside a spherical
shell, both within the incompressible and anelastic approximations,
and in the asymptotic limit of small viscosities. From boundary
layer analysis, we derive an analytical geostrophic solution for
the 3D incompressible steady flow, inside and outside the cylinder
$\mathcal{C}$ that is tangent to the inner shell. The Stewartson
layer that lies on $\mathcal{C}$ is composed of two nested shear
layers of thickness $O(E^{2/7})$ and $O(E^{1/3})$. We derive the
lowest order solution for the $E^{2/7}$-layer. A simple analysis of
the $E^{1/3}$-layer laying along the tangent cylinder, reveals it to
be the site of an upwelling flow of amplitude $O(E^{1/3})$. Despite
its narrowness, this shear layer concentrates most of the global
meridional kinetic energy of the spin-down flow. Furthermore, a stable
stratification does not perturb the spin-down flow provided the Prandtl
number is small enough. If this is not the case, the Stewartson layer
disappears and meridional circulation is confined within the thermal
layers. The scalings for the amplitude of the anelastic secondary flow
have been found to be the same as for the incompressible flow in all
three regions, at the lowest order. However, because the velocity
no longer conforms the Taylor-Proudman theorem, its shape differs
outside the tangent cylinder $\mathcal{C}$, that is, where differential
rotation takes place. Finally, we find the settling of the steady-state
to be reached on a viscous time for the weakly, strongly and thermally
unstratified incompressible flows. Large density variations relevant to
astro- and geophysical systems, tend to slightly shorten the transient.

---------------------------------------------------------
Title: A realistic two-dimensional model of Altair
Authors: Bouchaud, K.; Domiciano de Souza, A.; Rieutord, M.; Reese,
D. R.; Kervella, P.
2020A&A...633A..78B Altcode: 2019arXiv191203138B
Context. Fast rotation is responsible for important changes in the
structure and evolution of stars and the way we see them. Optical long
baseline interferometry now allows for the study of its effects on the
stellar surface, mainly gravity darkening and flattening. Aims:
We aim to determine the fundamental parameters of the fast-rotating
star Altair, in particular its evolutionary stage (represented here by
the core hydrogen mass fraction Xc), mass, and differential
rotation, using state-of-the-art stellar interior and atmosphere
models together with interferometric (ESO-VLTI), spectroscopic, and
asteroseismic observations. Methods: We use ESTER two-dimensional
stellar models to produce the relevant surface parameters needed to
create intensity maps from atmosphere models. Interferometric and
spectroscopic observables are computed from these intensity maps
and several stellar parameters are then adjusted using the publicly
available MCMC algorithm Emcee. Results: We determined Altair's
equatorial radius to be Req = 2.008 ± 0.006 R⊙,
the position angle PA = 301.1 ± 0.3°, the inclination i = 50.7 ±
1.2°, and the equatorial angular velocity Ω = 0.74 ± 0.01 times
the Keplerian angular velocity at equator. This angular velocity
leads to a flattening of ɛ = 0.220 ± 0.003. We also deduce from
the spectroscopically derived v sin i ≃ 243 km s-1,
a true equatorial velocity of ∼314 km s-1 corresponding
to a rotation period of 7h46m (∼3 cycles/day). The data also impose
a strong correlation between mass, metallicity, hydrogen abundance,
and core evolution. Thanks to asteroseismic data, and provided our
frequencies identification is correct, we constrain the mass of Altair
to 1.86 ± 0.03 M⊙ and further deduce its metallicity
Z = 0.019 and its core hydrogen mass fraction Xc = 0.71,
assuming an initial solar hydrogen mass fraction X = 0.739. These
values suggest that Altair is a young star ∼100 Myr old. Finally,
the 2D ESTER model also gives the internal differential rotation of
Altair, showing that its core rotates approximately 50% faster than
the envelope, while the surface differential rotation does not exceed
6%. Based on VLTI observations performed at ESO, Chile under
programme IDs 60.A-9164(A), 87.D-0150(A), and 094.C-0232(A).

---------------------------------------------------------
Title: Multi-Dimensional Processes In Stellar Physics
Authors: Rieutord, Michel; Baraffe, Isabelle; Lebreton, Yveline
2020mdps.conf.....R Altcode:
When one has to deal with fluid flows, magnetic fields or heat transfer
in stars, one faces the partial differential equations that govern
these processes. These phenomena are naturally multi-dimensional and
their study requires new and sophisticated models. This volume gathers
the lecture notes which summarize the essence of the lectures and
conferences given by world experts in the field of multi-dimensional
modelling of stars, during the 2018 Evry Schatzman School held in
Roscoff, France. It gives the present status of our understanding
of several processes that occur in stars, like thermal convection,
double-diffusive convection, dynamo effect or baroclinic flows. Every
subject is discussed under the light of the most recent results of
nowadays research and is made accessible to all newcomers, either
students or researchers who wish to join the field. HRA provides a
unique way to study regions of stellar formation, proto-planetary
discs as well as the surfaces of stars and their environments. This
volume offers lectures given by world experts in the field during the
Evry Schatzman School on Stellar Physics (EES 2017) held in Roscoff,
France. The addressed topics include a course of introduction to
optical/IR interferometry covering the history and basic principles,
a course on diffraction-dominated observational astronomy, and a course
presenting the principles and instrumentation of optical long baseline
interferometry. This book will be a valuable reference for researchers
and students in the coming years.

---------------------------------------------------------
Title: The first view of δ Scuti and γ Doradus stars with the
TESS mission
Authors: Antoci, V.; Cunha, M. S.; Bowman, D. M.; Murphy, S. J.;
Kurtz, D. W.; Bedding, T. R.; Borre, C. C.; Christophe, S.;
Daszyńska-Daszkiewicz, J.; Fox-Machado, L.; García Hernández,
A.; Ghasemi, H.; Handberg, R.; Hansen, H.; Hasanzadeh, A.; Houdek,
G.; Johnston, C.; Justesen, A. B.; Kahraman Alicavus, F.; Kotysz, K.;
Latham, D.; Matthews, J. M.; Mønster, J.; Niemczura, E.; Paunzen, E.;
Sánchez Arias, J. P.; Pigulski, A.; Pepper, J.; Richey-Yowell, T.;
Safari, H.; Seager, S.; Smalley, B.; Shutt, T.; Sódor, A.; Suárez,
J. -C.; Tkachenko, A.; Wu, T.; Zwintz, K.; Barceló Forteza, S.;
Brunsden, E.; Bognár, Z.; Buzasi, D. L.; Chowdhury, S.; De Cat,
P.; Evans, J. A.; Guo, Z.; Guzik, J. A.; Jevtic, N.; Lampens, P.;
Lares Martiz, M.; Lovekin, C.; Li, G.; Mirouh, G. M.; Mkrtichian,
D.; Monteiro, M. J. P. F. G.; Nemec, J. M.; Ouazzani, R. -M.;
Pascual-Granado, J.; Reese, D. R.; Rieutord, M.; Rodon, J. R.; Skarka,
M.; Sowicka, P.; Stateva, I.; Szabó, R.; Weiss, W. W.
2019MNRAS.490.4040A Altcode: 2019MNRAS.tmp.2389A; 2019arXiv190912018A
We present the first asteroseismic results for δ Scuti and γ
Doradus stars observed in Sectors 1 and 2 of the TESS mission. We
utilize the 2-min cadence TESS data for a sample of 117 stars to
classify their behaviour regarding variability and place them in the
Hertzsprung-Russell diagram using Gaia DR2 data. Included within our
sample are the eponymous members of two pulsator classes, γ Doradus
and SX Phoenicis. Our sample of pulsating intermediate-mass stars
observed by TESS also allows us to confront theoretical models of
pulsation driving in the classical instability strip for the first
time and show that mixing processes in the outer envelope play an
important role. We derive an empirical estimate of 74 per cent for the
relative amplitude suppression factor as a result of the redder TESS
passband compared to the Kepler mission using a pulsating eclipsing
binary system. Furthermore, our sample contains many high-frequency
pulsators, allowing us to probe the frequency variability of hot young
δ Scuti stars, which were lacking in the Kepler mission data set, and
identify promising targets for future asteroseismic modelling. The TESS
data also allow us to refine the stellar parameters of SX Phoenicis,
which is believed to be a blue straggler.

---------------------------------------------------------
Title: Classical and general relativistic post-Keplerian effects in
binary pulsars hosting fast rotating main sequence stars
Authors: Iorio, Lorenzo; Rieutord, Michel; Rozelot, Jean-Pierre;
Domiciano de Souza, Armando
2019EPJC...79..690I Altcode: 2019arXiv190505514I
We consider a binary system composed of a pulsar and a massive, fast
rotating, highly distorted main sequence star of mass M, spin angular
momentum S , dimensionless mass quadrupole moment J_2, equatorial and
polar radii R_ {e}, R_ {p}, flattening ν \doteq (R_ {e}-R_ {p})/R_
{e}, and ellipticity ɛ \doteq √{1-R_ {p}^2/R_ {e}^2} as a potential
scenario to dynamically put to the test certain post-Keplerian effects
of both Newtonian and post-Newtonian nature. We numerically produce time
series of the perturbations Δ ( δ τ ) of the Rømer-like, orbital
component of the pulsar's time delay δ τ induced over 10 years by
the pN gravitoelectric mass monopole ( {Schwarzschild}, GMc^{-2})
, quadrupole ( GMR^2_ {e}J_2 c^{-2}) , gravitomagnetic spin dipole (
{Lense}{-} {Thirring}, GSc^{-2}) and octupole ( GSR^2_ {e}ɛ ^2 c^{-2})
accelerations along with the Newtonian quadrupolar ( GMR^2_ {e}J_2)
one. We do not deal with the various propagation time delays due to
the travelling electromagnetic waves. It turns out that, for a Be-type
star with M = 15 {M}_⊙ , R_ {e} = 5.96 {R}_⊙ , ν = 0.203, S =
3.41× 10^{45} {J} {s}, J_2 = 1.92× 10^{-3} orbited by a pulsar with
an orbital period P_b∼ 40-70 days, the classical oblateness-driven
effects are at the ≲ 4-150 {s} level, while the pN shifts are of the
order of ≲ 1.5-20 {s} ( GMc^{-2}) , ≲ 10-40 {ms} ( GMR^2_ {e} J_2
c^{-2}) , ≲ 0.5-6 {ms} ( GSc^{-2}) , ≲ 5-20 μ {s} ( GSR^2_ {e}ɛ ^2
c^{-2}) , depending on their orbital configuration. The root-mean-square
(rms) timing residuals σ_{τ } of almost all the existing non-recycled,
non-millisecond pulsars orbiting massive, fast rotating main sequence
stars are ≲ {ms}. Thus, such kind of binaries have the potential to
become interesting laboratories to measure, or, at least, constrain,
some Newtonian and post-Newtonian (GMc^{-2}, GMJ_2c^{-2}, and, perhaps,
GSc^{-2} as well) key features of the distorted gravitational fields
of the fast rotating stars hosted by them.

---------------------------------------------------------
Title: Core overshooting under the light of fluid dynamics
Authors: Rieutord, M.
2019EAS....82..153R Altcode:
We discuss the possible contraints that are brought about by a fluid
mechanical analysis of the overshooting phenomenon at the interface
of convective cores and radiative envelopes of early-type stars. We
investigate an improvement of Roxburgh's criterion by taking into
account the viscous dissipation but show that this criterion remains
not stringent enough to be predictive. We then discuss the thickness
of the overshooting layer and show that all estimates, including the
one of Zahn (&lt;xref ref-type="bibr" rid="R21"&gt;1991&lt;/xref&gt;),
lead to a very thin mixing layer typically less than a percent of the
pressure scale height.

---------------------------------------------------------
Title: Evolution of rotation in rapidly rotating early-type stars
during the main sequence with 2D models
Authors: Gagnier, D.; Rieutord, M.; Charbonnel, C.; Putigny, B.;
Espinosa Lara, F.
2019A&A...625A..89G Altcode: 2019arXiv190405219G
The understanding of the rotational evolution of early-type stars
is deeply related to that of anisotropic mass and angular momentum
loss. In this paper, we aim to clarify the rotational evolution of
rapidly rotating early-type stars along the main sequence (MS). We have
used the 2D ESTER code to compute and evolve isolated rapidly rotating
early-type stellar models along the MS, with and without anisotropic
mass loss. We show that stars with Z = 0.02 and masses between 5 and
7 M⊙ reach criticality during the main sequence provided
their initial angular velocity is larger than 50% of the Keplerian
one. More massive stars are subject to radiation-driven winds and
to an associated loss of mass and angular momentum. We find that
this angular momentum extraction from the outer layers can prevent
massive stars from reaching critical rotation and greatly reduce the
degree of criticality at the end of the MS. Our model includes the
so-called bi-stability jump of the Ṁ - Teff relation of
1D-models. This discontinuity now shows up in the latitude variations
of the mass-flux surface density, endowing rotating massive stars with
either a single-wind regime (no discontinuity) or a two-wind regime
(a discontinuity). In the two-wind regime, mass loss and angular
momentum loss are strongly increased at low latitudes inducing a faster
slow-down of the rotation. However, predicting the rotational fate
of a massive star is difficult, mainly because of the non-linearity
of the phenomena involved and their strong dependence on uncertain
prescriptions. Moreover, the very existence of the bi-stability jump
in mass-loss rate remains to be substantiated by observations.

---------------------------------------------------------
Title: Critical angular velocity and anisotropic mass loss of rotating
stars with radiation-driven winds
Authors: Gagnier, D.; Rieutord, M.; Charbonnel, C.; Putigny, B.;
Espinosa Lara, F.
2019A&A...625A..88G Altcode: 2019arXiv190405089G
Context. The understanding of the evolution of early-type stars is
tightly related to that of the effects of rapid rotation. For massive
stars, rapid rotation combines with their strong radiation-driven
wind. Aims: The aim of this paper is to investigate two questions
that are prerequisite to the study of the evolution of massive rapidly
rotating stars: (i) What is the critical angular velocity of a star
when radiative acceleration is significant in its atmosphere? (ii) How
do mass and angular momentum loss depend on the rotation rate? 
Methods: To investigate fast rotation, which makes stars oblate, we used
the 2D ESTER models and a simplified approach, the ω-model, which gives
the latitudinal dependence of the radiative flux in a centrifugally
flattened radiative envelope. Results: We find that radiative
acceleration only mildly influences the critical angular velocity,
at least for stars with masses lower than 40 M⊙. For
instance, a 15 M⊙ star on the zero-age main sequence
would reach criticality at a rotation rate equal to 0.997 the Keplerian
equatorial rotation rate. We explain this mild reduction of the critical
angular velocity compared to the classical Keplerian angular velocity
by the combined effects of gravity darkening and a reduced equatorial
opacity that is due to the centrifugal acceleration. To answer the
second question, we first devised a model of the local surface mass
flux, which we calibrated with previously developed 1D models. The
discontinuity (the so-called bi-stability jump) included in the Ṁ -
Teff relation of 1D models means that the mass flux of a
fast-rotating star is controlled by either a single wind or a two-wind
regime. Mass and angular momentum losses are strong around the equator
if the star is in the two-wind regime. We also show that the difficulty
of selecting massive stars that are viewed pole-on makes detecting
the discontinuity in the relation between mass loss and effective
temperature also quite challenging.

---------------------------------------------------------
Title: Neural Network to Emulate Numerical Simulations of the Sun
and Infer Synthetic Observations for Data Assimilation
Authors: Tremblay, Benoit; Roudier, Thierry; Cossette, Jean-François;
Attié, Raphaël; Rieutord, Michel; Vincent, Alain
2019shin.confE..30T Altcode:
Satellites and ground-based observatories probe the Sun's photosphere
and atmosphere and are key in studying solar activity. Meanwhile,
numerical models have attempted to bridge the gap between the physics of
the solar interior and such observations. However, there are physical
quantities relevant to solar activity that can be modeled but that
cannot be directly measured and must be inferred. For example, direct
measurements of plasma motions at the photosphere are limited to the
line-of-sight component. Recently, neural network computing has been
used in conjunction with numerical models of the Sun to be able to
recover the full velocity vector in photospheric plasma of the Quiet
Sun. We used satellite observations as input in a fully convolutional
neural network to generate instantaneous synthetic plasma motions,
i.e. plasma motions that reflect the physics of a model but are made
to look as if they were observed by a specific instrument. A parallel
technique could then be invoked to eventually be able to derive the
plasma velocity vector maps of the Active Sun and, by extension, other
physical quantities of interest that can not yet be measured directly
at the photosphere or anywhere else in the solar atmosphere.

---------------------------------------------------------
Title: Do tidally-generated inertial waves heat the subsurface oceans
of Europa and Enceladus?
Authors: Rovira-Navarro, Marc; Rieutord, Michel; Gerkema, Theo; Maas,
Leo R. M.; van der Wal, Wouter; Vermeersen, Bert
2019Icar..321..126R Altcode:
Some of the moons of the outer solar system harbour subsurface liquid
oceans. Tidal dissipation plays an important role in preventing
these oceans from freezing. In the past, most studies considered
only tidal dissipation in the solid layers of these bodies (rock and
ice). Recently, new studies considering tidal dissipation in the oceans
of these moons have appeared. All of them make use of the shallow
water approximation. However, the use of this approximation might
not be adequate. Here we consider the linear non-hydrostatic three
dimensional response of these oceans to tidal forcing with the full
Coriolis force. To do so we consider an ocean of homogeneous density
contained within a perfectly spherical shell and neglect the effect of
the ice shell. We force the ocean with a time changing tidal potential
and observe patterns of periodic inertial waves that take energy from
the global tidal forcing and focus it along thin shear layers that
propagate in the fluid. We focus on Europa and Enceladus, showing
that inertial waves result in fluid flows of significant amplitude (a
few cm/s). Nevertheless, we find that under the previously mentioned
assumptions tidal dissipation due to inertial waves is several orders
of magnitude smaller than Europa's radiogenic heating and Enceladus'
observed heat flux. Finally, we propose additional dissipation
mechanisms that might play a relevant role in Europa and Enceladus
and could be further investigated.

---------------------------------------------------------
Title: Gravity darkening in late-type stars
Authors: Raynaud, R.; Rieutord, M.; Petitdemange, L.; Gastine, T.;
Putigny, B.
2018sf2a.conf...91R Altcode:
Recent interferometric data have been able to constrain the brightness
distribution at the surface of nearby stars, in particular the gravity
darkening that makes fast rotating stars brighter at their poles than
at their equator. However, good models of gravity darkening are missing
when the stars own a convective envelope. In order to better understand
how rotation affects the heat transfer in stellar convective envelopes,
we studied the heat flux distribution in latitude at the outer surface
of numerical models of anelastic convection in rotating sphericall
shells. We found that the variations of the surface brightness are
mainly controlled by the surface value of the local Rossby number: when
the Coriolis force dominates the dynamics, the heat flux is weakened
in the equatorial region by the zonal wind and enhanced at the poles by
convective motions inside the tangent cylinder. However, in presence of
a strong background density stratification, as expected in real stars,
the increase of the local Rossby number in the outer layers leads to
the uniformisation of the surface heat flux distribution.

---------------------------------------------------------
Title: Tidally-induced inertial waves in the subsurface ocean of
Enceladus
Authors: Rovira-Navarro, M.; Rieutord, M.; Gerkema, T.; Maas, L. R.;
van der Wal, W.; Vermeersen, B. L. A.
2018AGUFM.P33A..03R Altcode:
Recently, studies on tidally-induced ocean currents in Enceladus have
appeared [e.g., Tyler, Nature, 456 (7223), 770-772, 2008; Matsuyama et
al., Icarus, 312, 208-230, 2018]. All of these studies make use of the
shallow water approximation, which consists on assuming that currents
are predominantly horizontal. This approximation might not be adequate
to capture Enceladus' ocean dynamics. We study for the first time the
fully non-hydrostatic three dimensional response of Enceladus' ocean to
tidal forcing. To do so we consider an ocean of constant density inside
a perfectly spherical shell and force it with the tidal potential. We
observe patterns of internal inertial waves that take energy from the
global forcing and focus it along internal shear layers. We observe
that these waves are of significant amplitude and speculate that they
might be the origin of the characteristic "tiger stripes". We also
compute tidal dissipation due to inertial waves and show that for an
ocean of constant depth their contribution to Enceladus tidal budget is
not significant. We argue , however, that this estimation represents a
lower bound for tidal dissipation. Further study is required to assess
the importance of inertial waves in an ocean basin with variable depth.

---------------------------------------------------------
Title: The evolved fast rotator Sargas. Stellar parameters and
evolutionary status from VLTI/PIONIER and VLT/UVES
Authors: Domiciano de Souza, A.; Bouchaud, K.; Rieutord, M.; Espinosa
Lara, F.; Putigny, B.
2018A&A...619A.167D Altcode: 2018arXiv181009903D
Context. Gravity darkening (GD) and flattening are important
consequences of stellar rotation. The precise characterization of these
effects across the Hertzsprung-Russell (H-R) diagram is crucial to a
deeper understanding of stellar structure and evolution. Aims:
We seek to characterize such important effects on Sargas (θ Scorpii),
an evolved, fast-rotating, intermediate-mass (∼5 M⊙)
star, located in a region of the H-R diagram where they have never
been directly measured as far as we know. Methods: We use our
numerical model CHARRON to analyze interferometric (VLTI/PIONIER)
and spectroscopic (VLT/UVES) observations through a MCMC model-fitting
procedure. The visibilities and closure phases from the PIONIER data
are particularly sensitive to rotational flattening and GD. Adopting
the Roche approximation, we investigate two GD models: (1) the β-model
(Teff ∝ geff β&lt;!--inline-formula
id="FI1"&gt; &lt;alternatives&gt; g eff β &lt;![CDATA[
g{{eff}}^β ]]&gt; &lt;inline-graphic id="img_eq1"
mime-subtype="gif" mimetype="image" xlink:href="aa33450-18-eq1.gif"
xmlns:xlink="http://www.w3.org/1999/xlink"/&gt; &lt;/alternatives&gt; ),
which includes the classical von Zeipel's GD law, and (2) the ω-model,
where the flux is assumed to be anti-parallel to geff. 
Results: Using this approach we measure several physical parameters of
Sargas, namely, equatorial radius, mass, equatorial rotation velocity,
mean Teff, inclination and position angle of the rotation
axis, and β. In particular, we show that the measured β leads
to a surface flux distribution equivalent to the one given by the
ω-model. Thanks to our results, we also show that Sargas is most
probably located in a rare and interesting region of the H-R diagram:
within the Hertzsprung gap and over the hot edge of the instability
strip (equatorial regions inside it and polar regions outside it
because of GD). Conclusions: These results show once more
the power of optical/infrared long-baseline interferometry, combined
with high-resolution spectroscopy, to directly measure fast-rotation
effects and stellar parameters, in particular GD. As was the case
for a few fast rotators previously studied by interferometry, the
ω-model provides a physically more profound description of Sargas'
GD, without the need of a β exponent. It will also be interesting
to further investigate the implications of the singular location
of such a fast rotator as Sargas in the H-R diagram. Based on
observations performed at ESO, Chile under program IDs 097.D-0230(ABC)
and 266.D-5655(A). The OIFits files with the VLTI/PIONIER data
that we used are publicly available at the JMMC OIDB website service: <A
href="http://oidb.jmmc.fr/index.html">http://oidb.jmmc.fr/index.html</A>

---------------------------------------------------------
Title: The Sun's supergranulation
Authors: Rincon, François; Rieutord, Michel
2018LRSP...15....6R Altcode:
Supergranulation is a fluid-dynamical phenomenon taking place in
the solar photosphere, primarily detected in the form of a vigorous
cellular flow pattern with a typical horizontal scale of approximately
30-35 Mm, a dynamical evolution time of 24-48 h, a strong 300-400 m/s
(rms) horizontal flow component and a much weaker 20-30 m/s vertical
component. Supergranulation was discovered more than 60 years ago,
however, explaining its physical origin and most important observational
characteristics has proven extremely challenging ever since, as a
result of the intrinsic multiscale, nonlinear dynamical complexity of
the problem concurring with strong observational and computational
limitations. Key progress on this problem is now taking place with
the advent of twenty-first-century supercomputing resources and the
availability of global observations of the dynamics of the solar surface
with high spatial and temporal resolutions. This article provides
an exhaustive review of observational, numerical and theoretical
research on supergranulation, and discusses the current status of our
understanding of its origin and dynamics, most importantly in terms of
large-scale nonlinear thermal convection, in the light of a selection
of recent findings.

---------------------------------------------------------
Title: Mode Identification in Rapidly Rotating Stars from BRITE Data
Authors: Reese, Daniel R.; Dupret, Marc-Antoine; Rieutord, Michel
2018pas8.conf...80R Altcode: 2018arXiv180307442R
Apart from recent progress in Gamma Dor stars, identifying modes in
rapidly rotating stars is a formidable challenge due to the lack of
simple, easily identifiable frequency patterns. As a result, it is
necessary to look to observational methods for identifying modes. Two
popular techniques are spectroscopic mode identification based on line
profile variations (LPVs) and photometric mode identification based on
amplitude ratios and phase differences between multiple photometric
bands. In this respect, the BRITE constellation is particularly
interesting as it provides space-based multi-colour photometry. The
present contribution describes the latest developments in obtaining
theoretical predictions for amplitude ratios and phase differences
for pulsation modes in rapidly rotating stars. These developments are
based on full 2D non-adiabatic pulsation calculations, using models
from the ESTER code, the only code to treat in a self-consistent way
the thermal equilibrium of rapidly rotating stars. These predictions
are then specifically applied to the BRITE photometric bands to explore
the prospects of identifying modes based on BRITE observations.

---------------------------------------------------------
Title: Axisymmetric inertial modes in a spherical shell at low
Ekman numbers
Authors: Rieutord, M.; Valdettaro, L.
2018JFM...844..597R Altcode: 2018arXiv180207582R
We investigate the asymptotic properties of axisymmetric inertial modes
propagating in a spherical shell when viscosity tends to zero. We
identify three kinds of eigenmodes whose eigenvalues follow very
different laws as the Ekman number$E$becomes very small. First are
modes associated with attractors of characteristics that are made of
thin shear layers closely following the periodic orbit traced by the
characteristic attractor. Second are modes made of shear layers that
connect the critical latitude singularities of the two hemispheres of
the inner boundary of the spherical shell. Third are quasi-regular
modes associated with the frequency of neutral periodic orbits of
characteristics. We thoroughly analyse a subset of attractor modes
for which numerical solutions point to an asymptotic law governing
the eigenvalues. We show that three length scales proportional
to$E^{1/6}$,$E^{1/4}$and$E^{1/3}$control the shape of the shear
layers that are associated with these modes. These scales point out
the key role of the small parameter$E^{1/12}$in these oscillatory
flows. With a simplified model of the viscous Poincaré equation,
we can give an approximate analytical formula that reproduces the
velocity field in such shear layers. Finally, we also present an
analysis of the quasi-regular modes whose frequencies are close to$\sin
(\unicode[STIX]{x03C0}/4)$and explain why a fluid inside a spherical
shell cannot respond to any periodic forcing at this frequency when
viscosity vanishes.

---------------------------------------------------------
Title: Reconstruction of Horizontal Plasma Motions at the Photosphere
from Intensitygrams: A Comparison Between DeepVel, LCT, FLCT, and CST
Authors: Tremblay, Benoit; Roudier, Thierry; Rieutord, Michel;
Vincent, Alain
2018SoPh..293...57T Altcode:
Direct measurements of plasma motions in the photosphere are limited
to the line-of-sight component of the velocity. Several algorithms have
therefore been developed to reconstruct the transverse components from
observed continuum images or magnetograms. We compare the space and time
averages of horizontal velocity fields in the photosphere inferred from
pairs of consecutive intensitygrams by the LCT, FLCT, and CST methods
and the DeepVel neural network in order to identify the method that
is best suited for generating synthetic observations to be used for
data assimilation. The Stein and Nordlund (Astrophys. J. Lett.753, L13,
2012) magnetoconvection simulation is used to generate synthetic SDO/HMI
intensitygrams and reference flows to train DeepVel. Inferred velocity
fields show that DeepVel performs best at subgranular and granular
scales and is second only to FLCT at mesogranular and supergranular
scales.

---------------------------------------------------------
Title: Large-scale photospheric motions determined from granule
tracking and helioseismology from SDO/HMI data
Authors: Roudier, Th.; Švanda, M.; Ballot, J.; Malherbe, J. M.;
Rieutord, M.
2018A&A...611A..92R Altcode: 2017arXiv171205255R
Context. Large-scale flows in the Sun play an important role in the
dynamo process linked to the solar cycle. The important large-scale
flows are the differential rotation and the meridional circulation
with an amplitude of km s-1 and few m s-1,
respectively. These flows also have a cycle-related components,
namely the torsional oscillations. Aim. Our attempt is to determine
large-scale plasma flows on the solar surface by deriving horizontal
flow velocities using the techniques of solar granule tracking,
dopplergrams, and time-distance helioseismology. Methods:
Coherent structure tracking (CST) and time-distance helioseismology
were used to investigate the solar differential rotation and meridional
circulation at the solar surface on a 30-day HMI/SDO sequence. The
influence of a large sunspot on these large-scale flows with a specific
7-day HMI/SDO sequence has been also studied. Results: The
large-scale flows measured by the CST on the solar surface and the
same flow determined from the same data with the helioseismology in
the first 1 Mm below the surface are in good agreement in amplitude
and direction. The torsional waves are also located at the same
latitudes with amplitude of the same order. We are able to measure
the meridional circulation correctly using the CST method with only
3 days of data and after averaging between ± 15° in longitude. Conclusions: We conclude that the combination of CST and Doppler
velocities allows us to detect properly the differential solar rotation
and also smaller amplitude flows such as the meridional circulation
and torsional waves. The results of our methods are in good agreement
with helioseismic measurements.

---------------------------------------------------------
Title: Gravity darkening in late-type stars. I. The Coriolis effect
Authors: Raynaud, R.; Rieutord, M.; Petitdemange, L.; Gastine, T.;
Putigny, B.
2018A&A...609A.124R Altcode: 2017arXiv171203130R
Context. Recent interferometric data have been used to constrain
the brightness distribution at the surface of nearby stars, in
particular the so-called gravity darkening that makes fast rotating
stars brighter at their poles than at their equator. However, good
models of gravity darkening are missing for stars that posses a
convective envelope. Aim. In order to better understand how rotation
affects the heat transfer in stellar convective envelopes, we focus
on the heat flux distribution in latitude at the outer surface of
numerical models. Methods: We carry out a systematic parameter
study of three-dimensional, direct numerical simulations of anelastic
convection in rotating spherical shells. As a first step, we neglect
the centrifugal acceleration and retain only the Coriolis force. The
fluid instability is driven by a fixed entropy drop between the inner
and outer boundaries where stress-free boundary conditions are applied
for the velocity field. Restricting our investigations to hydrodynamical
models with a thermal Prandtl number fixed to unity, we consider both
thick and thin (solar-like) shells, and vary the stratification over
three orders of magnitude. We measure the heat transfer efficiency in
terms of the Nusselt number, defined as the output luminosity normalised
by the conductive state luminosity. Results: We report diverse
Nusselt number profiles in latitude, ranging from brighter (usually at
the onset of convection) to darker equator and uniform profiles. We find
that the variations of the surface brightness are mainly controlled by
the surface value of the local Rossby number: when the Coriolis force
dominates the dynamics, the heat flux is weakened in the equatorial
region by the zonal wind and enhanced at the poles by convective
motions inside the tangent cylinder. In the presence of a strong
background density stratification however, as expected in real stars,
the increase of the local Rossby number in the outer layers leads to
uniformisation of the surface heat flux distribution.

---------------------------------------------------------
Title: The 2D dynamics of radiative zones of low-mass stars
Authors: Hypolite, D.; Mathis, S.; Rieutord, M.
2018A&A...610A..35H Altcode: 2017arXiv171108544H
Context. Helioseismology and asteroseismology allow us to probe
the differential rotation deep within low-mass stars. In the solar
convective envelope, the rotation varies with latitude with an equator
rotating faster than the pole, which results in a shear applied
on the radiative zone below. However, a polar acceleration of the
convective envelope can be obtained through 3D numerical simulations
in other low-mass stars and the dynamical interaction of the surface
convective envelope with the radiative core needs to be investigated
in the general case. Aim. In the context of secular evolution, we
aim to describe the dynamics of the radiative core of low-mass stars
to get a deeper understanding of the internal transport of angular
momentum in such stars, which results in a solid rotation in the
Sun from 0.7R⊙ to 0.2R⊙ and a weak radial
core-envelope differential rotation in solar-type stars. This study
requires at least a 2D description to capture the latitudinal variations
of the differential rotation. Methods: We build 2D numerical
models of a radiative core on the top of which we impose a latitudinal
shear so as to reproduce a conical or cylindrical differential rotation
in a convective envelope. We perform a systematic study over the
Rossby number ℛo = ΔΩ/2Ω0 measuring the latitudinal
differential rotation at the radiative-convective interface. We provide
a 2D description of the differential rotation and the associated
meridional circulation in the incompressible and stably stratified
cases using the Boussinesq approximation. Results: The imposed
shear generates a geostrophic flow implying a cylindrical differential
rotation in the case of an isotropic viscosity. When compared to the
baroclinic flow that arises from the stable stratification, we find that
the geostrophic flow is dominant when the Rossby number is high enough
(ℛo ≥ 1) with a cylindrical rotation profile. For low Rossby numbers
(ℛo &lt; 1), the baroclinic solution dominates with a quasi-shellular
rotation profile. Using scaling laws from 3D simulations, we show that
slow rotators (Ω0 &lt; 30Ω⊙) are expected
to have a cylindrical rotation profile. Fast rotators (Ω0
&gt; 30Ω⊙) may have a shellular profile at the beginning
of the main sequence in stellar radiative zones. Conclusions:
This study enables us to predict different types of differential
rotation and emphasizes the need for a new generation of 2D rotating
stellar models developed in synergy with 3D numerical simulations. The
shear induced by a surface convective zone has a strong impact on the
dynamics of the underlying radiative zone in low-mass stars. However, it
cannot produce a flat internal rotation profile in a solar configuration
calling for additional processes for the transport of angular momentum
in both radial and latitudinal directions.

---------------------------------------------------------
Title: Non-adiabatic oscillations of fast-rotating stars: the example
of Rasalhague
Authors: Mirouh, G. M.; Reese, D. R.; Rieutord, M.; Ballot, J.
2017sf2a.conf..103M Altcode: 2017arXiv171106053M
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 α 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.

---------------------------------------------------------
Title: The 2D dynamics of the differentially rotating envelope of
massive stars
Authors: Hypolite, Delphine; Mathis, Stéphane; Rieutord, Michel
2017IAUS..329..409H Altcode:
We build a 2D model of the radiative envelope of main sequence
massive stars. We set a dynamical boundary condition at the bottom
of the radiative envelope at η = r C /R (where r 
C is the core size and R the radius of the star) to account
for the differential rotation of the convective core as computed in 3D
simulations (e.g. Browning et al. (2004, IAUS, 224, 149). We seek the
differential rotation and associated meridional circulation induced
by such a shear competing with the baroclinic flow of the stably
stratified radiative envelope using the Boussinesq approximation.

---------------------------------------------------------
Title: Gravity darkening in stars with surface differential rotation
Authors: Zorec, J.; Rieutord, M.; Espinosa Lara, F.; Frémat, Y.;
Domiciano de Souza, A.; Royer, F.
2017A&A...606A..32Z Altcode: 2017arXiv170506485Z
Context. The interpretation of stellar apparent fundamental parameters
(viewing-angle dependent) requires that they be treated consistently
with the characteristics of their surface rotation law. Aims: We
aim to develop a model to determine the distribution of the effective
temperature and gravity, which explicitly depend on the surface
differential rotation law and on the concomitant stellar external
geometry. Methods: The basic assumptions in this model are:
a) the external stellar layers are in radiative equilibrium; b) the
emergent bolometric flux is anti-parallel with the effective gravity;
c) the angular velocity in the surface obeys relations like Ω(θ) =
Ωo [ 1 + αΥ(θ,k) ] where Υ(θ,k) = coskθ or
sinkθ, and where (α,k) are free parameters. Results:
The effective temperature varies with co-latitude θ, with amplitudes
that depend on the differential-rotation law through the surface
effective gravity and the gravity-darkening function (GDF). Although
the derived expressions can be treated numerically, for some low
integer values of k, analytical forms of the integral of characteristic
curves, on which the determination of the GDF relies, are obtained. The
effects of the quantities (η,α,k) (η = ratio between centrifugal
and gravitational accelerations at the equator) on the determination
of the Vsini parameter and on the gravity-darkening exponent are
studied. Depending on the values of (η,α,k) the velocity V in the
derived Vsini may strongly deviate from the equatorial rotational
velocity. It is shown that the von Zeipel's-like gravity-darkening
exponent β1 depends on all parameters (η,α,k) and that
its value also depends on the viewing-angle i. Hence, there no unique
interpretation of this exponent determined empirically in terms of
(i,α). Conclusions: We stress that the data on rotating stars
should be analyzed by taking into account the rotational effects
through the GDF, by assuming k = 2 as a first approximation. Instead
of the classic pair (η,β1), it would be more useful to
determine the quantities (η,α,i) to characterize stellar rotation.

---------------------------------------------------------
Title: On the oscillation spectrum of a magnetized core in a
giant star
Authors: Rieutord, Michel
2017EPJWC.16002011R Altcode:
The spectrum of gravito-acoustic modes is depleted in dipolar modes
for a significant fraction of the giant stars observed by the Kepler
mission, a feature that has been explained by the presence of magnetic
fields in the core of these stars (Fuller et al. 2015, Cantiello et
al. 2016). We further investigate this possible scenario by considering
first the oscillation spectrum of the core of a giant star modeled by a
stably stratified, self-gravitating fluid of uniform density in a sphere
pervaded by a uniform magnetic field. Our results show that the first
effect of a magnetic field on the g-modes is to reduce their wavenumber
and therefore reduce their damping. The magnetic effect, on this model,
is therefore opposite Fuller's et al scenario. Moreover, the model shows
that it is not possible to change the damping rate without changing the
frequency of the modes and this latter change is not observed. Because
of the simplicity of our model, the magnetized core scenario cannot
be dismissed but further investigations are needed, and other ways of
explaining the presence of depressed modes should also be considered.

---------------------------------------------------------
Title: Non-adiabatic pulsations in ESTER models
Authors: Reese, Daniel Roy; Dupret, Marc-Antoine; Rieutord, Michel
2017EPJWC.16002007R Altcode: 2017arXiv171007451R
One of the greatest challenges in interpreting the pulsations
of rapidly rotating stars is mode identification, i.e. correctly
matching theoretical modes to observed pulsation frequencies. Indeed,
the latest observations as well as current theoretical results show
the complexity of pulsation spectra in such stars, and the lack of
easily recognisable patterns. In the present contribution, the latest
results on non-adiabatic effects in such pulsations are described,
and we show how these come into play when identifying modes. These
calculations fully take into account the effects of rapid rotation,
including centrifugal distortion, and are based on models from the ESTER
project, currently the only rapidly rotating models in which the energy
conservation equation is satisfied, a prerequisite for calculating
non-adiabatic effects. Non-adiabatic effects determine which modes
are excited and play a key role in the near-surface pulsation-induced
temperature variations which intervene in multi-colour amplitude ratios
and phase differences, as well as line profile variations.

---------------------------------------------------------
Title: 2D dynamics of the radiative core of low mass stars
Authors: Hypolite, Delphine; Mathis, Stéphane; Rieutord, Michel
2017EPJWC.16002006H Altcode: 2016arXiv161008798H
Understanding the internal rotation of low mass stars all along their
evolution is of primary interest when studying their rotational
dynamics, internal mixing and magnetic field generation. In this
context, helio- and asteroseismology probe angular velocity gradients
deep within solar type stars at different evolutionary stages. Still
the rotation close to the center of such stars on the main sequence
is hardly detectable and the dynamical interaction of the radiative
core with the surface convective envelope is not well understood. For
instance, the influence of the differential rotation profile sustained
by convection and applied as a boundary condition to the radiation
zone is very important in the formation of tachoclines. In this work,
we study a 2D hydrodynamical model of a radiative core when an imposed,
solar or anti-solar, differential rotation is applied at the upper
boundary. This model uses the Boussinesq approximation and we find
that the shear induces a cylindrical differential rotation associated
with a unique cell of meridional circulation in each hemisphere
(counterclockwise when the shear is solar-like and clockwise when it
is anti-solar). The results are discussed in the framework of seismic
observables (internal rotation rate, core-to-surface rotation ratio)
while perspectives to improve our modeling by including magnetic field
or transport by internal gravity waves will be discussed.

---------------------------------------------------------
Title: Completeness of inertial modes of an incompressible inviscid
fluid in a corotating ellipsoid
Authors: Backus, George; Rieutord, Michel
2017PhRvE..95e3116B Altcode: 2016arXiv160602114B
Inertial modes are the eigenmodes of contained rotating fluids restored
by the Coriolis force. When the fluid is incompressible, inviscid,
and contained in a rigid container, these modes satisfy Poincaré's
equation that has the peculiarity of being hyperbolic with boundary
conditions. Inertial modes are, therefore, solutions of an ill-posed
boundary-value problem. In this paper, we investigate the mathematical
side of this problem. We first show that the Poincaré problem can be
formulated in the Hilbert space of square-integrable functions, with
no hypothesis on the continuity or the differentiability of velocity
fields. We observe that with this formulation, the Poincaré operator
is bounded and self-adjoint, and as such, its spectrum is the union
of the point spectrum (the set of eigenvalues) and the continuous
spectrum only. When the fluid volume is an ellipsoid, we show that
the inertial modes form a complete base of polynomial velocity fields
for the square-integrable velocity fields defined over the ellipsoid
and meeting the boundary conditions. If the ellipsoid is axisymmetric,
then the base can be identified with the set of Poincaré modes, first
obtained by Bryan [Philos. Trans. R. Soc. London 180, 187 (1889),
10.1098/rsta.1889.0006], and completed with the geostrophic modes.

---------------------------------------------------------
Title: Supergranulation and multiscale flows in the solar
photosphere. Global observations vs. a theory of anisotropic turbulent
convection
Authors: Rincon, F.; Roudier, T.; Schekochihin, A. A.; Rieutord, M.
2017A&A...599A..69R Altcode: 2016arXiv160905785R
The Sun provides us with the only spatially well-resolved astrophysical
example of turbulent thermal convection. While various aspects of solar
photospheric turbulence, such as granulation (one-Megameter horizontal
scale), are well understood, the questions of the physical origin and
dynamical organization of larger-scale flows, such as the 30-Megameters
supergranulation and flows deep in the solar convection zone,
remain largely open in spite of their importance for solar dynamics
and magnetism. Here, we present a new critical global observational
characterization of multiscale photospheric flows and subsequently
formulate an anisotropic extension of the Bolgiano-Obukhov theory of
hydrodynamic stratified turbulence that may explain several of their
distinctive dynamical properties. Our combined analysis suggests
that photospheric flows in the horizontal range of scales between
supergranulation and granulation have a typical vertical correlation
scale of 2.5 to 4 Megameters and operate in a strongly anisotropic,
self-similar, nonlinear, buoyant dynamical regime. While the theory
remains speculative at this stage, it lends itself to quantitative
comparisons with future high-resolution acoustic tomography of
subsurface layers and advanced numerical models. Such a validation
exercise may also lead to new insights into the asymptotic dynamical
regimes in which other, unresolved turbulent anisotropic astrophysical
fluid systems supporting waves or instabilities operate.

---------------------------------------------------------
Title: Tidal dissipation by inertial waves in differentially rotating
convective envelopes of low-mass stars
Authors: Guenel, Mathieu; Mathis, Stéphane; Baruteau, Clément;
Rieutord, Michel
2016arXiv161205071G Altcode:
Tidal interactions in close star-planet or binary star systems
may excite inertial waves (their restoring force is the Coriolis
force) in the convective region of the stars. The dissipation of
these waves plays a prominent role in the long-term orbital and
rotational evolution of the bodies involved. If the primary star
rotates as a solid body, inertial waves have a Doppler-shifted
frequency restricted to the range $[-2\Omega, 2\Omega]$ ($\Omega$
being the angular velocity of the star), and they can propagate in the
entire convective region. However, turbulent convection can sustain
differential rotation with an equatorial acceleration (as in the Sun)
or deceleration that modifies the frequency range and propagation domain
of inertial waves and allows corotation resonances for non-axisymmetric
oscillations. In this work, we perform numerical simulations of tidally
excited inertial waves in a differentially rotating convective envelope
with a conical (or latitudinal) rotation profile. The tidal forcing
that we adopt contains spherical harmonics that correspond to the case
of a circular and coplanar orbit. We study the viscous dissipation of
the waves as a function of tidal frequency for various stellar masses
and differential rotation parameters, as well as its dependence on the
turbulent viscosity coefficient. We compare our results with previous
studies assuming solid-body rotation and point out the potential key
role of corotation resonances in the dynamical evolution of close-in
star-planet or binary systems.

---------------------------------------------------------
Title: 2D dynamics of the radiation zone of low mass stars
Authors: Hypolite, Delphine; Mathis, Stéphane; Rieutord, Michel
2016arXiv160900592H Altcode:
The internal rotation of low mass stars all along their evolution is
of primary interest when studying their rotational dynamics, internal
mixing and magnetic fields generation. In this context, helio- and
asteroseismology probe angular velocity gradients deep within solar
type stars. Still the rotation of the close center of such stars on the
main sequence is hardly detectable and the dynamical interactions of
the radiative core with the surface convective envelope is not well
understood. Among them, the influence of the differential rotation
profile sustained by convection and applied as a boundary condition to
the radiation zone may be very important leading to the formation of
tachoclines. In the solar convective region, the equator is rotating
faster than the pole while numerical simulations predict either a solar
or an anti-solar rotation in other low mass stars envelopes depending
on their convective Rossby number. In this work, we therefore build
for the first time 2D steady hydrodynamical models of low mass stars
radiation zone providing a full 2D description of their dynamics and
studying the influence of a general shear boundary condition accounting
for a solar or an anti-solar differential rotation in the convective
envelope. We compute coherently differential rotation and the associated
meridional circulation using the Boussinesq approximation.

---------------------------------------------------------
Title: Gravito-inertial waves in a differentially rotating spherical
shell
Authors: Mirouh, G. M.; Baruteau, C.; Rieutord, M.; Ballot, J.
2016JFM...800..213M Altcode: 2015arXiv151105832M
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.

---------------------------------------------------------
Title: An algorithm for computing the 2D structure of fast rotating
stars
Authors: Rieutord, Michel; Espinosa Lara, Francisco; Putigny, Bertrand
2016JCoPh.318..277R Altcode: 2016arXiv160502359R
Stars may be understood as self-gravitating masses of a compressible
fluid whose radiative cooling is compensated by nuclear reactions or
gravitational contraction. The understanding of their time evolution
requires the use of detailed models that account for a complex
microphysics including that of opacities, equation of state and nuclear
reactions. The present stellar models are essentially one-dimensional,
namely spherically symmetric. However, the interpretation of recent
data like the surface abundances of elements or the distribution of
internal rotation have reached the limits of validity of one-dimensional
models because of their very simplified representation of large-scale
fluid flows. In this article, we describe the ESTER code, which is
the first code able to compute in a consistent way a two-dimensional
model of a fast rotating star including its large-scale flows. Compared
to classical 1D stellar evolution codes, many numerical innovations
have been introduced to deal with this complex problem. First, the
spectral discretization based on spherical harmonics and Chebyshev
polynomials is used to represent the 2D axisymmetric fields. A
nonlinear mapping maps the spheroidal star and allows a smooth spectral
representation of the fields. The properties of Picard and Newton
iterations for solving the nonlinear partial differential equations
of the problem are discussed. It turns out that the Picard scheme
is efficient on the computation of the simple polytropic stars, but
Newton algorithm is unsurpassed when stellar models include complex
microphysics. Finally, we discuss the numerical efficiency of our
solver of Newton iterations. This linear solver combines the iterative
Conjugate Gradient Squared algorithm together with an LU-factorization
serving as a preconditioner of the Jacobian matrix.

---------------------------------------------------------
Title: Tidal inertial waves in differentially rotating convective
envelopes of low-mass stars. I. Free oscillation modes
Authors: Guenel, M.; Baruteau, C.; Mathis, S.; Rieutord, M.
2016A&A...589A..22G Altcode: 2016arXiv160104617G
Context. Star-planet tidal interactions may result in the excitation
of inertial waves in the convective region of stars. In low-mass
stars, their dissipation plays a prominent role in the long-term
orbital evolution of short-period planets. Turbulent convection can
sustain differential rotation in their envelopes with an equatorial
acceleration (as in the Sun) or deceleration, which can modify the
propagation properties of the waves. Aims: We explore in this
first paper the general propagation properties of free linear inertial
waves in a differentially rotating homogeneous fluid inside a spherical
shell. We assume that the angular velocity background flow depends on
the latitudinal coordinate alone, close to what is expected in the
external convective envelope of low-mass stars. Methods: We
use an analytical approach in the inviscid case to get the dispersion
relation, from which we compute the characteristic trajectories along
which energy propagates. This allows us to study the existence of
attractor cycles and infer the different families of inertial modes. We
also use high-resolution numerical calculations based on a spectral
method for the viscous problem. Results: We find that modes
that propagate in the whole shell (D modes) behave the same way as
with solid-body rotation. However, another family of inertial modes
exists (DT modes), which can only propagate in a restricted part of the
convective zone. Our study shows that they are less common than D modes
and that the characteristic rays and shear layers often focus towards
a wedge - or point-like attractor. More importantly, we find that for
non-axisymmetric oscillation modes, shear layers may cross a corotation
resonance with a local accumulation of kinetic energy. Their damping
rate scales very differently from the value we obtain for standard
D modes, and we show an example where it is independent of viscosity
(Ekman number) in the astrophysical regime in which it is small.

---------------------------------------------------------
Title: Relation between trees of fragmenting granules and
supergranulation evolution
Authors: Roudier, Th.; Malherbe, J. M.; Rieutord, M.; Frank, Z.
2016A&A...590A.121R Altcode: 2016arXiv160404118R
Context. The determination of the underlying mechanisms of the
magnetic elements diffusion over the solar surface is still a
challenge. Understanding the formation and evolution of the solar
network (NE) is a challenge, because it provides a magnetic flux over
the solar surface comparable to the flux of active regions at solar
maximum. Aims: We investigate the structure and evolution of
interior cells of solar supergranulation. From Hinode observations,
we explore the motions on solar surface at high spatial and temporal
resolution. We derive the main organization of the flows inside
supergranules and their effect on the magnetic elements. 
Methods: To probe the superganule interior cell, we used the trees of
fragmenting granules (TFG) evolution and their relations to horizontal
flows. Results: Evolution of TFG and their mutual interactions
result in cumulative effects able to build horizontal coherent flows
with longer lifetime than granulation (1 to 2 h) over a scale up to
12”. These flows clearly act on the diffusion of the intranetwork
(IN) magnetic elements and also on the location and shape of the
network. Conclusions: From our analysis during 24 h, TFG appear
as one of the major elements of the supergranules which diffuse
and advect the magnetic field on the Sun's surface. The strongest
supergranules contribute the most to magnetic flux diffusion in the
solar photosphere. Movies are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201628111/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Physical Processes Leading to Surface Inhomogeneities: The
Case of Rotation
Authors: Rieutord, Michel
2016LNP...914..101R Altcode: 2015arXiv150503997R
In this lecture I discuss the bulk surface heterogeneity of rotating
stars, namely gravity darkening. I especially detail the derivation of
the ω-model of Espinosa Lara and Rieutord (Astron Astrophys 533:A43,
2011), which gives the gravity darkening in early-type stars. I also
discuss the problem of deriving gravity darkening&lt;IndexTerm&gt;
&lt;Secondary&gt;darkening&lt;/Secondary&gt; in stars owning a
convective envelope and in those that are members of a binary system.

---------------------------------------------------------
Title: Two-dimensional models of early-type fast rotating stars:
the ESTER project
Authors: Rieutord, Michel
2016IAUFM..29B.147R Altcode: 2015arXiv150908609R
In this talk I present the latest results of the ESTER project that
has taken up the challenge of building two dimensional (axisymmetric)
models of stars rotating at any rotation rate. In particular, I focus
on main sequence massive and intermediate mass stars. I show what
should be expected in such stars as far as the differential rotation
and the associated meridional circulation are concerned, notably the
emergence of a Stewartson layer along the tangent cylinder of the
core. I also indicate what may be inferred about the evolution of an
intermediate-mass star at constant angular momentum and how Be stars may
form. I finally give some comparisons between models and observations
of the gravity darkening on some nearby fast rotators as it has been
derived from interferometric observations. In passing, I also discuss
how 2D models can help to recover the fundamental parameters of a star.

---------------------------------------------------------
Title: Free inertial modes in differentially rotating convective
envelopes of low-mass stars : numerical exploration
Authors: Guenel, M.; Baruteau, C.; Mathis, S.; Rieutord, M.
2015sf2a.conf..371G Altcode: 2015arXiv151005825G
Tidally-excited inertial waves in stellar convective regions are a key
mechanism for tidal dissipation in stars and therefore the evolution of
close-in binary or planetary systems. As a first step, we explore here
the impact of latitudinal differential rotation on the properties of
free inertial modes and identify the different families of modes. We
show that they differ from the case of solid-body rotation. Using an
analytical approach as well as numerical calculations, we conclude that
critical layers --- where the Doppler-shifted frequency vanishes ---
could play a very important role for tidal dissipation.

---------------------------------------------------------
Title: Using the Sun to estimate Earth-like planet detection
capabilities. VI. Simulation of granulation and supergranulation
radial velocity and photometric time series
Authors: Meunier, N.; Lagrange, A. -M.; Borgniet, S.; Rieutord, M.
2015A&A...583A.118M Altcode:
Context. Stellar variability, at a variety of timescales, can strongly
affect the ability to detect exoplanets, in particular when using radial
velocity (RV) techniques. Accurately characterized solar variations are
precious in this context to study the impact of stellar variations on
planet detectability. Here we focus on the impact of small timescale
variability. Aims: The objective of this paper is to model
realistic RV time series due to granulation and supergranulation and to
study in greater detail the impact of granulation and supergranulation
on RV times series in the solar case. Methods: We have simulated
a collection of granules and supergranules evolving in time to reproduce
solar photometric and RV time series. Synthetic time series are built
over the full hemisphere over one solar cycle. Results: We
obtain intensity and RV rms due to solar granulation of respectively
0.8 m/s and 67 ppm, with a strong variability at timescales up to more
than 1 h. The rms RV due to supergranulation is between 0.28 and 1.12
m/s. Conclusions: To minimize the effect of granulation, the best
strategy is to split the observing time during the night into several
periods instead of observing over a consecutive duration. However,
the best strategy depends on the precise nature of the signal. The
granulation RV remains large after even an hour of smoothing (about
0.4 m/s) while the supergranulation signal cannot be significantly
reduced on such timescales: a reduction of a factor 2 in rms RV can for
example be obtained over 7 nights (with 26 min/night). The activity
RV variability dominates at larger timescales. Detection limits can
easily be as high as 1 MEarth or above for periods of
tens or hundreds of days. The impact on detection limits is therefore
important and may prevent the detection of 1 MEarth planets
for long orbital periods, while the impact is much smaller at small
orbital periods. These results do not take the presence of pulsations
into account.

---------------------------------------------------------
Title: Gravito-inertial modes in a differentially rotating spherical
shell
Authors: Mirouh, Giovanni M.; Baruteau, Clément; Rieutord, Michel;
Ballot, Jérôme
2015EPJWC.10106046M Altcode:
While many intermediate- and high-mass main sequence stars are rapidly
and differentially rotating, the effects of rotation on oscillation
modes are poorly known. In this communication we present a first
study of axisymmetric gravito-inertial modes in the radiative zone of
a differentially rotating star. We consider a simplified model where
the radiative zone of the star is a linearly stratified rotating fluid
within a spherical shell, with differential rotation due to baroclinic
effects. We solve the eigenvalue problem with high-resolution spectral
computations and determine the propagation domain of the waves through
the theory of characteristics. We explore the propagation properties
of two kinds of modes: those that can propagate in the entire shell
and those that are restricted to a sub-domain. Some of the modes that
we find concentrate kinetic energy around short-period shear layers
known as attractors. We describe various geometries for the propagation
domains, conditioning the surface visibility of the corresponding modes.

---------------------------------------------------------
Title: Two-dimensional models of fast rotating early-type stars
Authors: Rieutord, Michel
2015IAUGA..2252835R Altcode:
Rotation has now become an unavoidable parameter of stellar models, but
for most massive or intermediate-mass stars rotation is fast, at least
of a significant fraction of the critical angular velocity. Current
spherically symmetric models try to cope with this feature of the
stars using various approximations, like for instance the so-called
shellular rotation usually accompanied with a diffusion that is meant
to represent the mixing induced by rotationally generated flows. Such
approximations may be justified in the limit of slow rotation where
anisotropies and associated flows are weak. However, when rotation
is fast, say larger than 50% of the critical velocities the use of a
spherically symmetric 1D-model is doubtful. This is not only because of
the centrifugal flattening of the star, but also because of the flows
that are induced by the baroclinic torque that naturally appears in
the radiative envelope of an early-type (rotating) star. These flows
face the cylindrical symmetry of the Coriolis force and the spheroidal
symmetry of the effective gravity.In this talk I shall present the
latest results of the ESTER project that has taken up the challenge of
making two-dimensional (axisymmetric) models of stars rotating at any
rotation rate. In particular, I will focus on main sequence massive
and intermediate-mass stars. I'll show what should be expected in such
stars as far as the differential rotation and the associated meridional
circulation are concerned, notably the emergence of a Stewartson layer
along the tangential cylinder of the core. I'll also indicate what
may be inferred about the evolution of an intermediate-mass star at
constant angular momentum and how Be stars may form. I shall finally
give some comparisons between models and observations of the gravity
darkening on some nearby fast rotators as it has been derived from
interferometric observations. In passing, I'll also discuss how 2D
models can help to recover the fundamental parameters of a star.

---------------------------------------------------------
Title: Families of Granules, Flows, and Acoustic Events in the Solar
Atmosphere from Hinode Observations
Authors: Malherbe, J. -M.; Roudier, T.; Frank, Z.; Rieutord, M.
2015SoPh..290..321M Altcode: 2014SoPh..tmp..189M
We investigate the relationship between trees of fragmenting granules
(TFG), horizontal and vertical flows, and acoustic events (AE) in the
photospheric network. AE are spatially concentrated and short-duration
locations of acoustic energy flux. We performed observations at disk
center of a 2D field of view (FOV) with high spatial and temporal
resolutions provided by the Solar Optical Telescope onboard Hinode. Line
profiles of Fe I 557.6 nm were recorded by the Narrow-band Filter
Imager on an 80″×36″ FOV during five hours with a cadence of
22 seconds and 0.08″ pixel size. Vertical velocities were derived
at two atmospheric levels allowing the determination of the energy
flux at the acoustic frequency of 3.3 mHz. Families of granules and
horizontal velocities were obtained from local correlation tracking
(LCT) after segmentation and labeling of either continuum intensities
or granular Doppler shifts. AE exhibit durations in the range 0.25
to 1 hour compatible with the lifetime of families (80 % do not last
more than two hours). High-energy AE have the shortest lifetimes. We
found that most AE occur in intergranular lanes located in or close
to the boundaries between different families (called inter families)
in regions with predominantly downward vertical motions and horizontal
converging flows. In contrast, diverging flows are observed inside
families, with a few AE in the intergranules. At the beginning of the
sequence, when families are not yet detected, the distribution of AE
is not uniform and is already organized at spatial lengths related to
the mesogranular scale, with maximum contribution in the range 5″
to 10″, fully compatible with the scale of the maximum contribution
of families in the TFG space. Although all sizes and durations seem
to exist for families, their number decreases with increasing size
and lifetime.

---------------------------------------------------------
Title: Fluid Dynamics: An Introduction
Authors: Rieutord, M.
2015fldy.book.....R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamics of the envelope of a rapidly rotating star or giant
planet in gravitational contraction
Authors: Hypolite, D.; Rieutord, M.
2014A&A...572A..15H Altcode: 2014arXiv1409.6129H
 Aims: We wish to understand the processes that control the
fluid flows of a gravitationally contracting and rotating star
or giant planet. Methods: We consider a spherical shell
containing an incompressible fluid that is slowly absorbed by the
core so as to mimic gravitational contraction. We also consider the
effects of a stable stratification that may modify the dynamics of
a pre-main-sequence star of intermediate mass. Results: This
simple model reveals the importance of both the Stewartson layer
attached to the core and the boundary conditions met by the fluid
at the surface of the object. In the case of a pre-main-sequence
star of intermediate mass where the envelope is stably stratified,
shortly after the birth line, the spin-up flow driven by contraction
overwhelms the baroclinic flow that would take place otherwise. This
model also shows that for a contracting envelope, a self-similar flow
of growing amplitude controls the dynamics. It suggests that initial
conditions on the birth line are most probably forgotten. Finally,
the model shows that the shear (Stewartson) layer that lies on the
tangent cylinder of the core is likely a key feature of the dynamics
that is missing in 1D models. This layer can explain the core and
envelope rotational coupling that is required to explain the slow
rotation of cores in giant and subgiant stars.

---------------------------------------------------------
Title: Gravito-inertial modes in a differentially rotating spherical
shell
Authors: Mirouh, G. M.; Baruteau, C.; Rieutord, M.; Ballot, J.
2014sf2a.conf..493M Altcode: 2014arXiv1411.3559M
Oscillations have been detected in a variety of stars, including
intermediate- and high-mass main sequence stars. While many of
these stars are rapidly and differentially rotating, the effects of
rotation on oscillation modes are poorly known. In this communication
we present a first study on axisymmetric gravito-inertial modes in
the radiative zone of a differentially rotating star. These modes
probe the deep layers of the star around its convective core. We
consider a simplified model where the radiative zone of a star
is a linearly stratified rotating fluid within a spherical shell,
with differential rotation due to baroclinic effects. We solve the
eigenvalue problem with high-resolution spectral simulations and
determine the propagation domain of the waves through the theory of
characteristics. We explore the propagation properties of two kinds
of modes: those that can propagate in the entire shell and those
that are restricted to a subdomain. Some of the modes that we find
concentrate kinetic energy around short-period shear layers known
as attractors. We characterise these attractors by the dependence of
their Lyapunov exponent with the BV frequency of the background and
the oscillation frequency of the mode. Finally, we note that, as modes
associated with short-period attractors form dissipative structures,
they could play an important role for tidal interactions but should
be dismissed in the interpretation of observed oscillation frequencies.

---------------------------------------------------------
Title: Magnetohydrodynamics and Solar Physics
Authors: Rieutord, M.
2014sf2a.conf...45R Altcode: 2014arXiv1410.3725R
In this short review, I present some of the recent progresses on the
pending questions of solar physics. These questions let us revisit the
solar wind, the solar dynamo problem, the dynamics of the photosphere
and finally have a glimpse at other solar type stars. Discussing the
use of direct numerical simulations in solar physics, I show that the
full numerical calculation of the flow in a single supergranule would
require more electric power than the luminosity of the sun itself with
present computer technology.

---------------------------------------------------------
Title: Inertial waves in differentially rotating low-mass starsand
tides
Authors: Guenel, M.; Baruteau, C.; Mathis, S.; Rieutord, M.
2014sf2a.conf..233G Altcode: 2014arXiv1410.3295G
Star-planet tidal interactions may result in the excitation of inertial
waves in the convective region of stars. Their dissipation plays a
prominent role in the long-term orbital evolution of short-period
planets. If the star is assumed to be rotating as a solid-body,
the waves' Doppler-shifted frequency is restricted to [-2 Ω, 2 Ω]
(Ω being the angular velocity of the star) and they can propagate
in the entire convective region. However, turbulent convection can
sustain differential rotation with an equatorial acceleration (as in
the Sun) or deceleration that may modify waves propagation. We thus
explore the properties of inertial modes of oscillation in a conically
differentially rotating background flow whose angular velocity depends
on the latitudinal coordinate only, close to what is expected in the
external convective envelope of low-mass stars. We find that their
frequency range is broadened by differential rotation, and that they
may propagate only in a restricted part of the envelope. In some
cases, inertial waves form shear layers around short-period attractor
cycles. In others, they exhibit a remarkable behavior when a turning
surface or a corotation layer exists in the star. We discuss how all
these cases can impact tidal dissipation in stars.

---------------------------------------------------------
Title: Gravito-inertial modes in a differentially rotating spherical
shell
Authors: Mirouh, Giovanni M.; Baruteau, Clément; Rieutord, Michel;
Ballot, Jérôme
2014arXiv1410.3655M Altcode:
Oscillations have been detected in a variety of stars, including
intermediate- and high-mass main sequence stars. While many of
these stars are rapidly and differentially rotating, the effects of
rotation on oscillation modes are poorly known. In this communication
we present a first study on axisymmetric gravito-inertial modes in
the radiative zone of a differentially rotating star. These modes
probe the deep layers of the star around its convective core. We
consider a simplified model where the radiative zone of a star
is a linearly stratified rotating fluid within a spherical shell,
with differential rotation due to baroclinic effects. We solve the
eigenvalue problem with high-resolution spectral simulations and
determine the propagation domain of the waves through the theory of
characteristics. We explore the propagation properties of two kinds
of modes: those that can propagate in the entire shell and those
that are restricted to a subdomain. Some of the modes that we find
concentrate kinetic energy around short-period shear layers known
as attractors. We characterise these attractors by the dependence of
their Lyapunov exponent with the \BV frequency of the background and
the oscillation frequency of the mode. Finally, we note that, as modes
associated with short-period attractors form dissipative structures,
they could play an important role for tidal interactions but should
be dismissed in the interpretation of observed oscillation frequencies.

---------------------------------------------------------
Title: Dynamics of the radiative envelope of rapidly rotating stars:
Effects of spin-down driven by mass loss
Authors: Rieutord, M.; Beth, A.
2014A&A...570A..42R Altcode: 2014arXiv1407.0946R
 Aims: This paper aims at deciphering the dynamics of the envelope
of a rotating star when some angular momentum loss due to mass loss
is present. We especially wish to know when the spin-down flow forced
by the mass loss supersedes the baroclinic flows that pervade the
radiative envelope of rotating stars. Methods: We consider a
Boussinesq fluid enclosed in a rigid sphere whose flows are forced
both by the baroclinic torque, the spin-down of an outer layer, and
an outward mass flux. The spin-down forcing is idealized in two ways:
either by a rigid layer that imposes its spinning down velocity at
some interface or by a turbulent layer that imposes a stress at this
same interface to the interior of the star. Results: In the
case where the layer is rigid and imposes its velocity, we find that,
as the mass-loss rate increases, the flow inside the star shows two
transitions: the meridional circulation associated with baroclinic flows
is first replaced by its spin-down counterpart, while at much stronger
mass-loss rates the baroclinic differential rotation is superseded by
the spin-down differential rotation. When boundary conditions specify
the stress instead of the velocity, we find just one transition as the
mass-loss rate increases. Besides the two foregoing transitions, we find
a third transition that separates an angular momentum flux dominated
by stresses from an angular momentum flux dominated by advection. Thus,
with this very simplified two-dimensional stellar model, we find three
wind regimes: weak (or no wind), moderate, and strong. In the weak wind
case, the flow in the radiative envelope is of baroclinic origin. In
the moderate case, the circulation results from the spin-down while
the differential rotation may either be of baroclinic or of spin-down
origin, depending on the boundary conditions or more generally on the
coupling between mass and angular momentum losses. For fast rotating
stars, our model says that the moderate wind regime starts when mass
loss is higher than ~ 10-11 M⊙/yr. In the
strong wind case, the flow in the radiative envelope is mainly driven
by angular momentum advection. This latter transition mass-loss rate
depends on the mass and the rotation rate of the star, being around
10-8 M⊙/yr for a 3 M⊙ ZAMS star
rotating at 200 km s-1 according to our model.

---------------------------------------------------------
Title: The environment of the fast rotating star
Achernar. III. Photospheric parameters revealed by the VLTI
Authors: Domiciano de Souza, A.; Kervella, P.; Moser Faes, D.; Dalla
Vedova, G.; Mérand, A.; Le Bouquin, J. -B.; Espinosa Lara, F.;
Rieutord, M.; Bendjoya, P.; Carciofi, A. C.; Hadjara, M.; Millour,
F.; Vakili, F.
2014A&A...569A..10D Altcode:
Context. Rotation significantly impacts on the structure and life
of stars. In phases of high rotation velocity (close to critical),
the photospheric structure can be highly modified, and present
in particular geometrical deformation (rotation flattening) and
latitudinal-dependent flux (gravity darkening). The fastest known
rotators among the nondegenerate stars close to the main sequence,
Be stars, are key targets for studying the effects of fast rotation
on stellar photospheres. Aims: We seek to determine the
purely photospheric parameters of Achernar based on observations
recorded during an emission-free phase (normal B phase). 
Methods: Several recent works proved that optical/IR long-baseline
interferometry is the only technique able to sufficiently spatially
resolve and measure photospheric parameters of fast rotating stars. We
thus analyzed ESO-VLTI (PIONIER and AMBER) interferometric observations
of Achernar to measure its photospheric parameters by fitting our
physical model CHARRON using a Markov chain Monte Carlo method. This
analysis was also complemented by spectroscopic, polarimetric, and
photometric observations to investigate the status of the circumstellar
environment of Achernar during the VLTI observations and to cross-check
our model-fitting results. Results: Based on VLTI observations
that partially resolve Achernar, we simultaneously measured five
photospheric parameters of a Be star for the first time: equatorial
radius (equatorial angular diameter), equatorial rotation velocity,
polar inclination, position angle of the rotation axis projected on the
sky, and the gravity darkening β coefficient (effective temperature
distribution). The close circumstellar environment of Achernar was
also investigated based on contemporaneous polarimetry, spectroscopy,
and interferometry, including image reconstruction. This analysis
did not reveal any important circumstellar contribution, so that
Achernar was essentially in a normal B phase at least from mid-2009
to end-2012, and the model parameters derived in this work provide
a fair description of its photosphere. Finally, because Achernar is
the flattest interferometrically resolved fast rotator to-date, the
measured β and flattening, combined with values from previous works,
provide a crucial test for a recently proposed gravity darkening
model. This model offers a promising explanation to the fact that the
measured β parameter decreases with flattening and shows significantly
lower values than the classical prediction of von Zeipel. Based
on observations performed at ESO, Chile under VLTI PIONIER and AMBER
programme IDs 087.D-0150 and 084.D-0456.

---------------------------------------------------------
Title: Pulsations of rapidly rotating stars with compositional
discontinuities
Authors: Reese, Daniel R.; Lara, Francisco Espinosa; Rieutord, Michel
2014IAUS..301..169R Altcode: 2013arXiv1310.3058R
Recent observations of rapidly rotating stars have revealed the
presence of regular patterns in their pulsation spectra. This has
raised the question as to their physical origin, and, in particular,
whether they can be explained by an asymptotic frequency formula for
low-degree acoustic modes, as recently discovered through numerical
calculations and theoretical considerations. In this context, a key
question is whether compositional/density gradients can adversely affect
such patterns to the point of hindering their identification. To answer
this question, we calculate frequency spectra using two-dimensional
ESTER stellar models. These models use a multi-domain spectral
approach, allowing us to easily insert a compositional discontinuity
while retaining a high numerical accuracy. We analyse the effects
of such discontinuities on both the frequencies and eigenfunctions
of pulsation modes in the asymptotic regime. We find that although
there is more scatter around the asymptotic frequency formula, the
semi-large frequency separation can still be clearly identified in a
spectrum of low-degree acoustic modes.

---------------------------------------------------------
Title: Asteroseismology of fast-rotating stars: the example of
α Ophiuchi
Authors: Mirouh, Giovanni M.; Reese, Daniel R.; Lara, Francisco
Espinosa; Ballot, Jérôme; Rieutord, Michel
2014IAUS..301..455M Altcode: 2013arXiv1309.3887M
Many early-type stars have been measured with high angular
velocities. In such stars, mode identification is difficult as the
effects of fast and differential rotation are not well known. Using
fundamental parameters measured by interferometry, the ESTER
structure code and the TOP oscillation code, we investigate the
oscillation spectrum of α Ophiuchi, for which observations by the
MOST satellite found 57 oscillations frequencies. Results do not show
a clear identification of the modes and highlight the difficulties
of asteroseismology for such stars with a very complex oscillation
spectrum.

---------------------------------------------------------
Title: Two-dimensional models of early-type fast rotating stars:
new challenges in stellar physics
Authors: Rieutord, M.; Espinosa Lara, F.
2013EAS....63..385R Altcode: 2013arXiv1310.1017R
Two-dimensional models of rapidly rotating stars are already
unavoidable for the interpretation of interferometric or asteroseismic
data of this kind of stars. When combined with time evolution,
they will allow the including of a more accurate physics for the
computation of element transport and the determination of surface
abundances. In addition, modeling the evolution of rotation will
improve gyrochronology. Presently, two-dimensional ESTER models
predict the structure and the large-scale flows (differential rotation
and meridional circulation) of stars with mass larger than 1.7
M⊙ at any rotation rate. Main sequence evolution can be
mimicked by varying the hydrogen content of the convective core. Models
have been successfully tested on half a dozen of nearby fast rotating
stars observed with optical or infra-red interferometers. They are now
the right tool to investigate the oscillation spectrum of early-type
fast rotators.

---------------------------------------------------------
Title: Present status of two-dimensional ESTER models: Application
to Be stars
Authors: Rieutord, M.; Espinosa, Lara F.
2013sf2a.conf..101R Altcode: 2013arXiv1310.0908R
ESTER two-dimensional models solve the steady state structure of fast
rotating early-type stars including the large scale flows associated
with the baroclinicity of the radiative zones. Models are compared
successfully to the fundamental parameters of the two main components of
the triple system δ Velorum that have been derived from interferometric
and orbit measurements. Testing the models on the Be star Achernar (α
Eri), we cannot reproduce the data and conclude that this star has left
the main sequence and is likely crossing the Herzsprung gap. Computing
main sequence evolution of fast rotating stars at constant angular
momentum shows that their criticality increases with time suggesting
that the Be phenomenon and the ensuing mass ejections is the result
of evolution.

---------------------------------------------------------
Title: Modeling rotating stars in two dimensions
Authors: Rieutord, M.
2013EAS....62..307R Altcode: 2013arXiv1305.0496R
In this lecture I present the way stars can be modeled in two dimensions
and especially the fluid flows that are driven by rotation. I discuss
some of the various ways of taking into account turbulence and conclude
this contribution by a short presentation of some of the first results
obtained with the ESTER code on the modeling of interferometrically
observed fast rotating early-type stars.

---------------------------------------------------------
Title: Comparison of Solar Surface Flows Inferred from Time-Distance
Helioseismology and Coherent Structure Tracking Using HMI/SDO
Observations
Authors: Švanda, Michal; Roudier, Thierry; Rieutord, Michel; Burston,
Raymond; Gizon, Laurent
2013ApJ...771...32S Altcode: 2013arXiv1305.0875S
We compare measurements of horizontal flows on the surface of the
Sun using helioseismic time-distance inversions and coherent structure
tracking of solar granules. Tracking provides two-dimensional horizontal
flows on the solar surface, whereas the time-distance inversions
estimate the full three-dimensional velocity flows in the shallow
near-surface layers. Both techniques use Helioseismic and Magnetic
Imager observations as input. We find good correlations between the
various measurements resulting from the two techniques. Further, we
find a good agreement between these measurements and the time-averaged
Doppler line-of-sight velocity, and also perform sanity checks on the
vertical flow that resulted from the three-dimensional time-distance
inversion.

---------------------------------------------------------
Title: ESTER: Evolution STEllaire en Rotation
Authors: Rieutord, Michel
2013ascl.soft05001R Altcode:
The ESTER code computes the steady state of an isolated star of mass
larger than two solar masses. The only convective region computed as
such is the core where isentropy is assumed. ESTER provides solutions
of the partial differential equations, for the pressure, density,
temperature, angular velocity and meridional velocity for the whole
volume. The angular velocity (differential rotation) and meridional
circulation are computed consistently with the structure and are
driven by the baroclinic torque. The code uses spectral methods, both
radially and horizontally, with spherical harmonics and Chebyshev
polynomials. The iterations follow Newton's algorithm. The code is
object-oriented and is written in C++; a python suite allows an easy
visualization of the results. While running, PGPLOT graphs are displayed
to show evolution of the iterations.

---------------------------------------------------------
Title: Comparison of solar horizontal velocity fields from SDO/HMI
and Hinode data
Authors: Roudier, Th.; Rieutord, M.; Prat, V.; Malherbe, J. M.; Renon,
N.; Frank, Z.; Švanda, M.; Berger, T.; Burston, R.; Gizon, L.
2013A&A...552A.113R Altcode: 2013arXiv1303.4271R
Context. The measurement of the Sun's surface motions with a high
spatial and temporal resolution is still a challenge. Aims:
We wish to validate horizontal velocity measurements all over the
visible disk of the Sun from Solar Dynamics Observatory/ Helioseismic
and Magnetic Imager (SDO/HMI) data. Methods: Horizontal velocity
fields are measured by following the proper motions of solar granules
using a newly developed version of the coherent structure tracking
(CST) code. The comparison of the surface flows measured at high
spatial resolution (Hinode, 0.1 arcsec) and low resolution (SDO/HMI,
0.5 arcsec) allows us to determine corrections to be applied to
the horizontal velocity measured from HMI white light data. 
Results: We derive horizontal velocity maps with spatial and temporal
resolutions of respectively 2.5 Mm and 30 min. From the two components
of the horizontal velocity vx and vy measured
in the sky plane and the simultaneous line of sight component from
SDO/HMI dopplergrams vD, we derive the spherical velocity
components (vr, vθ, vϕ). The
azimuthal component vϕ gives the solar differential rotation
with a high precision (± 0.037 km s-1) from a temporal
sequence of only three hours. Conclusions: By following the
proper motions of the solar granules, we can revisit the dynamics of
the solar surface at high spatial and temporal resolutions from hours
to months and years with the SDO data.

---------------------------------------------------------
Title: Self-consistent 2D models of fast-rotating early-type stars
Authors: Espinosa Lara, F.; Rieutord, M.
2013A&A...552A..35E Altcode: 2012arXiv1212.0778E
 Aims: This work aims at presenting the first two-dimensional
models of an isolated rapidly rotating star that include the
derivation of the differential rotation and meridional circulation
in a self-consistent way. Methods: We use spectral methods
in multidomains, together with a Newton algorithm to determine the
steady state solutions including differential rotation and meridional
circulation for an isolated non-magnetic, rapidly rotating early-type
star. In particular we devise an asymptotic method for small Ekman
numbers (small viscosities) that removes the Ekman boundary layer
and lifts the degeneracy of the inviscid baroclinic solutions. Results: For the first time, realistic two-dimensional models of
fast-rotating stars are computed with the actual baroclinic flows that
predict the differential rotation and the meridional circulation for
intermediate-mass and massive stars. These models nicely compare with
available data of some nearby fast-rotating early-type stars like Ras
Alhague (α Oph), Regulus (α Leo), and Vega (α Lyr). It is shown
that baroclinicity drives a differential rotation with a slow pole,
a fast equator, a fast core, and a slow envelope. The differential
rotation is found to increase with mass, with evolution (here measured
by the hydrogen mass fraction in the core), and with metallicity. The
core-envelope interface is found to be a place of strong shear
where mixing will be efficient. Conclusions: Two-dimensional
models offer a new view of fast-rotating stars, especially of their
differential rotation, which turns out to be strong at the core-envelope
interface. They also offer more accurate models for interpreting the
interferometric and spectroscopic data of early-type stars.

---------------------------------------------------------
Title: Inertial waves in a differentially rotating spherical shell
Authors: Baruteau, C.; Rieutord, M.
2013JFM...719...47B Altcode: 2012arXiv1203.4347B
We investigate the properties of small-amplitude inertial waves
propagating in a differentially rotating incompressible fluid contained
in a spherical shell. For cylindrical and shellular rotation profiles
and in the inviscid limit, inertial waves obey a second-order partial
differential equation of mixed type. Two kinds of inertial modes
therefore exist, depending on whether the hyperbolic domain where
characteristics propagate covers the whole shell or not. The occurrence
of these two kinds of inertial modes is examined, and we show that the
range of frequencies at which inertial waves may propagate is broader
than with solid-body rotation. Using high-resolution calculations
based on a spectral method, we show that, as with solid-body rotation,
singular modes with thin shear layers following short-period attractors
still exist with differential rotation. They exist even in the case
of a full sphere. In the limit of vanishing viscosities, the width
of the shear layers seems to weakly depend on the global background
shear, showing a scaling in E^{1/3} with the Ekman number E, as in the
solid-body rotation case. There also exist modes with thin detached
layers of width scaling with E^{1/2} as Ekman boundary layers. The
behavior of inertial waves with a corotation resonance within the shell
is also considered. For cylindrical rotation, waves get dramatically
absorbed at corotation. In contrast, for shellular rotation, waves may
cross a critical layer without visible absorption, and such modes can
be unstable for small enough Ekman numbers.

---------------------------------------------------------
Title: Ab Initio Modelling of Steady Rotating Stars
Authors: Rieutord, Michel; Espinosa Lara, Francisco
2013LNP...865...49R Altcode: 2012arXiv1208.4926R
Modelling isolated rotating stars at any rotation rate is a challenge
for the next generation of stellar models. These models will couple
dynamical aspects of rotating stars, like angular momentum and chemicals
transport, with classical chemical evolution, gravitational contraction
or mass-loss. Such modelling needs to be achieved in two dimensions,
combining the calculation of the structure of the star, its mean flows
and the time-evolution of the whole. We present here a first step
in this challenging programme. It leads to the first self-consistent
two-dimensional models of rotating stars in a steady state generated by
the ESTER code. In these models the structure (pressure, density and
temperature) and the flow fields are computed in a self-consistent
way allowing the prediction of the differential rotation and the
associated meridian circulation of the stars. After a presentation
of the physical properties of such models and the numerical methods
at work, we give the first grid of such models describing massive and
intermediate-mass stars for a selection of rotation rates up to 90 %
of the breakup angular velocity.

---------------------------------------------------------
Title: Rotation Effects as Possible Responsible for the Surface
Effects on the Sun and Solar-Type Stars
Authors: Suárez, J. C.; Goupil, M. J.; Reese, D. R.; Samadi, R.;
Lignières, F.; Rieutord, M.; Lochard, J.
2013ASSP...31..197S Altcode:
One of the main sources of uncertainty in the asteroseismic models
of the Sun and solar-like stars is the poor match between predicted
oscillation frequencies and observed ones in the very high frequency
domain. Today, effects of turbulence, diffusion, etc., i.e., the
so-called "surface effects" are signaled as possible responsible
for such a discrepancy. We show that the effect of the stellar
deformation due to rotation is of the same order or even larger than
these effects. We show that rotation effects, are important for the
asteroseismic analysis of the Sun and solar-like stars and they cannot
be neglected when modeling such stars.

---------------------------------------------------------
Title: Gravity darkening in binary stars
Authors: Espinosa Lara, F.; Rieutord, M.
2012A&A...547A..32E Altcode: 2012arXiv1210.4004E
Context. Interpretation of light curves of many types of binary stars
requires the inclusion of the (cor)relation between surface brightness
and local effective gravity. Until recently, this correlation has
always been modeled by a power law relating the flux or the effective
temperature and the effective gravity, namely Teff ∝
geffβ . Aims: We look for a simple
model that can describe the variations of the flux at the surface of
stars belonging to a binary system. Methods: This model assumes
that the energy flux is a divergence-free vector anti-parallel to the
effective gravity. The effective gravity is computed from the Roche
model. Results: After explaining in a simple manner the old
result of Lucy (1967, Z. Astrophys., 65, 89), which says that β ~
0.08 for solar type stars, we first argue that one-dimensional
models should no longer be used to evaluate gravity darkening
laws. We compute the correlation between log Teff and log
geff using a new approach that is valid for synchronous,
weakly magnetized, weakly irradiated binaries. We show that this
correlation is approximately linear, validating the use of a power
law relation between effective temperature and effective gravity as a
first approximation. We further show that the exponent β of this power
law is a slowly varying function, which we tabulate, of the mass ratio
of the binary star and the Roche lobe filling factor of the stars of
the system. The exponent β remains mostly in the interval [0.20,0.25]
if extreme mass ratios are eliminated. Conclusions: For binary
stars that are synchronous, weakly magnetized and weakly irradiated,
the gravity darkening exponent is well constrained and may be removed
from the free parameters of the models.

---------------------------------------------------------
Title: Solar Astronomy and its Wealth of Data
Authors: Rieutord, M.
2012ASPC..461..457R Altcode: 2012adass..21..457R
The only part of the Sun of which we have a detailed view is its
surface. But this surface is also that of its convection zone
which evolves on shorter and shorter time scales as smaller and
smaller length scales are detected. Monitoring the Sun at a given
(visible) wavelength thus generates a data flow which increases as
the cube of the spatial resolution. A series of images covering the
whole visible hemisphere of the Sun at the resolution of a 50 cm
aperture would generate a data flow rate of 0.8Tb per day. However,
knowing its physical state reduces to the knowledge of eight physical
quantities: vx, vy, vz, Bx,
By, Bz, ρ, T (velocity, magnetic, density and
temperature fields respectively). In this work, I give the orders of
magnitude of the data flow rates that should be expected for an ideal
monitoring of the Sun. I also discuss the effects of seeing or the
physical constraints that affect the measurement of quantities like
the horizontal component of the velocity field. Finally, I present
the various numerical simulations data sets that have emerged while
investigating the dynamics of the Sun.

---------------------------------------------------------
Title: 2D Computations of g-modes in Fast Rotating Stars
Authors: Ballot, J.; Lignières, F.; Prat, V.; Reese, D. R.;
Rieutord, M.
2012ASPC..462..389B Altcode: 2011arXiv1109.6856B
We present complete 2D computations of g-modes in distorted polytropic
models of stars performed with the Two-dimensional Oscillation Program
(TOP). We computed low-degree modes (ℓ =1 modes with radial order n =
-1,…,-14, and ℓ = 2, 3 modes with n = -1,…,-5 and -16,…,-20)
of a non-rotating model and followed them by slowly increasing the
rotation rate up to 70 % of the Keplerian break-up velocity. We use
these computations to determine the domain of validity of perturbative
methods up to the 3rd order. We study the evolution of the regularities
of the spectrum and show quantitative agreement with the traditional
approximation for not too large values of the ratio of the rotation
rate to the pulsation frequency. We also show the appearance of
new types of modes, called “rosette” modes due to their spatial
structure. Thanks to the ray theory for gravito-inertial waves that
we developed, we can associate these modes with stable periodic rays.

---------------------------------------------------------
Title: Excitation of inertial modes in an experimental spherical
Couette flow
Authors: Rieutord, Michel; Triana, Santiago Andrés; Zimmerman,
Daniel S.; Lathrop, Daniel P.
2012PhRvE..86b6304R Altcode: 2012arXiv1208.4937R
Spherical Couette flow (flow between concentric rotating spheres)
is one of flows under consideration for the laboratory magnetic
dynamos. Recent experiments have shown that such flows may excite
Coriolis restored inertial modes. The present work aims to better
understand the properties of the observed modes and the nature
of their excitation. Using numerical solutions describing forced
inertial modes of a uniformly rotating fluid inside a spherical shell,
we first identify the observed oscillations of the Couette flow with
nonaxisymmetric, retrograde, equatorially antisymmetric inertial modes,
confirming first attempts using a full sphere model. Although the model
has no differential rotation, identification is possible because a large
fraction of the fluid in a spherical Couette flow rotates rigidly. From
the observed sequence of the excited modes appearing when the inner
sphere is slowed down by step, we identify a critical Rossby number
associated with a given mode, below which it is excited. The matching
between this critical number and the one derived from the phase velocity
of the numerically computed modes shows that these modes are excited
by an instability likely driven by the critical layer that develops in
the shear layer, staying along the tangent cylinder of the inner sphere.

---------------------------------------------------------
Title: The Power Spectrum of the Solar Surface Flows from Hinode
Data and First Observations with MOF/CALAS Pic-du-Midi
Authors: Roudier, T.; Rieutord, M.; Rincon, F.; Malherbe, J. -M.;
Brito, D.; Berger, T.; Frank, Z.; Parés, L.; Bourrec, E.; Beigbeder,
F.
2012ASPC..454...47R Altcode:
Many features of solar surface turbulence, like the supergranulation,
are still poorly understood.We use long time series of images taken by
the Solar Optical Telescope on board the Hinode satellite to determine
the velocity fields. The dynamics in the subgranulation range can
be investigated with unprecedented precision thanks to the absence
of seeing effects and the use of the MTF of SOT for correcting the
spectra. The first MOF/CALAS/Pic-du-Midi results are also shown.

---------------------------------------------------------
Title: Acoustic Events in the Solar Atmosphere from Hinode/SOT
NFI Observations
Authors: Malherbe, J. -M.; Roudier, T.; Rieutord, M.; Berger, T.;
Franck, Z.
2012SoPh..278..241M Altcode: 2012arXiv1207.1170M
We investigate the properties of acoustic events (AEs), defined as
spatially concentrated and short duration energy flux, in the quiet
Sun, using observations of a 2D field of view (FOV) with high spatial
and temporal resolution provided by the Solar Optical Telescope (SOT)
onboard Hinode. Line profiles of Fe I 557.6 nm were recorded by the
Narrow-band Filter Imager (NFI) on a 82″×82″ FOV during 75 min with
a time step of 28.75 s and 0.08″ pixel size. Vertical velocities were
computed at three atmospheric levels (80, 130, and 180 km) using the
bisector technique, allowing the determination of energy flux to be
made in the range 3 - 10 mHz using two complementary methods (Hilbert
transform and Fourier power spectrum). Horizontal velocities were
computed using local correlation tracking (LCT) of continuum intensities
providing divergences. We found that the net energy flux is upward. In
the range 3 - 10 mHz, a full FOV space and time averaged flux of 2700 W
m−2 (lower layer 80 - 130 km) and 2000 W m−2
(upper layer 130 - 180 km) is concentrated in less than 1 % of the
solar surface in the form of narrow (0.3″) AE. Their total duration
(including rise and decay) is of the order of 103 s. Inside
each AE, the mean flux is 1.6×105 W m−2
(lower layer) and 1.2×105 W m−2 (upper). Each
event carries an average energy (flux integrated over space and time)
of 2.5×1019 J (lower layer) to 1.9×1019 J
(upper). More than 106 events could exist permanently on
the Sun, with a birth and decay rate of 3500 s−1. Most
events occur in intergranular lanes, downward velocity regions, and
areas of converging motions.

---------------------------------------------------------
Title: MHD simulations of the solar photosphere
Authors: Rieutord, M.; Rincon, F.; Roudier, T.
2012EAS....55....5R Altcode: 2012arXiv1206.6630R
We briefly review the observations of the solar photosphere and
pinpoint some open questions related to the magnetohydrodynamics of
this layer of the Sun. We then discuss the current modelling efforts,
addressing among other problems, that of the origin of supergranulation.

---------------------------------------------------------
Title: Some Dynamic Analysis of the Photosphere from Hinode/SOT and
SDO/HMI Observations
Authors: Roudier, T.; Malherbe, J.; Rieutord, M.; Berger, T.; Frank,
Z.; Prat, V.; Renon, N.; Gizon, L.; Svanda, M.
2012ASPC..456...65R Altcode:
We first present the important role played by the families of granule
(or Tree of Fragmenting granules) in the formation of the photospheric
network. Then, we describe the occurence and characteristics of
acoustic events (AE), defined as spatially concentrated energy flux,
in the quiet Sun. Finally, we present how horizontal velocities obtained
from SDO/HMI data are calibrated by using Hinode/SOT observations.

---------------------------------------------------------
Title: Quasi full-disk maps of solar horizontal velocities using
SDO/HMI data
Authors: Roudier, Th.; Rieutord, M.; Malherbe, J. M.; Renon, N.;
Berger, T.; Frank, Z.; Prat, V.; Gizon, L.; Švanda, M.
2012A&A...540A..88R Altcode: 2012arXiv1203.0514R
 Aims: For the first time, the motion of granules (solar plasma
on the surface on scales larger than 2.5 Mm) has been followed over
the entire visible surface of the Sun, using SDO/HMI white-light
data. Methods: Horizontal velocity fields are derived from image
correlation tracking using a new version of the coherent structure
tracking algorithm. The spatial and temporal resolutions of the
horizontal velocity map are 2.5 Mm and 30 min, respectively. Results: From this reconstruction, using the multi-resolution
analysis, one can obtain to the velocity field at different scales
with its derivatives such as the horizontal divergence or the vertical
component of the vorticity. The intrinsic error on the velocity is
~0.25 km s-1 for a time sequence of 30 min and a mesh size
of 2.5 Mm. This is acceptable compared to the granule velocities, which
range between 0.3 km s-1 and 1.8 km s-1. A high
correlation between velocities computed from Hinode and SDO/HMI has
been found (85%). From the data we derive the power spectrum of the
supergranulation horizontal velocity field, the solar differential
rotation, and the meridional velocity.

---------------------------------------------------------
Title: An r-mode in a magnetic rotating spherical layer: application
to neutron stars
Authors: Abbassi, S.; Rieutord, M.; Rezania, V.
2012MNRAS.419.2893A Altcode: 2011MNRAS.tmp.1917A; 2011arXiv1110.0277A
The combined impact of rotation and magnetic fields on oscillations of
stellar fluids is still not well known theoretically. It mixes Alfvén
and inertial waves. Neutron stars are a place where both effects
may be at work. We aim to solve this problem in the context of the
r-mode instability in neutron stars, as it appears when these modes
are coupled to gravitational radiation. We consider a rotating
spherical shell filled with a viscous fluid of infinite electrical
conductivity and analyse propagation of model perturbations when
a dipolar magnetic field is bathing the fluid layer. We perform an
extensive numerical analysis and find that the m= 2 r-mode oscillation
is influenced by the magnetic field when the Lehnert number (the ratio
of Alfvén speed to rotation speed) exceeds a value proportional to
the one-fourth power of the Ekman number (a non-dimensional measure
of viscosity). This scaling is interpreted as the coincidence of the
width of internal shear layers of inertial modes and the wavelength of
the Alfvén waves. Applied to the case of rotating magnetic neutron
stars, we find that dipolar magnetic fields above 1014
G are necessary to perturb the r-mode instability.

---------------------------------------------------------
Title: Rotation on the oscillation spectrum of solar-like stars
Authors: Suárez, J. C.; Goupil, M. -J.; Reese, D. R.; Samadi, R.;
Lignières, F.; Rieutord, M.; Lochard, J.
2011sf2a.conf..249S Altcode:
One of the main sources of uncertainty in the asteroseismic models
of solar-like stars is the poor match between predicted oscillation
frequencies and observed ones in the very high frequency domain. Today,
such deviation is usually corrected by fitting the affected
frequencies with polynomials which are then physically explained by
possible effects of turbulence, diffusion, etc., i.e., the so-called
“surface effects". In this work, we show that the effect of the stellar
deformation due to rotation is of the same order or even larger than
the aforementioned surface effects. Moreover, we show that rotation
effects, even for the low velocities generally observed in solar-like
stars, becomes important for the asteroseismic analysis and cannot be
neglected when modeling such stars.

---------------------------------------------------------
Title: Gravity darkening in rotating stars
Authors: Espinosa Lara, F.; Rieutord, M.
2011A&A...533A..43E Altcode: 2011arXiv1109.3038E
Context. Interpretation of interferometric observations of rapidly
rotating stars requires a good model of their surface effective
temperature. Until now, laws of the form T_eff∝ g_eff^β have been
used, but they are only valid for slowly rotating stars. Aims:
We propose a simple model that can describe the latitudinal variations
in the flux of rotating stars at any rotation rate. Methods:
This model assumes that the energy flux is a divergence-free vector
that is antiparallel to the effective gravity. Results: When
mass distribution can be described by a Roche model, the latitudinal
variations in the effective temperature only depend on a single
parameter, namely the ratio of the equatorial velocity to the Keplerian
velocity. We validate this model by comparing its predictions to those
of the most realistic two-dimensional models of rotating stars issued
from the ESTER code. The agreement is very good, as it is with the
observations of two rapidly rotating stars, α Aql and α Leo. Conclusions: We suggest that as long as a gray atmosphere can be
accepted, the inversion of data on flux distribution coming from
interferometric observations of rotating stars uses such a model,
which has just one free parameter.

---------------------------------------------------------
Title: The effects of μ gradients on pulsations of rapidly rotating
stars
Authors: Reese, Daniel R.; Espinosa Lara, Francisco; Rieutord, Michel
2011IAUS..272..535R Altcode:
Recently, Reese et al. (2008), Lignières &amp; Georgeot (2008)
and Lignières &amp; Georgeot (2009) showed that the frequencies of
low-degree acoustic modes in rapidly rotating stars, also known as
“island modes”, follow an asymptotic formula, the coefficients
of which can be deduced from ray dynamics. We investigate how this
asymptotic behaviour is affected by μ gradients by comparing pulsation
spectra from models with and without such a discontinuity.

---------------------------------------------------------
Title: The Sun's Supergranulation
Authors: Rieutord, Michel; Rincon, François
2010LRSP....7....2R Altcode: 2010arXiv1005.5376R
The Sun's supergranulation refers to a physical pattern covering
the surface of the quiet Sun with a typical horizontal scale of
approximately 30,000 km and a lifetime of around 1.8 d. Its most
noticeable observable signature is as a fluctuating velocity
field of 360 m st-1 rms whose components are mostly
horizontal. Supergranulation was discovered more than fifty years ago,
however explaining why and how it originates still represents one of
the main challenges of modern solar physics.

---------------------------------------------------------
Title: On the Interpretation of Echelle Diagrams for Solar-like
Oscillations Effect of Centrifugal Distortion
Authors: Suárez, J. C.; Goupil, M. J.; Reese, D. R.; Samadi, R.;
Lignières, F.; Rieutord, M.; Lochard, J.
2010ApJ...721..537S Altcode: 2010arXiv1009.0123S
This work aims at determining the impact of slow to moderate rotation on
the regular patterns often present in solar-like oscillation spectra,
i.e., the frequency spacings. We focus on the well-known asteroseismic
diagnostic echelle diagrams, examining how rotation may modify the
estimates of the large and small spacings, as well as the identification
of modes. We illustrate the work with a real case: the solar-like
star ηBootis. We study a main-sequence 1.3 M sun star
as a typical case. The modeling takes into account rotation effects
on the equilibrium models through an effective gravity and on the
oscillation frequencies through both perturbative and non-perturbative
calculations. We compare the results of both type of calculations in the
context of the regular spacings (like the small spacings and the scaled
small spacings) and echelle diagrams. We show that for echelle diagrams
the perturbative approach remains valid for rotational velocities
up to 40-50 km s-1. We show that for the rotational
velocities measured in solar-like stars, i.e., vsin i up to 20-30 km
s-1, rotation effects must be taken into account in the
modeling for a correct interpretation of the observed oscillations. In
particular, theoretical oscillation frequencies must be corrected up to
the second order in terms of rotation rate, including near degeneracy
effects. For rotational velocities of about 16 km s-1
and higher, diagnostics on large spacings and on modal identification
through echelle diagrams can be significantly altered by the presence of
the m ≠ 0 components of the rotationally split modes. We found these
effects to be detectable in the observed frequency range. Analysis of
the effects of rotation on small spacings and scaled small spacings
reveals that these can be of the order of or even larger than surface
effects, typically turbulence, microscopic diffusion, etc. Furthermore,
we show that scaled spacings are significantly affected by stellar
distortion even for small stellar rotational velocities (from 10 to
15 km s-1) and therefore some care must be taken when using
them as indicators for probing deep stellar interiors.

---------------------------------------------------------
Title: Gravity modes in rapidly rotating stars. Limits of perturbative
methods
Authors: Ballot, J.; Lignières, F.; Reese, D. R.; Rieutord, M.
2010A&A...518A..30B Altcode: 2010arXiv1005.0275B
Context. CoRoT and Kepler missions are now providing high-quality
asteroseismic data for a large number of stars. Among intermediate-mass
and massive stars, fast rotators are common objects. Taking the
rotation effects into account is needed to correctly understand,
identify, and interpret the observed oscillation frequencies of
these stars. A classical approach is to consider the rotation as a
perturbation. Aims: In this paper, we focus on gravity modes,
such as those occurring in γ Doradus, slowly pulsating B (SPB), or Be
stars. We aim to define the suitability of perturbative methods. Methods: With the two-dimensional oscillation program (TOP),
we performed complete computations of gravity modes - including the
Coriolis force, the centrifugal distortion, and compressible effects -
in 2D distorted polytropic models of stars. We started with the modes
ℓ = 1, n = 1-14, and ℓ = 2-3, n = 1-5, 16-20 of a nonrotating star,
and followed these modes by increasing the rotation rate up to 70% of
the break-up rotation rate. We then derived perturbative coefficients
and determined the domains of validity of the perturbative methods. Results: Second-order perturbative methods are suited to computing
low-order, low-degree mode frequencies up to rotation speeds ~100 km
s-1 for typical γ Dor stars or ~150 km s-1 for
B stars. The domains of validity can be extended by a few tens of km
s-1 thanks to the third-order terms. For higher order modes,
the domains of validity are noticeably reduced. Moreover, perturbative
methods are inefficient for modes with frequencies lower than the
Coriolis frequency 2Ω. We interpret this failure as a consequence of
a modification in the shape of the resonant cavity that is not taken
into account in the perturbative approach.

---------------------------------------------------------
Title: On the power spectrum of solar surface flows
Authors: Rieutord, M.; Roudier, T.; Rincon, F.; Malherbe, J. -M.;
Meunier, N.; Berger, T.; Frank, Z.
2010A&A...512A...4R Altcode: 2009arXiv0911.3319R
Context. The surface of the Sun provides us with a unique and
very detailed view of turbulent stellar convection. Studying its
dynamics can therefore help us make significant progress in stellar
convection modelling. Many features of solar surface turbulence like
the supergranulation are still poorly understood. Aims: The aim
of this work is to give new observational constraints on these flows
by determining the horizontal scale dependence of the velocity and
intensity fields, as represented by their power spectra, and to offer
some theoretical guidelines to interpret these spectra. Methods:
We use long time-series of images taken by the Solar Optical Telescope
(SOT) on board the Hinode satellite; we reconstruct both horizontal
(by granule tracking) and vertical (by Doppler effect) velocity
fields in a field-of-view of ~ 75 × 75 Mm2. The dynamics
in the subgranulation range can be investigated with unprecedented
precision thanks to the absence of seeing effects and the use of the
modulation transfer function of SOT for correcting the spectra. 
Results: At small subgranulation scales down to 0.4 Mm the spectral
density of kinetic energy associated with vertical motions exhibits
a k-10/3-like power law, while the intensity fluctuation
spectrum follows either a k-17/3 or a k-3-like
power law at the two continuum levels investigated (525 and 450
nm respectively). We discuss the possible physical origin of these
scalings and interpret the combined presence of k-17/3 and
k-10/3 power laws for the intensity and vertical velocity
as a signature of buoyancy-driven turbulent dynamics in a strongly
thermally diffusive regime. In the mesogranulation range and up to a
scale of 25 Mm, we find that the amplitude of the vertical velocity
field decreases like λ-3/2 with the horizontal scale
λ. This behaviour corresponds to a k2 spectral power
law. Still in the 2.5-10 Mm mesoscale range, we find that intensity
fluctuations in the blue continuum also follow a k2
power law. In passing we show that granule tracking cannot sample
scales below 2.5 Mm. We finally further confirm the presence of a
significant supergranulation energy peak at 30 Mm in the horizontal
velocity power spectrum and show that the emergence of a pore erases
this spectral peak. We tentatively estimate the scale height of the
vertical velocity field in the supergranulation range and find 1 Mm;
this value suggests that supergranulation flows are shallow.

---------------------------------------------------------
Title: Viscous dissipation by tidally forced inertial modes in a
rotating spherical shell
Authors: Rieutord, M.; Valdettaro, L.
2010JFM...643..363R Altcode: 2009arXiv0909.3922R
We investigate the properties of forced inertial modes of a rotating
fluid inside a spherical shell. Our forcing is tidal like, but its main
property is that it is on the large scales. Our solutions first confirm
some analytical results obtained on a two-dimensional model by Ogilvie
(2005). We also note that as the frequency of the forcing varies,
the dissipation varies drastically if the Ekman number E is low (as is
usually the case). We then investigate the three-dimensional case and
compare the results to the foregoing model. These solutions show, like
their 2D counterpart, a spiky dissipation curve when the frequency of
the forcing is varied; they also display small frequency intervals where
the viscous dissipation is independent of viscosity. However, we show
that the response of the fluid in these frequency intervals is crucially
dominated by the shear layer that is emitted at the critical latitude on
the inner sphere. The asymptotic regime is reached when an attractor has
been excited by this shear layer. This property is not shared by the
two-dimensional model. Finally, resonances of the three-dimensional
model correspond to some selected least-damped eigenmodes. Unlike
their two-dimensional counter parts these modes are not associated
with simple attractors; instead, they show up in frequency intervals
with a weakly contracting web of characteristics. Besides, we show that
the inner core is negligible when its relative radius is less than the
critical value 0.4E^{1/5}. For these spherical shells, the full sphere
solutions give a good approximation of the flows (abridged abstract).

---------------------------------------------------------
Title: Tidal instability in stellar and planetary binary systems
Authors: Le Bars, M.; Lacaze, L.; Le Dizès, S.; Le Gal, P.;
Rieutord, M.
2010PEPI..178...48L Altcode:
In this paper, we combine theoretical and experimental approaches to
study the tidal instability in planetary liquid cores and stars. We
demonstrate that numerous complex modes can be excited depending on
the relative values of the orbital angular velocity Ωorbit
and of the spinning angular velocity Ωspin, except in a
stable range characterized by Ωspin/ Ωorbit
∈ [-1;1/3]. Even if the tidal deformation is small, its subsequent
instability - coming from a resonance process - may induce motions
with large amplitude, which play a fundamental role at the planetary
scale. This general conclusion is illustrated in the case of Jupiter's
moon Io by a coupled model of synchronization, demonstrating the
importance of energy dissipation by elliptical instability.

---------------------------------------------------------
Title: Supergranulation, Network Formation, and TFGs Evolution from
Hinode Observations
Authors: Roudier, T.; Rincon, F.; Rieutord, M.; Brito, D.; Beigbeder,
F.; Parès, L.; Malherbe, J. -M.; Meunier, N.; Berger, T.; Frank, Z.
2009ASPC..415..203R Altcode:
In this paper, we analyse a a 48h high-resolution time sequence of the
quiet Sun photosphere obtained with the Solar Optical Telescope onboard
Hinode. Using floating corks advected by velocity fields inferred from
photometry measurements, we show that long-living Trees of Fragmenting
Granules play a crucial role in the advection of small-scale magnetic
fields and in the build-up of the magnetic network.

---------------------------------------------------------
Title: Gravity modes in rapidly rotating polytropic stars
Authors: Ballot, J.; Lignières, F.; Reese, D. R.; Rieutord, M.
2009arXiv0912.1679B Altcode:
Using the Two-dimensional Oscillation Program (TOP), we have
explored the effects of rapid rotation on gravity modes in polytropic
stars. Coriolis force, centrifugal distortion as well as compressible
effects have been taken into account. Thanks to our complete
calculation, we have first studied the validity domain of perturbative
methods and started to explore properties of these modes. We focus on
l=1 in this analysis.

---------------------------------------------------------
Title: On the dynamics of a radiative rapidly rotating star
Authors: Rieutord, M.; Espinosa Lara, F.
2009CoAst.158...99R Altcode:
The envelope of massive rapidly rotating stars is pervaded by
baroclinic fluid flows, namely a differential rotation with a meridional
circulation and likely a small-scale turbulence. We present here some
of the first results of the ESTER project (Evolution STEllaire en
Rotation). More specifically, using for the first time the spheroidal
geometry, we give the shape of the differential rotation and show that
the baroclinic torque imposes a fast rotating core and a slow envelope,
together with a slow pole and a fast equator. The angular velocity of
the core is 50% larger than that of the envelope.

---------------------------------------------------------
Title: Mesoscale dynamics on the Sun's surface from HINODE
observations
Authors: Roudier, Th.; Rieutord, M.; Brito, D.; Rincon, F.; Malherbe,
J. M.; Meunier, N.; Berger, T.; Frank, Z.
2009A&A...495..945R Altcode: 2009arXiv0902.2299R
Context: Aims: The interactions of velocity scales on the Sun's
surface, from granulation to supergranulation are still not understood,
nor are their interaction with magnetic fields. We thus aim at giving
a better description of dynamics in the mesoscale range which lies
between the two scales mentioned above. Methods: We analyse a
48 h high-resolution time sequence of the quiet Sun photosphere at
the disk center obtained with the Solar Optical Telescope onboard
Hinode. The observations, which have a field of view of 100´´ ×
100´´, typically contain four supergranules. We monitor in detail the
motion and evolution of granules as well as those of the radial magnetic
field. Results: This analysis allows us to better characterize
Trees of Fragmenting Granules issued from repeated fragmentation of
granules, especially their lifetime statistics. Using floating corks
advected by measured velocity fields, we show their crucial role
in the advection of the magnetic field and in the build up of the
network. Finally, thanks to the long duration of the time series, we
estimate that the turbulent diffusion coefficient induced by horizontal
motion is approximately 430 km2 s-1. Conclusions: These results demonstrate that the long living families
contribute to the formation of the magnetic network and suggest that
supergranulation could be an emergent length scale building up as small
magnetic elements are advected and concentrated by TFG flows. Our
estimate for the magnetic diffusion associated with this horizontal
motion might provide a useful input for mean-field dynamo models.

---------------------------------------------------------
Title: Development of large and fast cmos aps cameras at latt
Authors: Beigbeder, F.; Bourrec, E.; Dupieux, M.; Delaigue, G.; Rondi,
S.; Rieutord, M.; Meunier, N.; Roudier, T.
2009EAS....37..301B Altcode:
Since 2004, at the Laboratoire d'Astrophysique de Toulouse Tarbes
(LATT), we work with CMOS APS detectors, firstly to develop a
large-field,high-resolution camera for the observation of the solar
supergranulation, secondly to develop a fast camera for an adaptive
optics test bench. In these two projects, we use detectors from
FillFactory, now continued by Cypress Semiconductor Corporation:
IBIS4-14000, IBIS-16000, LUPA-4000, 14 Mpixels, 16 Mpixels, 4 Mpixels
respectively. The last one just reads in a 240 × 240 pixels window to
obtain readout rate of 1000 Image/s. For these purposes we developed
dedicated controllers to follow the high pixel rate and multi-output
readout of this type of detectors. We also studied the characterization
methods and measured the main parameters of these CMOS detectors to
know their behaviour. Using these kinds of APS detectors in these
two particular projects proves that we can already find niches to
use CMOS detectors in astronomy taking advantage of their present
specificities. Recent improvements like back illumination, noise
reduction, should rapidly open news possibilities.

---------------------------------------------------------
Title: Approaching the Low-Frequency Spectrum of Rotating Stars
Authors: Rieutord, M.
2009LNP...765..101R Altcode:
In this lecture I present the basic knowledge needed to understand
the properties of the low-frequency spectrum of rotating stars. This
spectrum is a mixture of inertial and gravity modes. These modes both
have singularities in the limit of vanishing diffusion for a generic
container. I explain the nature and the role of these singularities;
I also discuss the way these modes can be computed and the actual
difficulties that need to be circumvented to get sensible results.

---------------------------------------------------------
Title: Solar Convection Dynamics Derived from Long Time Series
Observations
Authors: Hanslmeier, A.; Roudier, Th.; Rieutord, M.; Muller, R.
2009CEAB...33...39H Altcode:
Long time series of solar granulation are extremely difficult to
be obtained from ground based observations because of the unstable
Earth's atmosphere. The Hinode-SOT instrument provided long term
stable time series of solar granulation at different wavelengths in
the visible. After appropriate calibration, these data can be used
for studies of long time series of several hours. In this study we
concentrate on the question of whether granulation can be considered
as an ergodic phenomenon. The answer to such question is very important
when comparing observational results with theoretical models since these
models are limited either in time or in the extension of the spatial
grid. We have analysed a series of 8 h of Hinode-SOT blue continuum
images, the average separation between the successive images was at
maximum about 50 sec. The images were aligned in order to minimize
tracking problems. A quiet region located near the solar disc centre at
the beginning of the observations was selected. The analysis shows that
it seems to be that solar granulation is at least near the behaviour
of ergodicity. That means, that the behaviour along the time axis and
along a spatial coordinate become similar on a long interval. From
sufficient spatial sampling the time behaviour could be derived.

---------------------------------------------------------
Title: Photospheric flows around a quiescent filament at Large and
small scale and their ffects on filament destabilization
Authors: Roudier, Th.; Malherbe, J. M.; Švanda, M.; Molodij, G.;
Keil, S.; Sütterlin, P.; Schmieder, B.; Bommier, V.; Aulanier, G.;
Meunier, N.; Rieutord, M.; Rondi, S.
2008sf2a.conf..569R Altcode:
We study the influence of large and small scales photospheric
motions on the destabilization of an eruptive filament, observed
on October 6, 7, and 8, 2004 as part of an international observing
campaign (JOP 178). Large-scale horizontal flows are invetigated
from a series of MDI/SOHO full-disc Dopplergrams and magnetograms
from THEMIS. Small-scale horizontal flows were derived using local
correlation tracking on TRACE satellite, Dutch Open Telescope (DOT)
and The Dunn Solar telescope (DST) data. The topology of the flow field
changed significantly during the filament eruptive phase, suggesting
a possible coupling between the surface flow field and the coronal
magnetic field. We measured an increase of the shear below the point
where the eruption starts and a decrease in shear after the eruption. We
conclude that there is probably a link between changes in surface flow
and the disappearance of the eruptive filament.

---------------------------------------------------------
Title: CoRoT Measures Solar-Like Oscillations and Granulation in
Stars Hotter Than the Sun
Authors: Michel, Eric; Baglin, Annie; Auvergne, Michel; Catala,
Claude; Samadi, Reza; Baudin, Frédéric; Appourchaux, Thierry; Barban,
Caroline; Weiss, Werner W.; Berthomieu, Gabrielle; Boumier, Patrick;
Dupret, Marc-Antoine; Garcia, Rafael A.; Fridlund, Malcolm; Garrido,
Rafael; Goupil, Marie-Jo; Kjeldsen, Hans; Lebreton, Yveline; Mosser,
Benoît; Grotsch-Noels, Arlette; Janot-Pacheco, Eduardo; Provost,
Janine; Roxburgh, Ian W.; Thoul, Anne; Toutain, Thierry; Tiphène,
Didier; Turck-Chieze, Sylvaine; Vauclair, Sylvie D.; Vauclair,
Gérard P.; Aerts, Conny; Alecian, Georges; Ballot, Jérôme;
Charpinet, Stéphane; Hubert, Anne-Marie; Lignières, François;
Mathias, Philippe; Monteiro, Mario J. P. F. G.; Neiner, Coralie;
Poretti, Ennio; Renan de Medeiros, José; Ribas, Ignasi; Rieutord,
Michel L.; Roca Cortés, Teodoro; Zwintz, Konstanze
2008Sci...322..558M Altcode: 2008arXiv0812.1267M
Oscillations of the Sun have been used to understand its interior
structure. The extension of similar studies to more distant stars
has raised many difficulties despite the strong efforts of the
international community over the past decades. The CoRoT (Convection
Rotation and Planetary Transits) satellite, launched in December 2006,
has now measured oscillations and the stellar granulation signature
in three main sequence stars that are noticeably hotter than the
sun. The oscillation amplitudes are about 1.5 times as large as
those in the Sun; the stellar granulation is up to three times as
high. The stellar amplitudes are about 25% below the theoretic values,
providing a measurement of the nonadiabaticity of the process ruling
the oscillations in the outer layers of the stars.

---------------------------------------------------------
Title: Supergranules over the solar cycle
Authors: Meunier, N.; Roudier, T.; Rieutord, M.
2008A&A...488.1109M Altcode:
Context: The origin of supergranulation has not been understood
yet. Contradictory results have been obtained in the past concerning
the relation between supergranule properties (mostly cell size) and
the solar cycle. Aims: We propose to study the variation in
supergranule cell sizes and velocity fields over the solar cycle, as
well as the intensity variation inside supergranules. Methods:
We define supergranule cells from maps of horizontal velocity field
divergences. The flow fields are derived from MDI/SOHO intensity
maps. The intensity variation in supergranules are compared to the
variation inside granules resulting from a numerical simulation. The
variation in these profiles with the cell size and over the solar cycle
is also analysed. Results: We find that cell sizes are smaller
on average at cycle maximum. We also find that the slope between Log
(V_rms) and Log (R) is weakly correlated with the spot number (i.e. the
global activity level) but anti-correlated with the local magnetic
field. We also confirm the decrease in the intensity variation from
cell centre to the boundary, which puts a lower limit on the temperature
variation of 0.57 ± 0.06 K. This temperature difference is of 1.03 ±
0.05 K when considering the areas of strongest divergence and strongest
convergence. We observe a strong similarity in the intensity variation
inside supergranules and granules. A significant variation with the
cell size is observed, also similar to that in granules, but the
variation over the solar cycle is not significant. Conclusions:
The sign of the variation in supergranule cell sizes over the solar
cycle is in agreement with what can be expected from the influence
of the magnetic field. The observed intensity variations show that a
common process could be the origin of both granules and supergranules.

---------------------------------------------------------
Title: Dynamics of the Solar Surface Reavealed by Granule Tracking
Authors: Rieutord, M.; Roudier, T.
2008ESPM...12..2.9R Altcode:
We present the recent results obtained by the Toulouse-Tarbes group
on the dynamics of the solar surface at meso and supergranular scale,
and their interactions with the magnetic field. First, using the
14-Mpixel CALAS camera at the Pic-du-Midi observatory, we obtained
a 7.5h-long sequence of high resolution images with unprecedented
field size (524"x350"). Tracking granules, we have determined the
velocity field at the Sun's surface in great detail from a scale of
2.5 Mm up to 250 Mm. The kinetic energy density spectrum shows that
supergranulation peaks at 36Mm and spans on scales ranging between 20Mm
and 75Mm. The decrease of supergranular flows in the small scales is
close to a 1/k^2-power law, steeper than the equipartition Kolmogorov
one. The probability distribution function of the divergence field shows
the signature of intermittency of the supergranulation and thus its
turbulent nature. Second, using a 48hours-long sequence of white
light images from the HINODE satellite, we determined the evolution of
granule families, which give birth to correlated motion at mesogranular
scale. Using passive cork and magnetic field observations, we show
that the surface velocity field via family organization is sufficient
to explain the magnetic field concentrations at the Sun's surface,
namely the magnetic network. We also find robust scaling laws for the
life time of granule families.

---------------------------------------------------------
Title: Regular patterns in the acoustic spectrum of rapidly rotating
stars
Authors: Reese, D.; Lignières, F.; Rieutord, M.
2008A&A...481..449R Altcode: 2008arXiv0801.4630R
Context: Rapid rotation modifies the structure of the frequency spectrum
of pulsating stars, thus making mode identification difficult. Aims: We look for new forms of organisation for the frequency spectrum
that can provide a basis for mode identification at high rotation
rates. Methods: Acoustic modes in uniformly rotating polytropic
models of stars are computed using a numerical code that fully takes the
effects of rotation (centrifugal distortion and Coriolis acceleration)
into account. All low-degree modes, ℓ = 0 to 3, with radial orders
n = 1-10 and 21-25 for N = 3 polytropic models and n = 1-10 for N =
1.5 polytropic models are followed from a zero rotation rate up to
59% of the break-up velocity. Results: We find an empirical
formula that gives a good description of the high-frequency range of
the computed acoustic spectrum for high rotation rates. Differences
between this formula and complete eigenmode calculations are shown
to be substantially smaller than those obtained with a 3rd order
perturbative method valid at low rotation rates.

---------------------------------------------------------
Title: Large-scale horizontal flows in the solar
photosphere. III. Effects on filament destabilization
Authors: Roudier, T.; Švanda, M.; Meunier, N.; Keil, S.; Rieutord,
M.; Malherbe, J. M.; Rondi, S.; Molodij, G.; Bommier, V.; Schmieder, B.
2008A&A...480..255R Altcode: 2007arXiv0711.3112R
Aims:We study the influence of large-scale photospheric motions on
the destabilization of an eruptive filament, observed on October 6,
7, and 8, 2004, as part of an international observing campaign (JOP
178). Methods: Large-scale horizontal flows were investigated
from a series of MDI full-disc Dopplergrams and magnetograms. From
the Dopplergrams, we tracked supergranular flow patterns using the
local correlation tracking (LCT) technique. We used both LCT and manual
tracking of isolated magnetic elements to obtain horizontal velocities
from magnetograms. Results: We find that the measured flow
fields obtained by the different methods are well-correlated on large
scales. The topology of the flow field changed significantly during
the filament eruptive phase, suggesting a possible coupling between
the surface flow field and the coronal magnetic field. We measured
an increase in the shear below the point where the eruption starts
and a decrease in shear after the eruption. We find a pattern in the
large-scale horizontal flows at the solar surface that interact with
differential rotation. Conclusions: We conclude that there is
probably a link between changes in surface flow and the disappearance
of the eruptive filament.

---------------------------------------------------------
Title: Solar supergranulation revealed by granule tracking
Authors: Rieutord, M.; Meunier, N.; Roudier, T.; Rondi, S.; Beigbeder,
F.; Parès, L.
2008A&A...479L..17R Altcode: 2008arXiv0801.1369R
Context: Supergranulation is a pattern of the velocity field at the
surface of the Sun, which has been known about for more than fifty
years, however, no satisfactory explanation of its origin has been
proposed. Aims: New observational constraints are therefore
needed to guide theoretical approaches which hesitate between scenarios
that either invoke a large-scale instability of the surface turbulent
convection or a direct forcing by buoyancy. Methods: Using the
14-Mpixel CALAS camera at the Pic-du-Midi observatory, we obtained
a 7.5 h-long sequence of high resolution images with unprecedented
field size. Tracking granules, we have determined the velocity field
at the Sun's surface in great detail from a scale of 2.5 Mm up to
250 Mm. Results: The kinetic energy density spectrum shows that
supergranulation peaks at 36 Mm and spans on scales ranging between 20
Mm and 75 Mm. The decrease of supergranular flows in the small scales
is close to a k-2-power law, steeper than the equipartition
Kolmogorov one. The probability distribution function of the divergence
field shows the signature of intermittency of the supergranulation
and thus its turbulent nature. Conclusions:

---------------------------------------------------------
Title: The dynamics of rotating fluids and binary stars
Authors: Rieutord, M.
2008EAS....29..127R Altcode:
In this lecture I try to explain the basic concepts related to fluid
motions when a background rotation dominates the flows. In particular,
the notions of geostrophic flow, Ekman layer, Ekman circulation are
explained. However, the main focus of this lecture is on the eigenmodes
which occur in such a context and I emphasize here the special effects
conveyed to inertial and gravito-inertial modes by the hyperbolic
nature of the governing operators. A final section introduces the case
of the elliptic instability which I recently suggested as one of the
processes of binary synchronization.

---------------------------------------------------------
Title: HINODE SOT Observations - First Preliminary Analysis
Authors: Hanslmeier, A.; Müller, R.; Roudier, Th.; Rieutord, M.
2008CEAB...32...25H Altcode:
In this paper we present some preliminary analysis of Hinode-SOT
data: time series as well as synoptic data. We show that the data
are influenced by periodic intensity variations as well as bad images
appear. This should be taken into account when analysing the data.

---------------------------------------------------------
Title: The dynamics of a fully radiative rapidly rotating star
enclosed within a spherical box
Authors: Espinosa Lara, F.; Rieutord, M.
2007A&A...470.1013E Altcode:
Context: Recent results from interferometry and asteroseismology require
models of rapidly rotating stars that are more and more precise. Aims: We describe the basic structure and the hydrodynamics of a fully
radiative star as a preliminary step towards more realistic models of
rotating stars. Methods: We consider a solar mass of perfect
gas enclosed in a spherical container. The gas is self-gravitating
and rotating, and is the seat of nuclear heating, and heat diffusion
is due to radiative diffusion with Kramers type opacities. Equations
are solved numerically with spectral methods in two dimensions with
a radial Gauss-Lobatto grid and spherical harmonics. Results:
We computed the centrifugally flattened structure of such a star: the
von Zeipel model, which says that the energy flux is proportional to
the local effective gravity is tested. We show that it overestimates
the ratio of the polar to the equatorial energy flux by almost a factor
2. We also determine the Brunt-Väisälä frequency distribution and
show that outer equatorial regions in a radiative zone are convectively
unstable when the rotation is fast enough. We compute the differential
rotation and meridional circulation stemming from the baroclinicity of
the star and show that, in such radiative zones, equatorial regions
rotate faster than polar ones. The surface differential rotation is
also shown to reach a universal profile when rotation is slow enough
(less than 36% of the breakup one), as long as viscosity and Prandlt
number remain small.

---------------------------------------------------------
Title: Convergence and round-off errors in a two-dimensional
eigenvalue problem using spectral methods and Arnoldi-Chebyshev
algorithm
Authors: Valdettaro, Lorenzo; Rieutord, Michel; Braconnier, Thierry;
Fraysse, Valerie
2007JCoAM.205..382V Altcode: 2006physics...4219V
An efficient way of solving 2D stability problems in fluid mechanics is
to use, after discretization of the equations that cast the problem
in the form of a generalized eigenvalue problem, the incomplete
Arnoldi-Chebyshev method. This method preserves the banded structure
sparsity of matrices of the algebraic eigenvalue problem and thus
decreases memory use and CPU time consumption. The errors that affect
computed eigenvalues and eigenvectors are due to the truncation in
the discretization and to finite precision in the computation of the
discretized problem. In this paper we analyze those two errors and
the interplay between them. We use as a test case the 2D eigenvalue
problem yielded by the computation of inertial modes in a spherical
shell. This problem contains many difficulties that make it a very good
test case. It turns out that single modes (especially most-damped modes
i.e. with high spatial frequency) can be very sensitive to roundoff
errors, even when apparently good spectral convergence is achieved. The
influence of roundoff errors is analyzed using the spectral portrait
technique and by comparison of double precision and extended precision
computations. Through the analysis we give practical recipes to control
the truncation and roundoff errors on eigenvalues and eigenvectors.

---------------------------------------------------------
Title: Tracking granules on the Sun's surface and reconstructing
velocity fields. I. The CST algorithm
Authors: Rieutord, M.; Roudier, T.; Roques, S.; Ducottet, C.
2007A&A...471..687R Altcode: 2007arXiv0707.1991R
Aims:Determination of horizontal velocity fields on the solar
surface is crucial for understanding the dynamics of structures like
mesogranulation or supergranulation or simply the distribution of
magnetic fields. Methods: We pursue here the development of a
method called CST for coherent structure tracking, which determines the
horizontal motion of granules in the field of view. Results: We
first devise a generalization of Strous method for the segmentation of
images and show that when segmentation follows the shape of granules
more closely, granule tracking is less effective for large granules
because of increased sensitivity to granule fragmentation. We then
introduce the multi-resolution analysis on the velocity field, based on
Daubechies wavelets, which provides a view of this field on different
scales. An algorithm for computing the field derivatives, like the
horizontal divergence and the vertical vorticity, is also devised. The
effects from the lack of data or from terrestrial atmospheric distortion
of the images are also briefly discussed.

---------------------------------------------------------
Title: Tracking granules on the Sun's surface and reconstructing
velocity fields. II. Error analysis
Authors: Tkaczuk, R.; Rieutord, M.; Meunier, N.; Roudier, T.
2007A&A...471..695T Altcode: 2007arXiv0707.1994T
Context: The determination of horizontal velocity fields at the solar
surface is crucial to understanding the dynamics and magnetism of the
convection zone of the sun. These measurements can be done by tracking
granules. Aims: Tracking granules from ground-based observations,
however, suffers from the Earth's atmospheric turbulence, which
induces image distortion. The focus of this paper is to evaluate the
influence of this noise on the maps of velocity fields. Methods:
We use the coherent structure tracking algorithm developed recently
and apply it to two independent series of images that contain the same
solar signal. Results: We first show that a k-ω filtering of
the times series of images is highly recommended as a pre-processing
to decrease the noise, while, in contrast, using destretching should
be avoided. We also demonstrate that the lifetime of granules has a
strong influence on the error bars of velocities and that a threshold
on the lifetime should be imposed to minimize errors. Finally, although
solar flow patterns are easily recognizable and image quality is very
good, it turns out that a time sampling of two images every 21 s is
not frequent enough, since image distortion still pollutes velocity
fields at a 30% level on the 2500 km scale, i.e. the scale on which
granules start to behave like passive scalars. Conclusions:
The coherent structure tracking algorithm is a useful tool for noise
control on the measurement of surface horizontal solar velocity fields
when at least two independent series are available.

---------------------------------------------------------
Title: The dynamics of a fully radiative rapidly rotating star
Authors: Espinosa Lara, F.; Rieutord, M.
2007astro.ph..2255E Altcode:
Recent results from interferometry and asteroseismology require
models of rapidly rotating stars that are more and more precise. We
describe the basic structure and the hydrodynamics of a fully radiative
star as a preliminary step towards more realistic models of rotating
stars. We consider a solar mass of perfect gas enclosed in a spherical
container. The gas is self-gravitating and rotating, and is the seat
of nuclear heating, and heat diffusion is due to radiative diffusion
with Kramers type opacities. Equations are solved numerically with
spectral methods in two dimensions with a radial Gauss-Lobatto grid and
spherical harmonics. We computed the centrifugally flattened structure
of such a star: the von Zeipel model, which says that the energy flux
is proportional to the local effective gravity is tested. We show that
it overestimates the ratio of the polar to the equatorial energy flux
by almost a factor 2. We also determine the Brunt-Vaisala frequency
distribution and show that outer equatorial regions in a radiative
zone are convectively unstable when the rotation is fast enough. We
compute the differential rotation and meridional circulation stemming
from the baroclinicity of the star and show that, in such radiative
zones, equatorial regions rotate faster than polar ones. The surface
differential rotation is also shown to reach a universal profile when
rotation is slow enough (less than 36% of the breakup one), as long
as viscosity and Prandlt number remain small.

---------------------------------------------------------
Title: Modeling rapidly rotating stars
Authors: Rieutord, M.
2007astro.ph..2384R Altcode:
We review the quest of modeling rapidly rotating stars during the
past 40 years and detail the challenges to be taken up by models
facing new data from interferometry, seismology, spectroscopy... We
then present the progress of the ESTER project aimed at giving a
physically self-consistent model for the structure and evolution of
rapidly rotating stars.

---------------------------------------------------------
Title: Photospheric flows around a quiescent filament and CALAS
first results .
Authors: Rondi, S.; Roudier, Th.; Molodij, G.; Bommier, V.; Malherbe,
J. M.; Schmieder, B.; Meunier, N.; Rieutord, M.; Beigbeder., F.
2007MmSAI..78..114R Altcode:
The horizontal photospheric flows below and around a filament are one of
the components in the formation and evolution of filaments. Few studies
have been done so far because this requires multiwalength time sequences
with high spatial resolution. We present observations obtained in 2004
during the international JOP 178 campaign in which eleven instruments
were involved, from space and ground based observatories. Several
supergranulation cells are crossing the Polarity Inversion Line (PIL)
allowing the transport of magnetic flux through the PIL, in particular
the parasitic polarities. Before the filament eruptive phase, parasitic
and normal polarities are swept by a continuous diverging horizontal
flow located in the filament gap where the disappearance of the filament
starts. In the future, observations at high spatial resolution on a
large field-of-view would be very useful to study filaments, as they are
very large structures. We also present the first images obtained with
the use of our new 14 MPixel camera CALAS (CAmera for the LArge Scales
of the Solar Surface) (10 arcmin× 6.7 arcmin) . These are the first
large-scale and high-resolution images of the solar surface ever made.

---------------------------------------------------------
Title: Velocities and divergences as a function of supergranule size
Authors: Meunier, N.; Tkaczuk, R.; Roudier, Th.; Rieutord, M.
2007A&A...461.1141M Altcode:
Context: The origin of supergranulation is not understood yet and many
scenarios, which range from large-scale deep convection to large-scale
instabilities of surface granular flows, are possible. Aims: We
characterize the velocities and divergences in supergranulation cells
as a function of their size. Methods: Using local correlation
tracking, we determine the horizontal flow fields from MDI intensity
maps and derive the divergences. The smoothed divergences are used to
determine the cells for various spatial smoothings, in particular at the
supergranular scale. Results: We find evidence of intermittency
in the supergranular range and a correlation between the size of
supergranules and the strength of the diverging flow. We also show
that the relation between rms velocities and scale (the supergranule
radius R) can be represented by a law V_rms∼ R0.66. Conclusions: . The results issued from our data point towards a
scenario where supergranulation is a surface phenomenon of the sun,
probably the consequence of a large-scale instability triggered by
strong positive correlated rising flows.

---------------------------------------------------------
Title: The Seismology Programme of CoRoT
Authors: Michel, E.; Baglin, A.; Auvergne, M.; Catala, C.; Aerts,
C.; Alecian, G.; Amado, P.; Appourchaux, T.; Ausseloos, M.; Ballot,
J.; Barban, C.; Baudin, F.; Berthomieu, G.; Boumier, P.; Bohm, T.;
Briquet, M.; Charpinet, S.; Cunha, M. S.; De Cat, P.; Dupret, M. A.;
Fabregat, J.; Floquet, M.; Fremat, Y.; Garrido, R.; Garcia, R. A.;
Goupil, M. -J.; Handler, G.; Hubert, A. -M.; Janot-Pacheco, E.;
Lambert, P.; Lebreton, Y.; Lignieres, F.; Lochard, J.; Martin-Ruiz,
S.; Mathias, P.; Mazumdar, A.; Mittermayer, P.; Montalban, J.;
Monteiro, M. J. P. F. G.; Morel, P.; Mosser, B.; Moya, A.; Neiner,
C.; Nghiem, P.; Noels, A.; Oehlinger, J.; Poretti, E.; Provost, J.;
Renan de Medeiros, J.; De Ridder, J.; Rieutord, M.; Roca-Cortes, T.;
Roxburgh, I.; Samadi, R.; Scuflaire, R.; Suarez, J. C.; Theado, S.;
Thoul, A.; Toutain, T.; Turck-Chieze, S.; Uytterhoeven, K.; Vauclair,
G.; Vauclair, S.; Weiss, W. W.; Zwintz, K.
2006ESASP1306...39M Altcode: 2008arXiv0811.1080M
We introduce the main lines and specificities of the CoRoT Seismology
Core Programme. The development and consolidation of this programme has
been made in the framework of the CoRoT Seismology Working Group. With a
few illustrative examples, we show how CoRoT data will help to address
various problems associated with present open questions of stellar
structure and evolution.

---------------------------------------------------------
Title: The DynaMICS perspective
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
2006ESASP.624E..24T Altcode: 2006soho...18E..24T
No abstract at ADS

---------------------------------------------------------
Title: Acoustic oscillations of rapidly rotating polytropic
stars. II. Effects of the Coriolis and centrifugal accelerations
Authors: Reese, D.; Lignières, F.; Rieutord, M.
2006A&A...455..621R Altcode: 2006astro.ph..5503R
Context.With the launch of space missions devoted to asteroseismology
(like COROT), the scientific community will soon have accurate
measurements of pulsation frequencies in many rapidly rotating
stars. Aims.The present work focuses on the effects of rotation
on pulsations of rapidly rotating stars when both the Coriolis and
centrifugal accelerations require a non-perturbative treatment. Methods.We develop a 2-dimensional spectral numerical approach
which allows us to compute acoustic modes in centrifugally distorted
polytropes including the full influence of the Coriolis force. This
method is validated through comparisons with previous studies, and
the results are shown to be highly accurate. Results.In the
frequency range considered and with COROT's accuracy, we establish a
domain of validity for perturbative methods, thus showing the need for
complete calculations beyond v sin i=50 km s-1 for a R =
2.3 R_⊙, M=1.9 M_⊙ polytropic star. Furthermore, it is shown that
the main differences between complete and perturbative calculations
come essentially from the centrifugal distortion.

---------------------------------------------------------
Title: Acoustic oscillations of rapidly rotating polytropic
stars. I. Effects of the centrifugal distortion
Authors: Lignières, F.; Rieutord, M.; Reese, D.
2006A&A...455..607L Altcode: 2006astro.ph..4312L
Aims.A new non-perturbative method to compute accurate oscillation
modes in rapidly rotating stars is presented. Methods.The effect
of the centrifugal force is fully taken into account while the Coriolis
force is neglected. This assumption is valid when the time scale of
the oscillation is much shorter than the inverse of the rotation rate
and is expected to be suitable for high radial order p-modes of δ
Scuti stars. Axisymmetric p-modes have been computed in uniformly
rotating polytropic models of stars. Results.In the frequency
and rotation range considered, we found that as rotation increases
(i) the asymptotic structure of the non-rotating frequency spectrum is
first destroyed then replaced by a new form of organization (ii) the
mode amplitude tends to concentrate near the equator (iii) differences
to perturbative methods become significant as soon as the rotation rate
exceeds about fifteen percent of the Keplerian limit. The implications
for the seismology of rapidly rotating stars are discussed.

---------------------------------------------------------
Title: Scientific Objectives of the Novel Formation Flying Mission
Aspiics
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
2006ESASP.617E.164L Altcode: 2006soho...17E.164L
No abstract at ADS

---------------------------------------------------------
Title: The EUV Variability Experiment (EVE) on the Solar Dynamics
Observatory (SDO): Science Plan and Instrument Overview
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
2006ESASP.617E.165W Altcode: 2006soho...17E.165W
No abstract at ADS

---------------------------------------------------------
Title: The Dynamics Project
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
2006ESASP.617E.162T Altcode: 2006soho...17E.162T
No abstract at ADS

---------------------------------------------------------
Title: The dynamics of the radiative envelope of rapidly rotating
stars. I. A spherical Boussinesq model
Authors: Rieutord, M.
2006A&A...451.1025R Altcode: 2006astro.ph..2040R
Context.The observations of rapidly rotating stars are increasingly
detailed and precise thanks to interferometry and asteroseismology;
two-dimensional models taking into account the hydrodynamics of these
stars are very much needed. Aims.A model to study the dynamics
of baroclinic stellar envelopes is presented. Methods.This
model treats the stellar fluid with the Boussinesq approximation
and assumes that it is contained in a rigid spherical domain. The
temperature field and the rotation of the system generate the
baroclinic flow. Results.We give an analytical solution to the
asymptotic problem at small Ekman and Prandtl numbers. We show that,
provided the Brunt-Väisälä frequency profile is smooth enough,
differential rotation of a stably stratified envelope takes the form
a fast rotating pole and a slow equator while it is the opposite in
a convective envelope. We also show that at low Prandtl numbers and
without μ-barriers, the jump in viscosity at the core-envelope
boundary generates a shear layer staying along the tangential
cylinder of the core. Its role in mixing processes is discussed. Conclusions.Such a model provides an interesting tool to investigate
the fluid dynamics of rotating stars in particular for the study of
the various instabilities affecting baroclinic flows or a dynamo effect.

---------------------------------------------------------
Title: Modeling rapidly rotating stars
Authors: Rieutord, M.
2006sf2a.conf..501R Altcode:
We review the quest of modeling rapidly rotating stars during the
past 40 years and detail the challenges to be taken up by models
facing new data from interferometry, seismology, spectroscopy... We
then present the progress of the ESTER project aimed at giving a
physically self-consistent model for the structure and evolution of
rapidly rotating stars.

---------------------------------------------------------
Title: A CMOS Sensor for Solar Observation
Authors: Beigbeder, Francis; Rondi, Sylvain; Meunier, Nadège;
Rieutord, Michel
2006ASSL..336..123B Altcode: 2006sda..conf..123B
No abstract at ADS

---------------------------------------------------------
Title: Preface
Authors: Rieutord, M.; Dubrulle, B.
2006EAS....21D...3R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the dynamics of radiative zones in rotating stars
Authors: Rieutord, M.
2006EAS....21..275R Altcode: 2006astro.ph..8431R
In this lecture I try to explain the basic dynamical processes at work
in a radiative zone of a rotating star. In particular, the notion of
baroclinicity is thoroughtly discussed. Attention is specially directed
to the case of circulations and the key role of angular momentum
conservation is stressed. The specific part played by viscosity is
also explained. The old approach of Eddington and Sweet is reviewed
and criticized in the light of the seminal papers of Busse (1981)
and Zahn (1992). Other examples taken in the recent literature are
also presented; finally, I summarize the important points.

---------------------------------------------------------
Title: An introduction to thermal convection
Authors: Rieutord, M.
2006EAS....21....5R Altcode:
In this lecture I propose a little tour of thermal convection and
its applications in astrophysics. The first part of the lecture is
devoted to a qualitative introduction to the convective instability
using the Schwarzschild criterion; then, concentrating on the equations
governing the fluid motions, I introduce the Boussinesq and anelastic
approximations which are so often used in these problems. The following
part focuses on the Rayleigh-Bénard model which is worked out in detail
up to the Landau equation and the Lorenz strange attractor. Finally,
I briefly sketch out some results on turbulent convection and end the
lecture with the case of stellar convection.

---------------------------------------------------------
Title: Oscillations of rapidly rotating stars
Authors: Reese, D.; Lignieres, F.; Rieutord, M.
2006CoAst.147...65R Altcode:
The effects of rapid rotation on stellar pulsation is examined using
an accurate 2D numerical method. We compare the results of these
non-perturbative calculations with those of perturbative methods
and find that frequency differences exceed 0.08 muHz on half of
the modes when the rotation rate is 15% of the keplerian (break-up)
limit. The differences between the two results is mainly attributed
to the approximate treatment of the centrifugal force in perturbative
methods. We also explore different levels of approximation for the
Coriolis force.

---------------------------------------------------------
Title: Acoustic oscillations in centrifugally flattened polytropic
star .
Authors: Lignières, F.; Rieutord, M.; Reese, D.
2006MmSAI..77..425L Altcode:
We present numerical calculations of axisymmetric acoustic modes in
polytropic models of star deformed by the centrifugal force. In the
range of flatness and frequencies considered, we found that, as flatness
increases, (i) differences with perturbative methods becomes rapidly
significant (ii) the structure of the spectrum is strongly modified
(iii) the mode amplitude at the surface tends to concentrate near
the equator.

---------------------------------------------------------
Title: Stellar fluid dynamics and numerical simulations : from the
sun to neutron stars
Authors: Rieutord, Michel; Dubrulle, Berengere
2006EAS....21.....R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The foundations of fluid dynamics
Authors: Rieutord, M.
2006EAS....21....3R Altcode:
In this lecture I introduce the basic concepts needed to understand
fluid motions at a macroscopic level. Hence, the idea of a continuous
medium and its kinematical properties are presented. Then the physical
laws governing the motion (mass, momentum and energy equation) are
derived and discussed. The concept of rheological law is introduced
and illustrated. I end this lecture with a discussion of the boundary
conditions that are needed to complete the equations of motion.

---------------------------------------------------------
Title: CALAS, A Camera for the Large-scale of the Solar Surface
Authors: Meunier, N.; Rondi, S.; Tkaczuk, R.; Rieutord, M.;
Beigbeder, F.
2005ASPC..346...53M Altcode:
The origin of supergranulation (convective or not) is still
much debated. Among various possible approaches, one way to study
supergranulation is to observe the horizontal motions of granules. A
combination of a very large field-of-view (in order to see as
many supergranules as possible), a very high spatial resolution
(to sample granules with a high accuracy, even the small ones)
and a high cadence is necessary to study this pattern in detail. We
plan to implement a 4k×4k CMOS camera at the Lunette Jean Rösch
(previously Coupole Tourelle) at the Pic du Midi Observatory (a
50 cm diameter refractor). The main objective of the camera, CALAS
(Camera for the Large Scale of the Solar Surface), is the study of
supergranulation. This will allow to sample granules with a high spatial
resolution on a 10'×10' field-of-view (∼ 100 supergranules). We
present our objectives, the instrumental set-up and organization,
and the status of the instrument.

---------------------------------------------------------
Title: The ESTER project
Authors: Rieutord, M.; Dintrans, B.; Lignières, F.; Corbard, T.;
Pichon, B.
2005sf2a.conf..759R Altcode:
The ESTER project aims at building a stellar evolution code in
two dimensions of space for the study of effects of rotation. The
numerical scheme is based on spectral methods with a spherical harmonic
decomposition in the horizontal direction and a Chebyshev polynomial
expansion in the vertical direction. Coordinates adapted to the
centrifugally distorted shape are mapped to spherical coordinates. First
tests on rotating polytropes are presented.

---------------------------------------------------------
Title: Mesoscale flows in large aspect ratio simulations of turbulent
compressible convection
Authors: Rincon, F.; Lignières, F.; Rieutord, M.
2005A&A...430L..57R Altcode: 2006astro.ph.11843R
We present the results of a very large aspect ratio (A=42.6) numerical
simulation of fully compressible turbulent convection in a polytropic
atmosphere, and focus on the properties of large-scale flows. Mesoscale
patterns dominate the turbulent energy spectrum. We show that these
structures, which had already been observed in Boussinesq simulations
by \cite{cattaneo01}, have a genuine convective origin and do not result
directly from collective interactions of the smaller scales of the flow,
even though their growth is strongly affected by nonlinear transfers. If
this result is relevant to the solar photosphere, it suggests that the
dominant convective mode below the Sun's surface may be at mesoscales.

---------------------------------------------------------
Title: Shear Alfvén modes in magnetized spherical shells
Authors: Reese, D.; Rincon, F.; Rieutord, M.
2004sf2a.conf..267R Altcode: 2004sf2a.confE.300R
An investigation of shear Alfvén waves inside a spherical shell is
carried out, in which the background magnetic field is dipolar and
resistive effects are taken into account. Numerical results indicate
two basic behaviours for both the axisymmetric and non-axisymmetric
cases. Poloidal modes appear to remain regular, except for internal
shear layers, when kinetic and magnetic diffusivities become arbitrarily
small, whereas toroidal modes become singular. Analytical results
are provided for the axisymmetric toroidal case. The corresponding
eigenvalues also exhibit different behaviours in the two cases.

---------------------------------------------------------
Title: Oscillations of magnetic stars. II. Axisymmetric toroidal
and non-axisymmetric shear Alfvén modes in a spherical shell
Authors: Reese, D.; Rincon, F.; Rieutord, M.
2004A&A...427..279R Altcode: 2004astro.ph..9120R
We carry out numerical and mathematical investigations of shear
Alfvén waves inside of a spherical shell filled with an incompressible
conducting fluid, and bathed in a strong dipolar magnetic field. We
focus on axisymmetric toroidal and non-axisymmetric modes, in
continuation of a previous work by Rincon &amp; Rieutord (2003, A&amp;A,
398, 663). Analytical expressions are obtained for toroidal eigenmodes
and their corresponding frequencies at low diffusivities. These
oscillations behave like magnetic shear layers, in which the magnetic
poles play a key role, and hence become singular when diffusivities
vanish. It is also demonstrated that non-axisymmetric modes are split
into two categories, namely poloidal or toroidal types, following
similar asymptotic behaviours as their axisymmetric counterparts when
the diffusivities become arbitrarily small. Appendices are only
available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: Evolution of Rotation in Binaries: Physical Processes
(Invited Review)
Authors: Rieutord, M.
2004IAUS..215..394R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Oscillations of Fast Rotating Stars: p-Modes in Centrifugally
Flattened Polytropes
Authors: Lignières, F.; Rieutord, M.
2004IAUS..215..414L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Statistical mechanics and phase diagrams of rotating
self-gravitating fermions
Authors: Chavanis, P. H.; Rieutord, M.
2003A&A...412....1C Altcode: 2003astro.ph..2594C
We compute statistical equilibrium states of rotating self-gravitating
fermions by maximizing the Fermi-Dirac entropy at fixed mass, energy
and angular momentum. We describe the phase transition from a gaseous
phase to a condensed phase (corresponding to white dwarfs, neutron
stars or fermion balls in dark matter models) as we vary energy and
temperature. We increase the rotation up to the Keplerian limit and
describe the flattening of the configuration until mass shedding
occurs. At the maximum rotation, the system develops a cusp at
the equator. We draw the equilibrium phase diagram of the rotating
self-gravitating Fermi gas and discuss the structure of the caloric
curve as a function of degeneracy parameter (or system size) and
angular velocity. We argue that systems described by the Fermi-Dirac
distribution in phase space do not bifurcate to non-axisymmetric
structures when rotation is increased, in continuity with the
case of polytropes with index n&gt;0.808 (the Fermi gas at T=0
corresponds to n=3/2). This differs from the study of Votyakov et
al. (\cite{Votyakov2002}) who consider a Fermi-Dirac distribution
in configuration space appropriate to stellar formation and find
“double star” structures (their model at T=0 corresponds to
n=0). We also consider the case of classical objects described by the
Boltzmann entropy and discuss the influence of rotation on the onset
of gravothermal catastrophe (for globular clusters) and isothermal
collapse (for molecular clouds). On general grounds, we complete
previous investigations concerning the nature of phase transitions in
self-gravitating systems. We emphasize the inequivalence of statistical
ensembles regarding the formation of binaries (or low-mass condensates)
in the microcanonical ensemble (MCE) and Dirac peaks (or massive
condensates) in the canonical ensemble (CE). We also describe an
hysteretic cycle between the gaseous phase and the condensed phase
that are connected by a “collapse” or an “explosion”.

---------------------------------------------------------
Title: Families of fragmenting granules and their relation to meso-
and supergranular flow fields
Authors: Roudier, Th.; Lignières, F.; Rieutord, M.; Brandt, P. N.;
Malherbe, J. M.
2003A&A...409..299R Altcode:
3D analysis (x,y,t) of the granular intensity field (11-hour time
sequence from the Swedish Vacuum Solar Telescope on La Palma, Canary
Islands), demonstrates that a significant fraction of the granules in
the photosphere are organized in the form of “Trees of Fragmenting
Granules" (TFGs). A TFG consists of a family of repeatedly splitting
granules, originating from a single granule at its beginning. A striking
result is that TFGs can live much longer (up to 8 h) than individual
granules (10 min). We find that 62% of the area covered by granules
belongs to TFGs of a lifetime &gt;1.5 h. When averaged in time, such
long-lived TFGs correspond to coherent diverging flows which may be
identified as mesogranules. We also find a correlation between the
network and the spatial distribution of TFGs.

---------------------------------------------------------
Title: Evolution of rotation in binaries: physical processes
Authors: Rieutord, Michel
2003astro.ph..8313R Altcode:
In this review, we describe the physical processes driving the dynamical
evolution of binary stars, namely the circularization of the orbit
and the synchronization of their spin and orbital rotation. We also
discuss the possible role of the elliptic instability which turns out
to be an unavoidable ingredient of the evolution of binary stars.

---------------------------------------------------------
Title: Oscillations of magnetic stars: I. Axisymmetric shear Alfvén
modes of a spherical shell in a dipolar magnetic field
Authors: Rincon, F.; Rieutord, M.
2003A&A...398..663R Altcode:
We carry out an investigation of axisymmetric shear Alfvén waves
in a spherical layer of an incompressible resistive fluid when a
strong dipolar magnetic field is applied. A decomposition on the
spherical harmonics base is used to compute the eigenmodes of the
system. Numerical results show that the least-damped Alfvénic modes
naturally concentrate near the magnetic polar axis. These modes also
show internal shear/magnetic layers associated with resonant field
lines. This model is useful when modelling planetary cores sustaining
a dynamo, magnetic neutron stars or to the magnetic layer of roAp
stars. In this latter case, it shows that shear Alfvén waves provide
a good instance of non-perturbative effects due to the strong magnetic
field of such stars.

---------------------------------------------------------
Title: A Study of Shear Alfvén Waves in Magnetic Stars: the Spherical
Shell Model
Authors: Rincon, F.; Rieutord, M.
2003aahd.conf..561R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stability of a compressible fluid layer in a magnetic field:
a simple model for supergranulation
Authors: Rincon, F.; Rieutord, M.
2003sf2a.conf..103R Altcode: 2003sf2a.confE..33R
We investigate the onset of steady Rayleigh-Benard magnetoconvection
enclosed between two thermally insulating plates in a polytropic
layer of index 1. In a non-magnetic Boussinesq context, such boundary
conditions, also called fixed flux conditions, are known to favor
an infinite horizontal scale as the first unstable mode. We show
that the situation can be very different when compressibility and
magnetic field combine, giving birth to a finite horizontal scale in
most cases. We explore different regimes and demonstrate in particular
that the transition to a non-zero wave number occurs for a Chandrasekhar
number Q=394.3 in the Boussinesq limit, while compressibility tends to
reduce this critical value. We discuss these results in the context of
large scale stellar convection such as supergranulation where fixed
flux conditions are physically relevant and for which the effects of
magnetic fields and stratification cannot be bypassed.

---------------------------------------------------------
Title: Recurrence of fragmenting granules and their relation to meso-
and supergranular flow fields
Authors: Roudier, Th.; Lignières, F.; Rieutord, M.; Brandt, P. N.;
Malherbe, J. -M.
2003EAS.....9..371R Altcode:
The 3D analysis (x, y, t) of the granulation intensity field (11-hour
time sequence from Swedish Vacuum Solar Telescope on La Palma, Canary
Islands), demonstrated that the granules in the phostosphere are
organized in “Trees of Fragmenting Granules" (TFGs). A TFG consists of
a family of repeatedly splitting granules, issued from one granule at
its beginning. A striking result is that TFGs can live much longer (up
to 8h10) than individual granules (10 mn). When averaged in time, such
long-lived TFGs can be identified to the mesogranules. We also found
a correlation between the network and the spatial distribution of TFGs.

---------------------------------------------------------
Title: CALAS: a CAmera for the LArge Scales of the solar surface
Authors: Meunier, N.; Rieutord, M.; Beigbeder, F.
2003sf2a.conf...93M Altcode: 2003sf2a.confE..29M
Supergranulation is constituted of cells of horizontal outflows with a
typical size of 30000 km at the surface of the Sun. The origin of this
pattern (convection or not) is still controversial. Usually observed
using Doppler shifts at the photospheric level, it can also be studied
using horizontal motions of solar granules. A combination of a large
field of view (containing as many supergranules as possible) and a
high spatial resolution (to sample granules with a high accuracy,
even the small ones) is then necessary to study this process in more
details. Our project is to implement a large scale CMOS camera, CALAS
(CAmera for the LArge Scales of the solar surface) at the Coupole
Tourelle at the Pic du Midi. The field of view will be 10'x10',
allowing to sample 100 hundred supergranules. We will present the
scientific objectives and the organisation of our project.

---------------------------------------------------------
Title: The oscillations of rapidly rotating stars
Authors: Rieutord, Michel
2003safd.book...99R Altcode:
We review the effects of rotation on the oscillation spectrum of rapidly
rotating stars. We particularly stress the novelties introduced by
rotation: for instance, the disappearance of modes in the low frequency
band due to the ill-posed natured of the underlying mathematical
problem. This is mainly an effect of the Coriolis acceleration. The
centrifugal effect changes the shape of the star in the first place. The
possible consequences of this deformation on the oscillation spectrum
are briefly analyzed. We also describe other possibly important effects
of the centrifugal acceleration which come about on the time scale of
star evolution.

---------------------------------------------------------
Title: More concerning the anelastic and subseismic approximations
for low-frequency modes in stars
Authors: Rieutord, Michel; Dintrans, Boris
2002MNRAS.337.1087R Altcode: 2002astro.ph..6357R
Two approximations, namely the subseismic approximation and the
anelastic approximation, are used to filter out the acoustic modes
when computing low-frequency modes of a star (gravity modes or
inertial modes). In a previous paper, we observed that the anelastic
approximation gave eigenfrequencies much closer to the exact ones than
the subseismic approximation. Here, we try to clarify this behaviour
and show that it is a result of the different physical approach taken
by each approximation. On the one hand, the subseismic approximation
considers the low-frequency part of the spectrum of (say) gravity modes
and turns out to be valid only in the central region of a star; on the
other hand, the anelastic approximation considers the Brunt-Väisälä
frequency to be asymptotically small and makes no assumption concerning
the order of the modes. Both approximations fail to describe the
modes in the surface layers but eigenmodes issued from the anelastic
approximation are closer to those including acoustic effects than their
subseismic equivalent. We conclude that, as far as stellar eigenvalue
problems are concerned, the anelastic approximation is better suited
for simplifying the eigenvalue problem when low-frequency modes of a
star are considered, while the subseismic approximation is a useful
concept when analytic solutions of high-order low-frequency modes are
needed in the central region of a star.

---------------------------------------------------------
Title: Adiabatic oscillations of non-rotating superfluid neutron stars
Authors: Prix, R.; Rieutord, M.
2002A&A...393..949P Altcode: 2002astro.ph..4520P
We present results concerning the linear (radial and non-radial)
oscillations of non-rotating superfluid neutron stars in Newtonian
physics. We use a simple two-fluid model to describe the superfluid
neutron star, where one fluid consists of the superfluid neutrons,
while the second fluid contains all the remaining constituents (protons,
electrons). The two fluids are assumed to be “free” in the sense of
absence of vortex-mediated forces like mutual friction or pinning,
but they can be coupled by the equation of state, in particular by
entrainment. We calculate numerically the eigen-frequencies and -modes
of adiabatic oscillations, neglecting beta-reactions that would lead to
dissipation. We find a doubling of all acoustic-type modes (f-modes,
p-modes), and confirm the absence of g-modes in these superfluid
models. We show analytically and numerically that only in the case of
non-stratified background models (i.e. with no composition gradient)
can these acoustic modes be separated into two distinct families, which
are characterized by either co- or counter-moving fluids respectively,
and which are sometimes referred to as “ordinary” and “superfluid”
modes. In the general, stratified case, however, this separation
is not possible, and these acoustic modes can not be classified as
being either purely “ordinary” or “superfluid”. We show how the
properties of the two-fluid modes change as functions of the coupling by
entrainment. We find avoided mode-crossings for the stratified models,
while the crossings are not avoided in the non-stratified, separable
case. The oscillations of normal-fluid neutron stars are recovered
as a special case simply by locking the two fluids together. In this
effective one-fluid case we find the usual singlet f- and p-modes,
and we also find the expected g-modes of stratified neutron star models.

---------------------------------------------------------
Title: A simulation of solar convection at supergranulation scale
Authors: Rieutord, M.; Ludwig, H. -G.; Roudier, T.; Nordlund, .;
Stein, R.
2002NCimC..25..523R Altcode: 2001astro.ph.10208R
We present here numerical simulations of surface solar convection
which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of
315$\times315\times$82, which is used to investigate the dynamics of
scales larger than granulation. No structure resembling supergranulation
is present; possibly higher Reynolds numbers (i.e. higher numerical
resolution), or magnetic fields, or greater depth are necessary. The
results also show interesting aspects of granular dynamics which are
briefly presented, like extensive p-mode ridges in the k-$\omega$
diagram and a ringlike distribution of horizontal vorticity around
granules. At large scales, the horizontal velocity is much larger
than the vertical velocity and the vertical motion is dominated by
p-mode oscillations.

---------------------------------------------------------
Title: Slichter modes of the Earth revisited
Authors: Rieutord, Michel
2002PEPI..131..269R Altcode:
Using the simple model of a spherical solid inner core oscillating in a
rotating liquid outer core, we compute the frequencies of the Slichter
modes of the earth. The fluid is assumed neutrally stratified (but with
a radially varying density) and viscous, viscosity being taken into
account non-perturbatively. The parameters are those given by earth
models like PREM, 1066A or CORE11. The computed resonant frequencies
are compared to those observed by Courtier et al. (2000) and claimed to
be the Slichter frequencies. We show that our model cannot reproduce
these frequencies and that previous models which did reproduce them
are not consistent with the observed quality factors of the resonances.

---------------------------------------------------------
Title: Photospheric flows measured with TRACE
Authors: Krijger, J. M.; Roudier, T.; Rieutord, M.
2002A&A...387..672K Altcode:
We analyse white-light image sequences taken with the Transition
Region and Coronal Explorer (TRACE) using an optimised local
correlation tracking (LCT) method to measure the horizontal flows
in the quiet solar photosphere with high spatial (1 arcsec) and
temporal (5 min) resolution. Simultaneously taken near-ultraviolet
images from TRACE confirm that our LCT-determined flows recover the
actual supergranulation pattern, thus proving that the topology of the
horizontal flow distribution and network assembly may be studied from
long-duration TRACE white-light sequences with our method.

---------------------------------------------------------
Title: On the Theory of Oscillations of Rapidly Rotating Stars
Authors: Rieutord, M.; Lignières, F.
2002ASPC..259..190R Altcode: 2002rnpp.conf..190R; 2002IAUCo.185..190R
No abstract at ADS

---------------------------------------------------------
Title: A Comparison of the Anelastic and Subseismic Approximations
for Low-Frequency Stellar Oscillations: an Application to Rapidly
Rotating Stars
Authors: Dintrans, B.; Rieutord, M.
2002ASPC..259..186D Altcode: 2002IAUCo.185..186D; 2002rnpp.conf..186D; 2001astro.ph.11446D
After showing that the anelastic approximation is better than the
subseismic one to filter out acoustic waves when studying low-frequency
stellar oscillations, we compute gravito-inertial modes of a typical
Gamma-Doradus star using this approximation. We show that eigenmodes
can be regular or singular, according to the possible focusing towards
attractors of the underlying characteristics. Consequences on the
oscillations spectrum are then discussed.

---------------------------------------------------------
Title: Are granules good tracers of solar surface velocity fields?
Authors: Rieutord, M.; Roudier, T.; Ludwig, H. -G.; Nordlund, Å.;
Stein, R.
2001A&A...377L..14R Altcode: 2001astro.ph..8284R
Using a numerical simulation of compressible convection with radiative
transfer mimicking the solar photosphere, we compare the velocity
field derived from granule motions to the actual velocity field of
the plasma. We thus test the idea that granules may be used to trace
large-scale velocity fields at the sun's surface. Our results show that
this is indeed the case provided the scale separation is sufficient. We
thus estimate that neither velocity fields at scales less than 2500
km nor time evolution at scales shorter than 0.5 hr can be faithfully
described by granules. At larger scales the granular motions correlate
linearly with the underlying fluid motions with a slope of ≲2 reaching
correlation coefficients up to ~ 0.9.

---------------------------------------------------------
Title: Erratum: Ekman Layers and the Damping of Inertial r-Modes in
a Spherical Shell: Application to Neutron Stars
Authors: Rieutord, Michel
2001ApJ...557..493R Altcode:
In the paper “Ekman Layers and the Damping of Inertial r-Modes in
a Spherical Shell: Application to Neutron Stars” by Michel Rieutord
(<A href="/abs/2001ApJ...550..443">ApJ, 550, 443 [2001]</A>), Figure
2 was erroneously reproduced as Figure 1. The correct Figure 1 appears
below. The Press sincerely regrets this error.

---------------------------------------------------------
Title: A comparison of the anelastic and subseismic approximations
for low-frequency gravity modes in stars
Authors: Dintrans, B.; Rieutord, M.
2001MNRAS.324..635D Altcode: 2000astro.ph.11273D
A comparative study of the properties of the anelastic and subseismic
approximations is presented. The anelastic approximation is commonly
used in astrophysics in compressible convection studies, whereas the
subseismic approximation comes from geophysics where it is used to
describe long-period seismic oscillations propagating in the Earth's
outer fluid core. Both approximations aim to filter out the acoustic
waves while retaining the density variations of the equilibrium
configuration. However, they disagree on the form of the equation of
mass conservation. We show here that the anelastic approximation is in
fact the only consistent approximation as far as stellar low-frequency
oscillations are concerned. We also show that this approximation
implies Cowling's approximation which neglects perturbations of the
gravity field. Examples are given to demonstrate the efficiency of
the anelastic approximation.

---------------------------------------------------------
Title: Acoustic modes in spheroidal cavities
Authors: Lignières, Francois; Rieutord, M.; Valdettaro, L.
2001sf2a.conf..127L Altcode: 2001astro.ph.10214L
The calculation of the oscillation modes of a star in fast rotation has
not yet been carried out with precision, the effect of rotation has been
considered only by means of perturbation developpements. We develop
a numerical method able to fully take into account the deformation
of the star by the centrifugal force and, in a first step, we have
determined the acoustic modes of an ellipsoid of uniform density and
studied the effect of the ellipticity on these modes. The consequences
of these first results for the seismology of stars in fast rotation
as the delta Scuti delta will be discussed.

---------------------------------------------------------
Title: Inertial waves in a rotating spherical shell: attractors and
asymptotic spectrum
Authors: Rieutord, M.; Georgeot, B.; Valdettaro, L.
2001JFM...435..103R Altcode: 2000physics...7007R
We investigate the asymptotic properties of inertial modes confined
in a spherical shell when viscosity tends to zero. We first consider
the mapping made by the characteristics of the hyperbolic equation
(Poincaré's equation) satisfied by inviscid solutions. Characteristics
are straight lines in a meridional section of the shell, and the
mapping shows that, generically, these lines converge towards a periodic
orbit which acts like an attractor (the associated Lyapunov exponent
is always negative or zero). We show that these attractors exist in
bands of frequencies the size of which decreases with the number of
reflection points of the attractor. At the bounding frequencies the
associated Lyapunov exponent is generically either zero or minus
infinity. We further show that for a given frequency the number of
coexisting attractors is finite.

---------------------------------------------------------
Title: Ekman Layers and the Damping of Inertial R-Modes in a Spherical
Shell: Application to Neutron Stars
Authors: Rieutord, Michel
2001ApJ...550..443R Altcode: 2000astro.ph..3171R
Recently, eigenmodes of rotating fluids, namely, inertial modes,
have received much attention in relation to their destabilization when
coupled to gravitational radiation within neutron stars. However, these
modes have been known for a long time in fluid dynamics. We give a short
account of their history and review our present understanding of their
properties. Considering the case of a spherical container, we then give
the exact solution of the boundary (Ekman) layer flow associated with
inertial r-modes and show that previous estimations all underestimated
the dissipation by these layers. We also show that the presence of an
inner core has little influence on this dissipation. As a conclusion, we
compute the window of instability in the temperature/rotation plane for
a crusted neutron star when it is modeled by an incompressible fluid.

---------------------------------------------------------
Title: Temporal height properties of the exploding granules
Authors: Roudier, Th.; Eibe, M. T.; Malherbe, J. M.; Rieutord, M.;
Mein, P.; Mein, N.; Faurobert, M.
2001A&A...368..652R Altcode:
Based on time series of 2D MSDP spectrograms, taken at the Turret Dome
in Pic du Midi, we present the temporal evolution of exploding granules
in intensity and Doppler velocity through the solar photosphere. We
describe the penetration of exploding granules in the solar photosphere
during their lifes and the related phenomena like the “Bright Plumes”
located in the downflowing plasma just on the edge of the granule. We
suggest a possible scenario of the exploding granule evolution in the
solar photosphere.

---------------------------------------------------------
Title: Wave Attractors in Rotating Fluids: A Paradigm for Ill-Posed
Cauchy Problems
Authors: Rieutord, M.; Georgeot, B.; Valdettaro, L.
2000PhRvL..85.4277R Altcode: 2000physics..11031R
In the limit of low viscosity, we show that the amplitude of the modes
of oscillation of a rotating fluid, namely inertial modes, concentrate
along an attractor formed by a periodic orbit of characteristics
of the underlying hyperbolic Poincaré equation. The dynamics of
characteristics is used to elaborate a scenario for the asymptotic
behavior of the eigenmodes and eigenspectrum in the physically
relevant régime of very low viscosities which are out of reach
numerically. This problem offers a canonical ill-posed Cauchy problem
which has applications in other fields.

---------------------------------------------------------
Title: On mesogranulation, network formation and supergranulation
Authors: Rieutord, M.; Roudier, T.; Malherbe, J. M.; Rincon, F.
2000A&A...357.1063R Altcode:
We present arguments which show that in all likelihood mesogranulation
is not a true scale of solar convection but the combination of
the effects of both highly energetic granules, which give birth to
strong positive divergences (SPDs) among which we find exploders, and
averaging effects of data processing. The important role played by SPDs
in horizontal velocity fields appears in the spectra of these fields
where the scale ~ 4 Mm is most energetic; we illustrate the effect
of averaging with a one-dimensional toy model which shows how two
independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are
degraded. The role of SPDs in the formation of the photospheric network
is shown by computing the advection of floating corks by the granular
flow. The coincidence of the network bright points distribution and
that of the corks is remarkable. We conclude with the possibility that
supergranulation is not a proper scale of convection but the result
of a large-scale instability of the granular flow, which manifests
itself through a correlation of the flows generated by SPDs.

---------------------------------------------------------
Title: Oscillations of a rotating star: a non-perturbative theory
Authors: Dintrans, Boris; Rieutord, Michel
2000A&A...354...86D Altcode:
Nonradial gravity modes of a 1.5 Mscriptstyleun 
rotating ZAMS star are investigated using the anelastic
approximation. Formulating the oscillation equations as a generalized
eigenvalue problem, we first show that the usual second-order
perturbative theory reaches its limits for rotation periods of about
three days. Studying the rapid rotation régime, we develop a geometric
formalism based on the integration of the characteristics of the
governing mixed-type operator. These characteristics propagate in the
star interior and the resulting web can be either ergodic (the web fills
the whole domain) or periodic (the web reduces to an attractor along
which characteristics focus). We further show the deep relation existing
between the orbits of characteristics and the corresponding eigenmodes:
{(i)} with ergodic orbits are associated regular eigenmodes which are
similar to the usual gravity modes; {(ii)} with periodic orbits are
associated singular eigenmodes for which the velocity diverges along the
attractor. If diffusivity is taken into account, this singularity turns
into internal shear layers tracing the attractor. As a consequence, the
classical organization of eigenvalues along families with fixed (l,n)
disappears and leaves the place to an intricate low-frequency spectrum.

---------------------------------------------------------
Title: Oscillations of Rapidly Rotating Stars
Authors: Dintrans, B.; Rieutord, M.
2000ASPC..203..373D Altcode: 2001astro.ph.11453D; 2000IAUCo.176..373D; 2000ilss.conf..373D
We present numerical simulations of gravito-inertial waves propagating
in radiative zones of rapidly rotating stars. A first model, using
the Boussinesq approximation, allows us to study the oscillations of a
quasi-incompressible stratified fluid embedded in a rapidly rotating
sphere or spherical shell. In a second step, we investigate the case
of a γ Doradus-type star using the anelastic approximation. Some
fascinating features of rapidly rotating fluids, such as wave
attractors, appear in both cases.

---------------------------------------------------------
Title: A note on inertial modes in the core of the Earth
Authors: Rieutord, Michel
2000PEPI..117...63R Altcode:
We analyse the consequences of the singular (at zero viscosity) nature
of inertial modes in a spherical shell for the dynamics of the Earth's
liquid core on the time scale of a day. We show that the singularities,
essentially appearing as internal shear layers, although increasing the
damping rates of the modes, cannot be invoked to rule out the possible
role of the elliptical instability in the geodynamo. We also show that
the search for core modes in the spectrum of oscillations of the Earth
should be guided by the Lyapunov exponents associated with the maps
built up by characteristics propagating in the shell.

---------------------------------------------------------
Title: Determination of horizontal velocity fields at the sun's
surface with high spatial and temporal resolution
Authors: Roudier, Th.; Rieutord, M.; Malherbe, J. M.; Vigneau, J.
1999A&A...349..301R Altcode:
We analyse the two algorithms which have been used in the past few years
to determine the horizontal flow fields at the Sun's surface, namely
the Local Correlation Tracking (LCT) of L. November and the Feature
Tracking of L. Strous. Analysing the systematic errors introduced by
LCT, we show that these errors come from the averaging processes. More
precisely, they arise from the interpolating step of the algorithm:
granules' motions determine the flow on an irregular grid which is
then interpolated to derive quantities such as horizontal divergence
or vertical vorticity. Interpolation is therefore a crucial step since
mesoscale structures have mainly been studied through divergences and
vorticities. We conclude that a reliable algorithm should be based
on the tracking of coherent structures, like granules, since they are
representative of the fluid motion, and should contain an interpolator
which keeps track of the errors introduced either by location of the
data (the shape of the irregular grid) or by the noise in the data.

---------------------------------------------------------
Title: Internal intermittency on the Sun surface?
Authors: Roudier, T.; Rieutord, M.; Malherbe, J. M.; Vigneau, J.
1999joso.proc..197R Altcode:
An improved version of LCT and FT is presented and used to determine
high spatial and temporal resolution horizontal flow fields at the
Sun's surface. The granule diplacements and group evolution can be
followed. The horizontal flow fields calculated by these methods show
a structured flow at mean scale (4 to 8 arcsec) by a group of granules
which is probably related to the local intermittency.

---------------------------------------------------------
Title: Ekman Pumping and Tidal Dissipation in Close Binaries:
A Refutation of Tassoul's Mechanism
Authors: Rieutord, Michel; Zahn, Jean-Paul
1997ApJ...474..760R Altcode:
We show that the existence of an Ekman boundary layer does not enhance
the tidal dissipation in a close binary star because the tides do not
exert a stress on the stellar surface. The synchronization timescale
is of order (ɛT)-2tadj, where
tadj is the (global) viscous damping time and ɛT
is the tidal deformation caused by the companion (Darwin 1879; Zahn
1966; Scharlemann 1982; Rieutord &amp; Bonazzola 1987). We thus refute
the claim made by Tassoul (1987), who thought to have found a very
efficient mechanism for the synchronization and circularization of
binary systems. We analyze the paper by Tassoul &amp; Tassoul (1992b)
and prove that the alleged magnitude of their Ekman pumping is due to an
improper treatment of the surface boundary conditions. Their mechanism
would have dramatic, yet unverified consequences, as illustrated by
two examples of tidal interaction: between Io and Jupiter, and in the
newly discovered planetary system 51 Peg.

---------------------------------------------------------
Title: On the internal dynamics of turbulent plumes in the context
of stellar convection.
Authors: Bonin, P.; Rieutord, M.
1996A&A...310..221B Altcode:
We investigate in some details the modeling of turbulent plumes which
have been proposed by Rieutord and Zahn to represent the downflows in a
stellar convective zone. We show in particular the limits of Taylor's
hypothesis about turbulent entrainment and emphasize its connection
with the flow's self-similarity. The role of the dissipation is shown
to be important in the choice of the final asymptotic regime. This is
illustrated by a paradox which is solved when dissipation is correctly
taken into account. It is concluded that in stellar conditions, the
lack of self-similarity implies the replacement of Taylor's hypothesis
by a proper closure of the mean-field equations in order to obtain a
reliable prediction on the large-scale dynamics.

---------------------------------------------------------
Title: Magnetic structures in a dynamo simulation
Authors: Brandenburg, A.; Jennings, R. L.; Nordlund, Å.; Rieutord,
M.; Stein, R. F.; Tuominen, I.
1996JFM...306..325B Altcode:
We use three-dimensional simulations to study compressible convection
in a rotating frame with magnetic fields and overshoot into surrounding
stable layers. The, initially weak, magnetic field is amplified and
maintained by dynamo action and becomes organized into flux tubes
that are wrapped around vortex tubes. We also observe vortex buoyancy
which causes upward flows in the cores of extended downdraughts. An
analysis of the angles between various vector fields shows that there
is a tendency for the magnetic field to be parallel or antiparallel
to the vorticity vector, especially when the magnetic field is
strong. The magnetic energy spectrum has a short inertial range with
a slope compatible with k(+1/3) during the early growth phase of the
dynamo. During the saturated state the slope is compatible with k(-1). A
simple analysis based on various characteristic timescales and energy
transfer rates highlights important qualitative ideas regarding the
energy budget of hydromagnetic dynamos.

---------------------------------------------------------
Title: Turbulent plumes in stellar convective envelopes.
Authors: Rieutord, M.; Zahn, J. -P.
1995A&A...296..127R Altcode:
Recent numerical simulations of compressible convection in a stratified
medium suggest that strong downwards directed flows may play an
important role in stellar convective envelopes, both in the dynamics and
in the energy transport. We transpose this idea to stellar convective
envelopes by assuming that these plumes are turbulent plumes which may
be described by Taylor's entrainment hypothesis, whose validity is well
established in various geophysical conditions. We consider first the
ideal case of turbulent plumes occurring in an isentropic atmosphere,
and ignore all types of feedback. Thereafter we include the effect
of the backflow generated by the plumes, and take into account the
contribution of the radiative flux. The main result is that plumes
originating from the upper layers of a star are able to reach the base
of its convective envelope. Their number is necessarily limited because
of their conical shape; the backflow further reduces their number to
a maximum of about 1000. In these plumes the flux of kinetic energy is
directed downwards, but it is less than the upwards directed enthalpy
flux, so that the plumes always carry a net energy flux towards the
surface. Our plume model is not applicable near the surface, where
the departures from adiabaticity become important due to radiative
leaking; therefore it cannot predict the depth of the convection
zone, which is determined mainly by the transition from the radiative
regime above to the nearly adiabatic conditions below. Neither does it
permit to evaluate the extent of penetration, which strongly depends
on the (unknown) number of plumes. We conclude that, to be complete, a
phenomenological model of stellar convection must have a dual character:
it should include both the advective transport through diving plumes,
which is outlined in this paper, and the turbulent diffusion achieved
by the interstitial medium. Only the latter process is apprehended by
the familiar mixing-length treatment.

---------------------------------------------------------
Title: Turbulent Jets: Reichardt's Inductive Theory and Intermittency
Corrections
Authors: Bonin, P.; Rieutord, M.
1995LNP...462..165B Altcode: 1995ssst.conf..165B
We investigate the so-called inductive closure relation given by
Reichardt for mean turbulent momentum transport in jets (1941). It is
shown to be in good agreement with measurements in the axisymmetric
jet by Panchapakesan and Lumley (1993). Despite these results, it is
conjectured that there is a lack of account for intermittency effects
which is hidden by the experimental procedure of Panchapakesan and
Lumley. Intermittency corrections are then proposed, based on the
classical concept of intermittency factor and the corrected model is
compared with the results of Bradbury (1965) for the plane jet. The
agreement with experimental measurements appears to be very good. The
underlying picture of the Reichardt 1941 model is discussed, mainly
the fact that it expresses the self-injection of kinetic energy by
the jet and the conversion of longitudinal into lateral momentum thus
providing entrainment of the surrounding fluid.

---------------------------------------------------------
Title: Inertial modes in the liquid core of the Earth
Authors: Rieutord, Michel
1995PEPI...91...41R Altcode:
Using the simple model of an incompressible fluid, we have computed
the eigenfrequencies of the lowest-order inertial modes (azimuthal
wavenumber m = 0,1,2) in a spherical shell with the same aspect ratio
as the liquid core of the Earth. The computed eigenfunctions show that
all inertial modes have strong oscillating shear layers. For the very
low Ekman number appropriate to the core, these layers might be the
origin of some small-scale turbulence through shear instabilities. We
have also studied the effect of a thin stable layer lying just below
the core-mantle boundary, with the remainder of the core being neutrally
stratified, as suggested by recent work. For plausible Nusselt numbers
(0.8-0.9), the frequencies of the large-scale modes are only slightly
increased (at best by 10 -4).

---------------------------------------------------------
Title: Reynolds stresses and differential rotation in Boussinesq
convection in a rotating spherical shell.
Authors: Rieutord, Michel; Brandenburg, Axel; Mangeney, Andre;
Drossart, Pierre
1994A&A...286..471R Altcode:
We consider the problem of how numerical simulations of convection
in a spherical shell can be used to estimate turbulent transport
coefficients that may be used in mean field theory. For this purpose
we analyse data from simulations of three dimensional Boussinesq
convection. The rotational influence on convection is described in
terms of the {LAMBDA}-effect and anisotropic eddy conductivity. When
the resulting transport coefficients are used in a mean field model,
the original rotation law is recovered approximately. We thus conclude
that the flow can be described in terms of a {LAMBDA}-effect. Our
results are also compared with analytical theories and observations.

---------------------------------------------------------
Title: Does solar differential rotation ARISE from a large scale
instability?.
Authors: Tuominen, Ilkka; Brandenburg, Axel; Moss, David; Rieutord,
Michel
1994A&A...284..259T Altcode:
The suggestion by several authors that the solar differential rotation
is caused by a large scale instability of the basic convective state
is examined. We find that the proposed mean-field models are unstable
to a Rayleigh-Benard type instability, but argue that this cannot
explain the differential rotation of the Sun, because such a flow would
become nonaxisymmetric. We discuss the applicability of the mean-field
equations to the problem. hydrodynamics - Sun: rotation

---------------------------------------------------------
Title: Non linear stability of slender accretion disks by bifurcation
method
Authors: Dubrulle, B.; Chomaz, J. M.; Kumar, S.; Rieutord, M.
1993GApFD..70..235D Altcode:
The nonlinear evolution of a constant angular momentum accretion disk
subject to Papaloizou and Pringle (1984; hereafter PP) instability is
investigated. The analysis is performed on an inviscid incompressible
two-dimensional model using a formalism suitable for Hamiltonian
systems. Only the most unstable modes are taken into account. It is
found that relevant nonlinear terms have a stabilizing influence on the
system. This supports recent numerical experiments showing a transition
towards a quasi-stable planet configuration. The extension of the method
to fat disk instabilities, more relevant to AGN disks, is discussed.

---------------------------------------------------------
Title: Coherent Structures and the Differential Rotation of the Sun
Authors: Rieutord, M.
1993ASPC...42...65R Altcode: 1993gong.conf...65R
No abstract at ADS

---------------------------------------------------------
Title: Ekman circulation and the synchronization of binary stars
Authors: Rieutord, M.
1992A&A...259..581R Altcode:
We show that large-scale flows driven by Ekman pumping in the
spin-up/down of a tidally distorted star is not efficient enough
to reduce the synchronization time. This latter time remains of the
order of the viscous time, if the star is made of an incompressible
viscous fluid. The computation of the synchronization time scale of
early-type binaries should follow the approach proposed by Zahn and
recently improved by Rocca, and Goldreich and Nicholson.

---------------------------------------------------------
Title: Dynamo Action in Stratified Convection with Overshoot
Authors: Nordlund, Ake; Brandenburg, Axel; Jennings, Richard L.;
Rieutord, Michel; Ruokolainen, Juha; Stein, Robert F.; Tuominen, Ilkka
1992ApJ...392..647N Altcode:
Results are presented from direct simulations of turbulent compressible
hydromagnetic convection above a stable overshoot layer. Spontaneous
dynamo action occurs followed by saturation, with most of the generated
magnetic field appearing as coherent flux tubes in the vicinity
of strong downdrafts, where both the generation and destruction of
magnetic field is most vigorous. Whether or not this field is amplified
depends on the sizes of the magnetic Reynolds and magnetic Prandtl
numbers. Joule dissipation is balanced mainly by the work done against
the magnetic curvature force. It is this curvature force which is also
responsible for the saturation of the dynamo.

---------------------------------------------------------
Title: Ekman layers and tidal synchronization of binary stars.
Authors: Rieutord, M.
1992btsf.work..229R Altcode: 1992bats.proc..229R
No abstract at ADS

---------------------------------------------------------
Title: Linear theory of rotating fluids using spherical harmonics
part II, time-periodic flows
Authors: Rieutord, Michel
1991GApFD..59..185R Altcode:
Expansion in spherical harmonics is used to solve linear equations
of flows of homogeneous viscous fluids in a rotating frame. For a
truncated series, analytical solutions are obtained for the radial
functions. These solutions are used to investigate the modal properties
of a viscous incompressible fluid in a spherical shell. The results
are compared to the experimental data of Aldridge. The problem of
identification of inertial modes in the Earth's outer core is also
discussed.

---------------------------------------------------------
Title: αΛ-dynamos
Authors: Brandenburg, A.; Moss, D.; Rieutord, M.; Rüdiger, G.;
Tuominen, I.
1991LNP...380..147B Altcode: 1991sacs.coll..147B; 1991IAUCo.130..147B
In contrast to -dynamos, where the angular velocity is arbitrarily
prescribed, we consider here -dynamos, for which the differential
rotation and meridional circulation are solutions of the momentum
equation. The non-diffusive parts of the Reynolds stress tensor are
parameterized by the -effect. In earlier investigations we have shown
that the turbulent magnetic diffusivity has to be much smaller than
the eddy viscosity, otherwise the dynamo is not oscillatory or else
the contours of constant angular velocity are cylindrical, contrary
to observations. In the present paper we investigate the effects of
compressibility.

---------------------------------------------------------
Title: Large scale convection in stars : Towards a model for the
action of coherent structures
Authors: Rieutord, Michel; Zahn, Jean-Paul
1991LNP...380...33R Altcode: 1991sacs.coll...33R; 1991IAUCo.130...33R
We show that, representing the descending fluid in a convection zone
by a porous medium, the differential rotation of the (rising) fluid
is very close to that in an axisymmetric model of the convection zone
with anisotropic viscosity

---------------------------------------------------------
Title: Tidal heating in close binary stellar systems
Authors: Rieutord, Michel; Bonazzola, Silvano
1987MNRAS.227..295R Altcode:
The present consideration of tidal heating in a close binary system's
low-mass star due to the conjugate effect of angular momentum loss
and tidal action, with attention to the flow within the secondary,
notes in the case of cataclismic binaries that viscous dissipation is
at most one-thousandth of the nuclear luminosity of a star, thereby
confirming by a more exact model the results of Verbunt and Hut
(1981). It is shown, however, that the dissipation is very sensitive
to the turbulent viscosity of the secondary's envelope. Attention is
also given to the case of very close pairs of white dwarfs, which may
dissipate a power as great as 10 to the 38th erg/sec if they reach
synchronization, but whose detection will be very difficult.

---------------------------------------------------------
Title: Contribution à l'étude de l'évolution des étoiles doubles
serrées : étude du spin-up par effet de marée

---------------------------------------------------------
Title: Contribution
à l'étude de l'évolution des étoiles doubles serrées : étude du
spin-up par effet de marée

---------------------------------------------------------
Title: Contribution to the study of the
evolution of close binary stars: study of the spin-up by tidal effect;
Authors: Rieutord, Michel
1987PhDT.......152R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Linear theory of rotating fluids using spherical harmonics
part I: Steady flows
Authors: Rieutord, Michel
1987GApFD..39..163R Altcode:
It is shown that a systematic development of physical quantities using
spherical harmonics provides analytical solutions to a whole class
of linear problems of rotating fluids. These solutions are regular
throughout the whole domain of the fluid and are not much affected
by the equatorial singularity of steady boundary layers in spherical
geometries. A comparison between this method and the one based on
boundary layer theory is carried out in the case of the steady spin-up
of a fluid inside a sphere.