zahn

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Author name code: zahn
ADS astronomy entries on 2022-09-14
author:"Zahn, Jean-Paul"

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Title: Integrating Single Sign-On into the STScI Archive: Lessons
Learned
Authors: Alexov, A.; Abney, F.; Kyprianou, M.; Levay, K.; Zahn, J.
2017ASPC..512...93A Altcode: 2017adass..25...93A; 2018ASPC..512...93A
The Space Telescope Science Institute (STScI) migrated a handful
of web services to a Single Sign-On (SSO) solution in 2014, using
the Shibboleth and CAS software solutions. SSO was implemented in
order to consolidate usernames and passwords used for a variety
of web services, to improve security at STScI and to better the
user experience. It took an additional year to integrate the STScI
Archive into the SSO Framework, including multiple web services and
a non-web service authentication back end which allowed for one set
of user credentials. The SSO Framework was expanded to allow external
users to register with STScI and use those SSO credentials to access
multiple services at STScI, including the Archive. We took great care
in migrating and informing over 13,000 STScI Archive users of their
new SSO credentials and new user interfaces. We coordinated help
desks between IT, the instrument science groups and the Archive to
have a successful, seamless transition of the STScI Archive users to
their new SSO credentials. We outline our SSO architecture, hurdles,
lessons learned and implementation solutions which we have taken in
order to migrate STScI's Archive services to using SSO.

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Title: Confinement of the solar tachocline by a cyclic dynamo
magnetic field
Authors: Barnabé, Roxane; Strugarek, Antoine; Charbonneau, Paul;
Brun, Allan Sacha; Zahn, Jean-Paul
2017A&A...601A..47B Altcode: 2017arXiv170302374B
Context. The surprising thinness of the solar tachocline is still not
understood with certainty today. Among the numerous possible scenarios
suggested to explain its radial confinement, one hypothesis is based on
Maxwell stresses that are exerted by the cyclic dynamo magnetic field of
the Sun penetrating over a skin depth below the turbulent convection
zone. Aims: Our goal is to assess under which conditions
(turbulence level in the tachocline, strength of the dynamo-generated
field, spreading mechanism) this scenario can be realized in the
solar tachocline. Methods: We develop a simplified 1D model of
the upper tachocline under the influence of an oscillating magnetic
field imposed from above. The turbulent transport is parametrized with
enhanced turbulent diffusion (or anti-diffusion) coefficients. Two main
processes that thicken the tachocline are considered; either turbulent
viscous spreading or radiative spreading. An extensive parameter study
is carried out to establish the physical parameter regimes under which
magnetic confinement of the tachocline that is due to a surface dynamo
field can be realized. Results: We have explored a large range
of magnetic field amplitudes, viscosities, ohmic diffusivities and
thermal diffusivities. We find that, for large but still realistic
magnetic field strengths, the differential rotation can be suppressed
in the upper radiative zone (and hence the tachocline confined)
if weak turbulence is present (with an enhanced ohmic diffusivity
of η&gt; 107-8 cm2/ s), even in the presence
of radiative spreading. Conclusions: Our results show that a
dynamo magnetic field can, in the presence of weak turbulence, prevent
the inward burrowing of a tachocline subject to viscous diffusion or
radiative spreading.

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Title: New constraints on Saturn's interior from Cassini astrometric
data
Authors: Lainey, Valéry; Jacobson, Robert A.; Tajeddine, Radwan;
Cooper, Nicholas J.; Murray, Carl; Robert, Vincent; Tobie, Gabriel;
Guillot, Tristan; Mathis, Stéphane; Remus, Françoise; Desmars,
Josselin; Arlot, Jean-Eudes; De Cuyper, Jean-Pierre; Dehant,
Véronique; Pascu, Dan; Thuillot, William; Le Poncin-Lafitte,
Christophe; Zahn, Jean-Paul
2017Icar..281..286L Altcode: 2015arXiv151005870L
Using astrometric observations spanning more than a century and
including a large set of Cassini data, we determine Saturn's tidal
parameters through their current effects on the orbits of the eight
main and four coorbital Moons. We have used the latter to make the
first determination of Saturn's Love number from observations,
k2=0.390 ± 0.024, a value larger than the commonly
used theoretical value of 0.341 (Gavrilov &amp; Zharkov, 1977), but
compatible with more recent models (Helled &amp; Guillot, 2013) for
which the static k2 ranges from 0.355 to 0.382. Depending
on the assumed spin for Saturn's interior, the new constraint can
lead to a significant reduction in the number of potential models,
offering great opportunities to probe the planet's interior. In
addition, significant tidal dissipation within Saturn is confirmed
(Lainey et al., 2012) corresponding to a high present-day tidal ratio
k2/Q=(1.59 ± 0.74) × 10-4 and implying fast
orbital expansions of the Moons. This high dissipation, with no obvious
variations for tidal frequencies corresponding to those of Enceladus
and Dione, may be explained by viscous friction in a solid core,
implying a core viscosity typically ranging between 1014
and 1016 Pa.s (Remus et al., 2012). However, a dissipation
increase by one order of magnitude at Rhea's frequency could suggest the
existence of an additional, frequency-dependent, dissipation process,
possibly from turbulent friction acting on tidal waves in the fluid
envelope of Saturn (Ogilvie &amp; Lin, 2004; Fuller et al. 2016).

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Title: The magnetic fields at the surface of active single G-K giants
Authors: Aurière, M.; Konstantinova-Antova, R.; Charbonnel, C.;
Wade, G. A.; Tsvetkova, S.; Petit, P.; Dintrans, B.; Drake, N. A.;
Decressin, T.; Lagarde, N.; Donati, J. -F.; Roudier, T.; Lignières,
F.; Schröder, K. -P.; Landstreet, J. D.; Lèbre, A.; Weiss, W. W.;
Zahn, J. -P.
2015A&A...574A..90A Altcode: 2014arXiv1411.6230A
 Aims: We investigate the magnetic field at the surface of 48
red giants selected as promising for detection of Stokes V Zeeman
signatures in their spectral lines. In our sample, 24 stars are
identified from the literature as presenting moderate to strong
signs of magnetic activity. An additional 7 stars are identified
as those in which thermohaline mixing appears not to have occured,
which could be due to hosting a strong magnetic field. Finally, we
observed 17 additional very bright stars which enable a sensitive
search to be performed with the spectropolarimetric technique. 
Methods: We use the spectropolarimeters Narval and ESPaDOnS to detect
circular polarization within the photospheric absorption lines of our
targets. We treat the spectropolarimetric data using the least-squares
deconvolution method to create high signal-to-noise ratio mean Stokes
V profiles. We also measure the classical S-index activity indicator
for the Ca ii H&amp;K lines, and the stellar radial velocity. To infer
the evolutionary status of our giants and to interpret our results,
we use state-of-the-art stellar evolutionary models with predictions
of convective turnover times. Results: We unambiguously
detect magnetic fields via Zeeman signatures in 29 of the 48 red
giants in our sample. Zeeman signatures are found in all but one of
the 24 red giants exhibiting signs of activity, as well as 6 out of
17 bright giant stars. However no detections were obtained in the 7
thermohaline deviant giants. The majority of the magnetically detected
giants are either in the first dredge up phase or at the beginning of
core He burning, i.e. phases when the convective turnover time is at a
maximum: this corresponds to a "magnetic strip" for red giants in the
Hertzsprung-Russell diagram. A close study of the 16 giants with known
rotational periods shows that the measured magnetic field strength
is tightly correlated with the rotational properties, namely to the
rotational period and to the Rossby number Ro. Our results show that
the magnetic fields of these giants are produced by a dynamo, possibly
of α-ω origin since Ro is in general smaller than unity. Four stars
for which the magnetic field is measured to be outstandingly strong
with respect to that expected from the rotational period/magnetic field
relation or their evolutionary status are interpreted as being probable
descendants of magnetic Ap stars. In addition to the weak-field giant
Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, η Psc) are
detected with magnetic field strength at the sub-Gauss level. Besides
Arcturus, these stars were not considered to be active giants before
this study and are very similar in other respects to ordinary giants,
with S-index indicating consistency with basal chromospheric
flux. Tables 6-8 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424579/olm">http://www.aanda.org</A>Based
on observations obtained at the Télescope Bernard Lyot (TBL) at
Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse,
France, and at the Canada-France-Hawaii Telescope (CFHT) which is
operated by the National Research Council of Canada, CNRS/INSU and
the University of Hawaii.

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Title: The surface signature of the tidal dissipation of the core
in a two-layer planet
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.; Lainey, V.
2015A&A...573A..23R Altcode: 2014arXiv1409.8343R
Context. Tidal dissipation, which is directly linked to internal
structure, is one of the key physical mechanisms that drive the
evolution of systems and govern their architecture. A robust evaluation
of its amplitude is thus needed to predict the evolution time for spins
and orbits and their final states. Aims: The purpose of this
paper is to refine a recent model of the anelastic tidal dissipation in
the central dense region of giant planets, which are commonly assumed to
retain a large amount of heavy elements, which constitute an important
source of dissipation. Methods: The previous paper evaluated
the impact of the static fluid envelope on the tidal deformation of
the core and on the associated anelastic tidal dissipation through
the tidal quality factor Qc. We examine here its impact
on the corresponding effective anelastic tidal dissipation through
the effective tidal quality factor Qp. Results:
We show that the strength of this mechanism mainly depends on mass
concentration. In the case of Jupiter- and Saturn-like planets, it can
increase their effective tidal dissipation by, around, factors 2.4 and
2, respectively. In particular, the range of the rheologies compatible
with the observations is enlarged compared to the results issued from
previous formulations. Conclusions: We derive here an improved
expression of the tidal effective factor Qp in terms of the
tidal dissipation factor of the core Qc, without assuming
the commonly used assumptions. When applied to giant planets, the
formulation obtained here allows a better match between the anelastic
core's tidal dissipation of a two-layer model and the observations.

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Title: Tidal dissipation in stars and fluid planetary layers and
their impact on the evolution of star-planet systems
Authors: Mathis, S.; Auclair-Desrotour, P.; Guenel, M.; Remus, F.;
Le Poncin-Lafitte, C.; Lainey, V.; Zahn, J. -P.
2014spih.confE...6M Altcode:
No abstract at ADS

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Title: Anelastic Tidal Dissipation in Multi-Layer Planets
Authors: Remus, Françcoise; Mathis, Stéphane; Zahn, Jean-Paul;
Lainey, Valéry
2014IAUS..293..362R Altcode:
Earth-like planets have anelastic mantles, whereas giant planets may
have anelastic cores. As for the fluid parts, the tidal dissipation of
these regions, gravitationally perturbed by a companion, highly depends
on its internal friction and thus its internal structure. Therefore,
modeling this kind of interaction presents a high interest to constrain
planetary interiors, whose properties are still quite uncertain. Here,
we examine the anelastic tidal dissipation in deep planetary interiors,
in presence of a fluid envelope, and taking into account its dependence
on the rheology. Taking plausible values for the anelastic
parameters, and discussing the frequency-dependence of the anelastic
dissipation, we show how this mechanism may compete with the dissipation
in fluid layers, when applied to Jupiter- and Saturn-like planets. We
also discuss the case of the icy giants Uranus and Neptune. Finally,
we show how the results may be implemented to describe the dynamical
evolution of planetary systems.

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Title: Rotation induced mixing in stellar interiors
Authors: Zahn, J. -P.
2013EAS....63..245Z Altcode:
The standard model of stellar structure is unable to account for various
observational facts, such as anomalies in the surface composition,
and there is now a broad consensus that some extra mixing must occur
in the radiation zones, in addition to the always present convective
overshoot or penetration. The search for the causes of this extra
mixing started in the late seventies, and it was quickly realized -
in particular by Sylvie Vauclair and her co-workers - that some mild
turbulence must be present to counteract the effect of gravitational
settling and radiative levitation. What could be responsible for this
turbulence? One suggestion was the internal gravity waves emitted
at the boundary of convection zones, but it is still not established
whether these waves will lead to true mixing. However they transport
angular momentum, and therefore they generate differential rotation,
which may be shear-unstable and thus lead to turbulence. Another way to
transport angular momentum and produce an unstable rotation profile is
through the large-scale circulation which is induced by the structural
adjustments as the star evolves, or by the torques applied to it (due
to stellar wind, accretion, tides). These processes participate in what
is called the "rotational mixing"; their implementation in stellar
evolution codes - again under Sylvie's impulse - has given birth to
a new generation of stellar models, which agree much better with the
observational constraints, although there is still room for improvement.

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Title: Seismic diagnostics for transport of angular momentum in
stars. I. Rotational splittings from the pre-main sequence to the
red-giant branch
Authors: Marques, J. P.; Goupil, M. J.; Lebreton, Y.; Talon, S.;
Palacios, A.; Belkacem, K.; Ouazzani, R. -M.; Mosser, B.; Moya, A.;
Morel, P.; Pichon, B.; Mathis, S.; Zahn, J. -P.; Turck-Chièze, S.;
Nghiem, P. A. P.
2013A&A...549A..74M Altcode: 2012arXiv1211.1271M
Context. Rotational splittings are currently measured for several
main sequence stars and a large number of red giants with the space
mission Kepler. This will provide stringent constraints on rotation
profiles. Aims: Our aim is to obtain seismic constraints on
the internal transport and surface loss of the angular momentum of
oscillating solar-like stars. To this end, we study the evolution
of rotational splittings from the pre-main sequence to the red-giant
branch for stochastically excited oscillation modes. Methods:
We modified the evolutionary code CESAM2K to take rotationally induced
transport in radiative zones into account. Linear rotational splittings
were computed for a sequence of 1.3 M⊙ models. Rotation
profiles were derived from our evolutionary models and eigenfunctions
from linear adiabatic oscillation calculations. Results: We find
that transport by meridional circulation and shear turbulence yields
far too high a core rotation rate for red-giant models compared with
recent seismic observations. We discuss several uncertainties in the
physical description of stars that could have an impact on the rotation
profiles. For instance, we find that the Goldreich-Schubert-Fricke
instability does not extract enough angular momentum from the core
to account for the discrepancy. In contrast, an increase of the
horizontal turbulent viscosity by 2 orders of magnitude is able to
significantly decrease the central rotation rate on the red-giant
branch. Conclusions: Our results indicate that it is possible
that the prescription for the horizontal turbulent viscosity largely
underestimates its actual value or else a mechanism not included in
current stellar models of low mass stars is needed to slow down the
rotation in the radiative core of red-giant stars.

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Title: Stellar Tides
Authors: Zahn, Jean-Paul
2013LNP...861..301Z Altcode:
To a first approximation, a binary star behaves as a closed system;
therefore it conserves its angular momentum while evolving to its state
of minimum kinetic energy, where the orbits are circular, all spins
are aligned, and the components rotate in synchronism with the orbital
motion. The pace at which this final state is reached depends on the
physical processes responsible for the dissipation of the tidal kinetic
energy. For stars with an outer convection zone, the dominant mechanism
is presumably the turbulent dissipation acting on the equilibrium
tide. For stars with an outer radiation zone, the major dissipative
process is radiative damping operating on the dynamical tide.

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Title: The anelastic equilibrium tide in exoplanetary systems
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.; Lainey, V.
2012sf2a.conf..245R Altcode: 2012arXiv1208.0377R
Earth-like planets have anelastic mantles, whereas giant planets may
have anelastic cores. As for the fluid parts of a body, the tidal
dissipation of such solid regions, gravitationally perturbed by a
companion body, highly depends on its internal friction, and thus on
its internal structure. Therefore, modelling this kind of interaction
presents a high interest to provide constraints on planet interiors,
whose properties are still quite uncertain. Here, we examine the
equilibrium tide in the solid central region of a planet, taking
into account the presence of a fluid envelope. We discuss how the
quality factor Q depends on the rheological parameters, and the size
of the core. Taking plausible values for the anelastic parameters, and
examinig the frequency-dependence of the solid dissipation, we show
how this mechanism may compete with the dissipation in fluid layers,
when applied to Jupiter- and Saturn-like planets. We also discuss the
case of the icy giants Uranus and Neptune.

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Title: The Anelastic Equilibrium Tide in Giant Planets
Authors: Remus, Francoise; Mathis, S.; Zahn, J.; Lainey, V.
2012DPS....4411319R Altcode:
Once a planetary system is formed, its dynamical evolution is
governed by gravitational interactions between its components, be it
a star-planet or planet-satellite interaction. By converting kinetic
energy into heat, the tides pertub their orbital and rotational
properties. The rate at which the system evolves depends on the
physical properties of tidal dissipation. Therefore, to understand
the past history and predict the fate of a binary system, one has to
identify the dissipative processes that achieve this conversion of
energy. Planetary systems display a large diversity of planets by their
composition. Since tidal mechanism is closely related with the internal
structure of the perturbed body, one has to investigate its effects
on either its fluid and solid layers. Studies have been carried out
on tidal effects in fluid bodies such as stars and envelopes of giant
planets. However, the planetary solid regions may also contribute to
tidal dissipation, be it the mantles of Earth-like planets that have
been investigated by many works, or the cores of giant planets. The
purpose of our study is to determine the tidal dissipation in the solid
central regions of giant planets, taking into account the presence of a
fluid envelope. We derive the different Love numbers that describe the
anelastic deformation and discuss the dependence of the quality factor Q
on the rheological parameters and the size of the core. Taking plausible
values for these parameters, and discussing the frequency-dependence
of the solid dissipation, we show how this mechanism may compete
with the dissipation in fluid layers, when applied to Jupiter- and
Saturn-like planets. We also discuss the case of the icy giants Uranus
and Neptune. Finally, we present the way to implement the results in the
equations that describe the dynamical evolution of planetary systems.

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Title: The equilibrium tide in stars and giant planets. I. The
coplanar case
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.
2012A&A...544A.132R Altcode: 2012arXiv1205.3536R
Context. Since 1995, more than 500 extrasolar planets have been
discovered orbiting very close to their parent star, where they
experience strong tidal interactions. Their orbital evolution depends
on the physical mechanisms that cause tidal dissipation, which remain
poorly understood. Aims: We refine the theory of the equilibrium
tide in fluid bodies that are partly or entirely convective, to predict
the dynamical evolution of the systems. In particular, we examine the
validity of modeling the tidal dissipation using the quality factor Q,
which is commonly done. We consider here the simplest case where the
considered star or planet rotates uniformly, all spins are aligned,
and the companion is reduced to a point mass. Methods: We
expand the tidal potential as a Fourier series, and express the
hydrodynamical equations in the reference frame, which rotates with
the corresponding Fourier component. The results are cast in the form
of a complex disturbing function, which may be implemented directly in
the equations governing the dynamical evolution of the system. 
Results: The first manifestation of the tide is to distort the shape
of the star or planet adiabatically along the line of centers. This
generates the divergence-free velocity field of the adiabatic
equilibrium tide, which is stationary in the frame rotating with the
considered Fourier component of the tidal potential; this large-scale
velocity field is decoupled from the dynamical tide. The tidal kinetic
energy is dissipated into heat by means of turbulent friction, which
is modeled here as an eddy-viscosity acting on the adiabatic tidal
flow. This dissipation induces a second velocity field, the dissipative
equilibrium tide, which is in quadrature with the exciting potential;
this field is responsible for the imaginary part of the disturbing
function, which is implemented in the dynamical evolution equations,
from which one derives the characteristic evolutionary times. Conclusions: The rate at which the system evolves depends on the
physical properties of the tidal dissipation, and specifically on both
how the eddy viscosity varies with tidal frequency and the thickness
of the convective envelope for the fluid equilibrium tide. At low
frequency, this tide is retarded by a constant time delay, whereas it
lags behind by a constant angle when the tidal frequency exceeds the
convective turnover rate.

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Title: Strong Tidal Dissipation in Saturn and Constraints on
Enceladus' Thermal State from Astrometry
Authors: Lainey, Valéry; Karatekin, Özgür; Desmars, Josselin;
Charnoz, Sébastien; Arlot, Jean-Eudes; Emelyanov, Nicolai; Le
Poncin-Lafitte, Christophe; Mathis, Stéphane; Remus, Françoise;
Tobie, Gabriel; Zahn, Jean-Paul
2012ApJ...752...14L Altcode: 2012arXiv1204.0895L
Tidal interactions between Saturn and its satellites play a crucial
role in both the orbital migration of the satellites and the heating of
their interiors. Therefore, constraining the tidal dissipation of Saturn
(here the ratio k 2/Q) opens the door to the past evolution
of the whole system. If Saturn's tidal ratio can be determined at
different frequencies, it may also be possible to constrain the giant
planet's interior structure, which is still uncertain. Here, we try
to determine Saturn's tidal ratio through its current effect on the
orbits of the main moons, using astrometric data spanning more than
a century. We find an intense tidal dissipation (k 2/Q =
(2.3 ± 0.7) × 10-4), which is about 10 times higher than
the usual value estimated from theoretical arguments. As a consequence,
eccentricity equilibrium for Enceladus can now account for the huge
heat emitted from Enceladus' south pole. Moreover, the measured k
2/Q is found to be poorly sensitive to the tidal frequency,
on the short frequency interval considered. This suggests that Saturn's
dissipation may not be controlled by turbulent friction in the fluid
envelope as commonly believed. If correct, the large tidal expansion
of the moon orbits due to this strong Saturnian dissipation would be
inconsistent with the moon formations 4.5 Byr ago above the synchronous
orbit in the Saturnian subnebulae. But it would be compatible with a
new model of satellite formation in which the Saturnian satellites
formed possibly over a longer timescale at the outer edge of the
main rings. In an attempt to take into account possible significant
torques exerted by the rings on Mimas, we fitted a constant rate da/dt
on Mimas' semi-major axis as well. We obtained an unexpected large
acceleration related to a negative value of da/dt = -(15.7 ± 4.4)
× 10-15 AU day-1. Such acceleration is about an
order of magnitude larger than the tidal deceleration rates observed
for the other moons. If not coming from an astrometric artifact
associated with the proximity of Saturn's halo, such orbital decay
may have significant implications on the Saturn's rings.

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Title: Anelastic tidal dissipation in multi-layer planets
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.; Lainey, V.
2012A&A...541A.165R Altcode: 2012arXiv1204.1468R
Context. Earth-like planets have viscoelastic mantles, whereas giant
planets may have viscoelastic cores. The tidal dissipation of these
solid regions, which are gravitationally perturbed by a companion body,
strongly depends on their rheology and the tidal frequency. Therefore,
modeling tidal interactions provides constraints on planets' properties
and helps us to understand their history and evolution, in either
our solar system or exoplanetary systems. Aims: We examine the
equilibrium tide in the anelastic parts of a planet for every rheology,
and by taking into account the presence of a fluid envelope of constant
density. We show how to obtain the different Love numbers describing its
tidal deformation, and discuss how the tidal dissipation in the solid
parts depends on the planet's internal structure and rheology. Finally,
we show how our results may be implemented to describe the dynamical
evolution of planetary systems. Methods: We expand in Fourier
series the tidal potential exerted by a point mass companion, and
express the dynamical equations in the orbital reference frame. The
results are cast in the form of a complex disturbing function, which
may be implemented directly in the equations governing the dynamical
evolution of the system. Results: The first manifestation of the
tide is to distort the shape of the planet adiabatically along the line
of centers. The response potential of the body to the tidal potential
then defines the complex Love numbers, whose real part corresponds
to the purely adiabatic elastic deformation and the imaginary part
accounts for dissipation. The tidal kinetic energy is dissipated
into heat by means of anelastic friction, which is modeled here by
the imaginary part of the complex shear modulus. This dissipation is
responsible for the imaginary part of the disturbing function, which is
implemented in the dynamical evolution equations, from which we derive
the characteristic evolution times. Conclusions: The rate at
which the system evolves depends on the physical properties of the tidal
dissipation, and specifically on (1) how the shear modulus varies with
tidal frequency, (2) the radius, and (3) the rheological properties
of the solid core. The quantification of the tidal dissipation in the
solid core of giant planets reveals a possible high dissipation that
may compete with dissipation in fluid layers.

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Title: The fluid equilibrium tide in stars and giant planets
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.
2011sf2a.conf..515R Altcode:
Many extrasolar planets orbit very close to their parent star, so that
they experience strong tidal interactions; by converting mechanical
energy into heat, these tides contribute to the dynamical evolution
of such systems. This motivates us to seek a deeper understanding
of the processes that cause tidal dissipation, which depend both on
the internal structure and the physical properties of the considered
bodies. Here, we examine the equilibrium tide, i.e. the hydrostatic
adjustment to the tidal potential, in a rotating fluid planet or
star. We derive the adiabatic velocity field induced by the tidal
perturbation and discuss in particular how the quality factor Q
characterizing the dissipation is linked whith the turbulent viscosity
of convection zones. Finally, we show how the results may be implemented
to describe the dynamical evolution of the system.

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Title: The equilibrium tide in viscoelastic parts of planets
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.; Lainey, V.
2011sf2a.conf..519R Altcode:
Earth-like planets have viscoelastic mantles, whereas giant planets
may have viscoelastic cores. As for the fluid parts of a body, the
tidal dissipation of such solid regions, gravitationally perturbed
by a companion body, highly depends on the tidal frequency, as well
as on the rheology. Therefore, modelling tidal interactions presents
a high interest to provide constraints on planet properties, and to
understand their history and their evolution. Here, we examine the
equilibrium tide in the solid core of a planet, taking into account the
presence of a fluid envelope. We explain how to obtain the different
Love numbers that describe its deformation. Next, we discuss how the
quality factor Q depends on the chosen viscoelastic model. Finally,
we show how the results may be implemented to describe the dynamical
evolution of planetary systems.

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Title: Magnetic confinement of the solar tachocline: The oblique
dipole
Authors: Strugarek , A.; Brun, A. S.; Zahn, J. -P.
2011AN....332..891S Altcode: 2011arXiv1112.1319A
3D MHD global solar simulations coupling the turbulent convective
zone and the radiative zone have been carried out. Essential features
of the Sun such as differential rotation, meridional circulation and
internal waves excitation are recovered. These realistic models are
used to test the possibility of having the solar tachocline confined by
a primordial inner magnetic field. We find that the initially confined
magnetic fields we consider open into the convective envelope. Angular
momentum is transported across the two zones by magnetic torques and
stresses, establishing the so-called Ferarro's law of isorotation. In
the parameter space studied, the confinement of the magnetic field by
meridional circulation penetration fails, also implying the failure of
the tachocline confinement by the magnetic field. Three-dimensional
convective motions are proven responsible for the lack of magnetic
field confinement. Those results are robust for the different magnetic
field topologies considered, i.e. aligned or oblique dipole.

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Title: The fluid equilibrium tide in stars and giant planets
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.
2011epsc.conf.1366R Altcode: 2011DPS....43.1366R
Many extrasolar planets orbit very close to their parent star, so that
they experience strong tidal interactions; by converting mechanical
energy into heat, these tides contribute to the dynamical evolution of
such systems. This motivates us to acquire a deeper understanding of
the processes that cause tidal dissipation, which depend both on the
structure and the physical properties of the considered body. Here
we examine the equilibrium tide, i.e. the hydrostatic adjustment
to the tidal potential, in a rotating fluid planet or star. We first
present the equations governing the problem, and show how to rigorously
separate the equilibrium tide from the dynamical tide, which is due
to the excited eigenmodes. We discuss in particular how the quality
factor Q is linked whith the turbulent viscosity of the convection
zone. Finally we show how the results may be implemented to describe
the dynamical evolution of the system.

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Title: The elasto-viscous equilibrium tide in exoplanetary systems
Authors: Remus, F.; Mathis, S.; Zahn, J. -P.; Lainey, V.
2011epsc.conf.1372R Altcode: 2011DPS....43.1372R
Earth-like planets have viscoelastic mantles. Moreover, giant planets
may have viscoelastic cores. As for the fluid parts of a body, the
tidal dissipation of such solid regions, gravitationally perturbed by
a companion body, highly depends on its internal friction, and thus on
the rheology, as well as on its size. Therefore, modelling this kind of
interaction presents a high interest to provide constraints on planets
properties. Here, we examine the equilibrium tide in the solid part
of a planet, taking into account the presence of a fluid envelope. We
first present the equations governing the problem, and show how to
obtain the different Love numbers that describe its deformation. We
discuss how the quality factor Q depends on the chosen viscoelastic
model. Finally we show how the results may be implemented to describe
the dynamical evolution of planetary systems.

---------------------------------------------------------
Title: Mixing in Stars
Authors: Zahn, Jean-Paul
2011iasa.confE..18Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The fluid Equilibrium Tide In Stars And Giant Planets
Authors: Remus, Françoise; Mathis, S.; Zahn, J.
2011ESS.....2.3401R Altcode:
Many extrasolar planets orbit very close to their parent star, so that
they experience strong tidal interactions; by converting mechanical
energy into heat, these tides contribute to the dynamical evolution
of such systems. This motivates us to acquire a deeper understanding
of the processes that cause tidal dissipation, which depend both on
the structure and the physical properties of the considered body. Here we examine the equilibrium tide, i.e. the hydrostatic adjustment
to the tidal potential, in a rotating fluid planet or star. We first
present the equations governing the problem, and show how to rigorously
separate the equilibrium tide from the dynamical tide, which is due
to the excited eigenmodes. We discuss in particular how the quality
factor Q is linked whith the turbulent viscosity of the convection
zone. Finally we show how the results may be implemented to describe
the dynamical evolution of the system.

---------------------------------------------------------
Title: Magnetic confinement of the solar tachocline: influence of
turbulent convective motions
Authors: Strugarek, Antoine; Brun, Allan Sacha; Zahn, Jean-Paul
2011IAUS..271..399S Altcode:
We present the results of 3D simulations, performed with the ASH
code, of the nonlinear, magnetic coupling between the convective and
radiative zones in the Sun, through the tachocline. Contrary to the
predictions of Gough &amp; McIntyre (1998), a fossil magnetic field,
deeply buried initially in the solar interior, will penetrate into
the convection zone. According to Ferraro's law of iso-rotation, the
differential rotation of the convective zone will thus expand into
the radiation zone, along the field lines of the poloidal field.

---------------------------------------------------------
Title: Magnetic confinement of the solar tachocline: II. Coupling
to a convection zone
Authors: Strugarek, A.; Brun, A. S.; Zahn, J. -P.
2011A&A...532A..34S Altcode: 2011arXiv1107.3665S
Context. The reason for the observed thinness of the solar tachocline
is still not well understood. One of the explanations that have been
proposed is that a primordial magnetic field renders the rotation
uniform in the radiation zone. Aims: We test here the validity
of this magnetic scenario through 3D numerical MHD simulations that
encompass both the radiation zone and the convection zone. 
Methods: The numerical simulations are performed with the anelastic
spherical harmonics (ASH) code. The computational domain extends from
0.07R⊙ to 0.97R⊙. Results: In the
parameter regime we explored, a dipolar fossil field aligned with
the rotation axis cannot remain confined in the radiation zone. When
the field lines are allowed to interact with turbulent unstationary
convective motions at the base of the convection zone, 3D effects
prevent the field confinement. Conclusions: In agreement with
previous work, we find that a dipolar fossil field, even when it is
initially buried deep inside the radiation zone, will spread into the
convective zone. According to Ferraro's law of iso-rotation, it then
imprints on the radiation zone the latitudinal differential rotation
of the convection zone, which is not observed.

---------------------------------------------------------
Title: Juri Toomre and the art of modeling convection zones
Authors: Zahn, Jean-Paul
2011IAUS..271..339Z Altcode:
Thermal convection plays a very important role in the structure and
evolution of stars, as it is one of the main physical processes that
transport heat from their interior where it is released, to the surface
where it is radiated into space. Much progress has been achieved in
modeling that process during the past 60 years, and I shall recall
here how Juri Toomre has greatly contributed to it.

---------------------------------------------------------
Title: Rapid rotation and mixing in active OB stars - Physical
processes
Authors: Zahn, Jean-Paul
2011IAUS..272...14Z Altcode:
In the standard description of stellar interiors, O and B stars possess
a thoroughly mixed convective core surrounded by a stable radiative
envelope in which no mixing occurs. But as is well known, this model
disagrees strongly with the spectroscopic diagnostic of these stars,
which reveals the presence at their surface of chemical elements that
have been synthesized in the core. Hence the radiation zone must be the
seat of some mild mixing mechanisms. The most likely to operate there
are linked with the rotation: these are the shear instabilites triggered
by the differential rotation, and the meridional circulation caused
by the changes in the rotation profile accompanying the non-homologous
evolution of the star. In addition to these hydrodynamical processes,
magnetic stresses may play an important role in active stars, which
host a magnetic field. These physical processes will be critically
examined, together with some others that have been suggested.

---------------------------------------------------------
Title: On the shape of rapidly rotating stars
Authors: Zahn, J. -P.; Ranc, C.; Morel, P.
2010A&A...517A...7Z Altcode:
 Aims: The critical surface of a rapidly rotating star is
determined, assuming that the rotation is either uniform or shellular
(angular velocity constant on level surfaces, but increasing with
depth). Methods: A step beyond the classical Roche model, where
the entire mass is assumed to be gathered at the center of the star,
here the quadrupolar moment of the mass distribution is taken into
account through a linear perturbation method. Results: The
flattening (defined here as the ratio between the equatorial and the
polar radius) can somewhat exceed the 3/2 value of the Roche model,
depending on the strength of the interior rotation. The result is
applied to a star of 7 solar masses, which is the mass of Achernar,
the star with the largest flattening detected so far through optical
interferometry.

---------------------------------------------------------
Title: The impact of magnetic fields on stellar structure and
evolution
Authors: Zahn, J. -P.
2009CoAst.158...27Z Altcode:
Magnetic fields responsible for various activity phenomena have been
detected at the surface of many stars. Presumably they are also present
in the stars' deep interior. They channel stellar winds to large
distance, thus increasing the loss of angular momentum. In stably
stratified radiation zones, they tend to impose uniform rotation,
and may thus interfere with the rotational mixing operating in these
regions and influence the evolution of the host star. Moreover, certain
magnetic configurations are known to be unstable, and they could produce
some mixing of their own. We shall discuss these problems in the light
of the latest developments.

---------------------------------------------------------
Title: Spanish Relativity Meeting Encuentros Relativistas Espanoles
Authors: Mediavilla, Evencio; Oscoz, Alex; Serra-Ricart, Miquel;
Zahn, Jean-Paul
2009srme.book.....M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On MHD rotational transport, instabilities and dynamo action
in stellar radiation zones
Authors: Mathis, Stéphane; Brun, A. -S.; Zahn, J. -P.
2009IAUS..259..421M Altcode:
Magnetic field and their related dynamical effects are thought
to be important in stellar radiation zones. For instance, it has
been suggested that a dynamo, sustained by a m = 1 MHD instability
of toroidal magnetic fields (discovered by Tayler in 1973), could
lead to a strong transport of angular momentum and of chemicals in
such stable regions. We wish here to recall the different magnetic
transport processes present in radiative zone and show how the dynamo
can operate by recalling the conditions required to close the dynamo
loop (BPol → BTor → BPol). Helped
by high-resolution 3D MHD simulations using the ASH code in the solar
case, we confirm the existence of the m = 1 instability, study its
non-linear saturation, but we do not detect, up to a magnetic Reylnods
number of 105, any dynamo action.

---------------------------------------------------------
Title: ISSI Workshop on Solar Magnetism: Concluding Remarks
Authors: Zahn, Jean-Paul
2009SSRv..144..423Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Diagnoses to unravel secular hydrodynamical processes in
rotating main sequence stars
Authors: Decressin, T.; Mathis, S.; Palacios, A.; Siess, L.; Talon,
S.; Charbonnel, C.; Zahn, J. -P.
2009A&A...495..271D Altcode: 2008arXiv0812.0363D
Context: Recent progress and constraints brought by helio and
asteroseismology call for a better description of stellar interiors
and an accurate description of rotation-driven mechanisms in stars. Aims: We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species
in the radiative regions of rotating stars. We focus on cases where
meridional circulation and shear-induced turbulence all that are
included in the simulations (i.e., no either internal gravity waves
nor magnetic fields). We put special emphasis on analysing the angular
momentum transport loop and on identifying the contribution of each of
the physical process involved. Methods: We develop a variety of
diagnostic tools designed to help disentangle the role of the various
transport mechanisms. Our analysis is based on a 2-D representation
of the secular hydrodynamics, which is treated using expansions in
spherical harmonics. By taking appropriate horizontal averages, the
problem reduces to one dimension, making it implementable in a 1D
stellar evolution code, while preserving the advective character of
angular momentum transport. We present a full reconstruction of the
meridional circulation and of the associated fluctuations of temperature
and mean molecular weight, along with diagnosis for the transport of
angular momentum, heat, and chemicals. In the present paper these tools
are used to validate the analysis of two main sequence stellar models
of 1.5 and 20 M_⊙, for which the hydrodynamics has previously been
extensively studied in the literature. Results: We obtain a clear
visualisation and a precise estimation of the different terms entering
the angular momentum and heat transport equations in radiative zones
of rotating stars. This enables us to corroborate the main results
obtained over the past decade by Zahn, Maeder, and collaborators
concerning the secular hydrodynamics of such objects. We focus on the
meridional circulation driven by angular momentum losses and structural
readjustments. We confirm quantitatively for the first time through
detailed computations and separation of the various components that
the advection of entropy by this circulation is balanced very well by
the barotropic effects and the thermal relaxation during most of the
main sequence evolution. This enables us to simplify for the thermal
relaxation on this phase. The meridional currents in turn advect heat
and generate temperature fluctuations that induce differential rotation
through thermal wind, thus closing the transport loop. We plan to make
use of our refined diagnosis tools in forthcoming studies of secular
(magneto-)hydrodynamics of stars at various evolutionary stages.

---------------------------------------------------------
Title: CoRot observations of active giants: preliminary results
Authors: Gondoin, P.; Fridlund, M.; Goupil, M. J.; Baudin, F.; Samadi,
R.; Barban, C.; Belkacem, K.; Corbard, T.; Dupret, M. A.; Foing, B.;
den Hartog, R.; Lebreton, Y.; Lochard, J.; Mathias, P.; Michel, E.;
Morel, P.; Moya, A.; Palacios, A.; Zahn, J. P.
2009AIPC.1094..864G Altcode: 2009csss...15..864G
We have analysed rotation modulated light-curves of active giants
observed with CoRot using spots model. Preliminary results suggest
an increase of the surface spot coverage with decreasing rotation
period. A maximum of the surface spot coverage seems to occur on
giants with effective temperature around 5100 K. Confirmation and
interpretation of these preliminary results require groundbased
follow-up observations to measure activity indicators, to identify
binary systems, and to determine the stellar parameters and evolutionary
status of the sample giants.

---------------------------------------------------------
Title: Stellar Magnetism
Authors: Neiner, C.; Zahn, J. -P.
2009EAS....39.....N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The impact of magnetic fields on stellar structure and
evolution
Authors: Zahn, J. -P.
2009EAS....39..211Z Altcode:
Magnetic fields have been detected at the surface of many stars, where
they are responsible for various activity phenomena, and presumably they
are present also in their deep interior. They channel stellar winds
to large distance, thus increasing the loss of angular momentum. In
stably stratified radiation zones, they tend to impose uniform rotation,
and may thus interfere with the rotational mixing operating in these
regions and influence the evolution of the host star. Moreover, certain
magnetic configurations are known to be unstable, and they could produce
some mixing of their own. We shall discuss these problems in the light
of the latest developments.

---------------------------------------------------------
Title: ISSI Workshop on Solar Magnetism: Concluding Remarks
Authors: Zahn, Jean-Paul
2009odsm.book..423Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Origin and Dynamics of Solar Magnetism
Authors: Thompson, M. J.; Balogh, A.; Culhane, J. L.; Nordlund, Å.;
Solanki, S. K.; Zahn, J. -P.
2009odsm.book.....T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Foreword
Authors: Neiner, Coralie; Zahn, Jean-Paul
2009EAS....39D...3N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Sun: A Slowly Rotating Star
Authors: Zahn, J. -P.
2009LNP...765....1Z Altcode:
After a brief historical sketch, we compare the slowly rotating Sun
to other stars and explain how it was able to lose so much of its
angular momentum. We next discuss the physical processes that may be
responsible for its internal rotation profile and conclude that the
most efficient of them is the transport of angular momentum by the
gravito-inertial waves excited at the base of the convection zone.

---------------------------------------------------------
Title: Effects of rotation on stellar structure: rotation induced
mixing
Authors: Zahn, J. -P.
2008CoAst.157..196Z Altcode:
Standard models of stellar structure are unable to account for various
observational facts, such as the appearance at the surface of chemical
elements that have been produced in the nuclear core. Thus there
is now a large consensus that some ‘extra mixing’ must occur
in the radiation zones. This mixing is achieved mainly through the
shear-turbulence generated by the differential rotation, which itself
results from the transport of angular momentum by a large-scale
circulation that is induced either by the structural adjustments
accompanying the evolution or by the applied torques (stellar wind,
accretion, tides). These processes are now being implemented in stellar
evolution codes, and they provide a much better agreement with the
observations.

---------------------------------------------------------
Title: Stellar Nucleosynthesis: 50 years after B2FH
Authors: Charbonnel, C.; Zahn, J. -P.
2008EAS....32.....C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamical aspects of stellar physics
Authors: Zahn, J. -P.; Brun, A. -S.; Mathis, S.
2008sf2a.conf..341Z Altcode:
Several manifestations of the dynamics of stellar interiors are
briefly presented, with emphasis on the most recent developments in
their numerical simulation.

---------------------------------------------------------
Title: Foreword
Authors: Charbonnel, C.; Zahn, J. -P.
2008EAS....32D...3C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Thermohaline mixing and fossil magnetic fields in red giant
stars
Authors: Charbonnel, Corinne; Zahn, Jean-Paul
2008IAUS..252..245C Altcode:
We discuss the occurence and consequences of thermohaline mixing in
RGB stars, as well as the possible inhibition of this process by a
fossil magnetic field in Ap star descendants.

---------------------------------------------------------
Title: On MHD rotational transport, instabilities and dynamo action
in stellar radiation zones
Authors: Mathis, S.; Zahn, J. -P.; Brun, A. -S.
2008IAUS..252..255M Altcode:
Magnetic field is an essential dynamical process in stellar radiation
zones. Moreover, it has been suggested that a dynamo action, sustained
by a MHD instability which affects the toroidal axisymmetric magnetic
field, could lead to a strong transport of angular momentum and of
chemicals in such regions. Here, we recall the different magnetic
transport and mixing processes in radiative regions. Next, we show
that the dynamo cannot operate as described by Spruit (2002) and
recall the condition required to close the dynamo loop. We perform
high-resolution 3D simulations with the ASH code, where we observe
indeed the MHD instability, but where we do not detect any dynamo
action, contrary to J. Braithwaite (2006). We conclude on the picture
we get for magnetic transport mechanisms in radiation zones and the
associated consequences for stellar evolution.

---------------------------------------------------------
Title: Wave transport in stellar radiation zone influenced by the
Coriolis acceleration
Authors: Mathis, S.; Talon, S.; Pantillon, F. P.; Charbonnel, C.;
Zahn, J. P.
2008JPhCS.118a2025M Altcode:
Internal gravity waves constitute an efficient process for angular
momentum transport over large distances. They are now seen as an
important ingredient in understanding the evolution of rotation, and
could explain the Sun's quasi-flat rotation profile. Because the Sun's
rotation frequency is of the same order as that of the waves, it is
necessary to refine our description of wave propagation and to take
into account the action of the Coriolis acceleration in a coherent
way. To achieve this aim, we adopt the Traditional Approximation
which is verified in stellar radiation zones. We present the modified
transport equations and their numerical evaluation in a parameter range
that is significant for the Sun. The effectiveness of gravity waves,
which become gravito-inertial waves, is reduced while new type of
waves, namely the Rossby, the Yanai and the Kelvin waves appear with
their associated transport.

---------------------------------------------------------
Title: Instabilities and mixing in stellar radiation zones
Authors: Zahn, Jean-Paul
2008IAUS..252...47Z Altcode:
The standard model of stellar structure is unable to account for
various observational facts, and there is now a large consensus that
some ‘extra mixing’ must occur in the radiation zones. The possible
causes for such mixing are briefly reviewed. The most efficient among
them is probably the shear-turbulence generated by the differential
rotation, which itself results from the transport of angular momentum
that can be mediated through the large-scale circulation induced
by structural adjustments or by the applied torques (stellar wind,
accretion, tides). In solar-type stars this angular momentum transport
is ensured mainly by internal gravity waves that are excited at the
boundary with convection zones. Another cause of mixing manifests
itself in the red giant phase, namely the thermohaline instability due
to an inversion of the molecular weight gradient. The implementation
of these processes in stellar evolution codes is giving rise to a new
generation of stellar models, which are in much better agreement with
the observational constraints.

---------------------------------------------------------
Title: Angular Momentum Transport in the Sun's Radiative Zone by
Gravito-Inertial Waves
Authors: Mathis, S.; Talon, S.; Pantillon, F. -P.; Zahn, J. -P.
2008SoPh..251..101M Altcode: 2008SoPh..tmp...48M
Internal gravity waves constitute an efficient process for angular
momentum transport over large distances. They are now seen as an
important ingredient in understanding the evolution of stellar rotation
and can explain the Sun's quasi-flat internal-rotation profile. Because
the Sun's rotation frequency is of the same order as that of the waves,
it is now necessary to refine our description of wave propagation and to
take into account the action of the Coriolis acceleration in a coherent
way. To achieve this goal, we adopt the traditional approximation,
which can be applied to stellar radiation zones under conditions
that are given. We present the modified transport equations and their
numerical evaluation in a parameter range that is significant for the
Sun. Consequences for the transport of angular momentum inside solar
and stellar radiative regions are discussed.

---------------------------------------------------------
Title: Tidal dissipation in binary systems
Authors: Zahn, J. -P.
2008EAS....29...67Z Altcode: 2008arXiv0807.4870Z
To first approximation, a binary system conserves its angular momentum
while it evolves to its state of minimum kinetic energy: circular orbit,
all spins aligned, and components rotating in synchronism with the
orbital motion. The pace at which this final state is achieved depends
on the physical processes that are responsible for the dissipation
of the tidal kinetic energy. For stars (or planets) with an outer
convection zone, the dominant mechanism identified so far is the
viscous dissipation acting on the equilibrium tide. For stars with
an outer radiation zone, it is the radiative damping operating on the
dynamical tide. After a brief presentation of the tides, I shall
review these physical processes; I shall discuss the uncertainties of
their present treatment, describe the latest developments, and compare
the theoretical predictions with the observed properties concerning
the orbital circularization of close binaries.

---------------------------------------------------------
Title: Star formation in accretion discs: from the Galactic center
to active galactic nuclei
Authors: Collin, S.; Zahn, J. -P.
2008A&A...477..419C Altcode: 2007arXiv0709.3772C
Context: Keplerian accretion discs around massive black holes (MBHs) are
gravitationally unstable beyond a few hundredths of a parsec, and they
should collapse to form stars. It has indeed been shown recently that an
accretion/star formation episode took place a few million years ago in
the Galactic center (GC). This raises the question of how the disc can
survive in AGN and quasars and continue to transport matter towards the
black hole. Aims: We study the accretion/star formation process in
quasars and AGN with one aim in mind: to show that a spectrum similar
to the observed one can be produced by the disc. Methods: We
compute models of stationary accretion discs that are either continuous
or clumpy. Continuous discs must be maintained in a state of marginal
stability so that the rate of star formation remains modest and the
disc is not immediately destroyed. The disc then requires additional
heating and additional transport of angular momentum. In clumpy discs,
the momentum transport is provided by cloud interactions. Results:
Non-viscous heating can be provided by stellar illumination, but in
the case of continuous discs, even momentum transport by supernovae
is insufficient for sustaining a marginal state, except at the very
periphery of the disc. In clumpy discs it is possible to account
for the required accretion rate through interactions between clouds,
but this model is unsatisfactory because its parameters are tightly
constrained without any physical justification. Conclusions:
Finally one must appeal to non-stationary discs with intermittent
accretion episodes like those that occurred in the GC, but such a
model is probably not applicable either to luminous high redshift
quasars or to radio-loud quasars.

---------------------------------------------------------
Title: Foreword
Authors: Goupil, M. -J.; Zahn, J. -P.
2008EAS....29D...3G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamic stability and mode coupling in Keplerian flows:
local strato-rotational analysis
Authors: Tevzadze, A. G.; Chagelishvili, G. D.; Zahn, J. -P.
2008A&A...478....9T Altcode: 2007arXiv0710.3648T
Aims:We present a qualitative analysis of key (but yet unappreciated)
linear phenomena in stratified hydrodynamic Keplerian flows: (i) the
occurrence of a vortex mode, as a consequence of strato-rotational
balance, with its transient dynamics; (ii) the generation of
spiral-density waves (also called inertia-gravity or gΩ waves) by the
vortex mode through linear mode coupling in shear flows. Methods:
Non-modal analysis of linearized Boussinesq equations were written in
the shearing sheet approximation of accretion disk flows. Results:
It is shown that the combined action of rotation and stratification
introduces a new degree of freedom, vortex mode perturbation, which
is in turn linearly coupled with the spiral-density waves. These two
modes are jointly able to extract energy from the background flow, and
they govern the disk dynamics in the small-scale range. The transient
behavior of these modes is determined by the non-normality of the
Keplerian shear flow. Tightly leading vortex mode perturbations undergo
substantial transient growth, then, becoming trailing, inevitably
generate trailing spiral-density waves by linear mode coupling. This
course of events - transient growth plus coupling - is particularly
pronounced for perturbation harmonics with comparable azimuthal and
vertical scales, and it renders the energy dynamics similar to the 3D
unbounded plane Couette flow case. Conclusions: Our investigation
strongly suggests that the so-called bypass concept of turbulence,
which has been recently developed by the hydrodynamic community for
spectrally stable shear flows, can also be applied to Keplerian
disks. This conjecture may be confirmed by appropriate numerical
simulations that take the vertical stratification and consequent mode
coupling into account in the high Reynolds number regime.

---------------------------------------------------------
Title: Tidal Effects in Stars, Planets and Disks
Authors: Goupil, M. -J.; Zahn, J. -P.
2008EAS....29.....G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Meridional circulation in the radiation zones of rotating
stars: Origins, behaviors and consequences on stellar evolution
Authors: Mathis, S.; Decressin, T.; Palacios, A.; Eggenberger, P.;
Siess, L.; Talon, S.; Charbonnel, C.; Turck-Chièze, S.; Zahn, J. -P.
2007AN....328.1062M Altcode:
Stellar radiation zones are the seat of meridional currents. This
circulation has a strong impact on the transport of angular momentum
and the mixing of chemicals that modify the evolution of stars. First,
we recall in details the dynamical processes that are taking place in
differentially rotating stellar radiation zones and the assumptions
which are adopted for their modelling in stellar evolution. Then, we
present our new results of numerical simulations which allow us to
follow in 2D the secular hydrodynamics of rotating stars, assuming
that anisotropic turbulence enforces a shellular rotation law and
taking into account the transport of angular momentum by internal
gravity waves. The different behaviors of the meridional circulation
in function of the type of stars which is studied are discussed with
their physical origin and their consequences on the transport of
angular momentum and of chemicals. Finally, we show how this work is
leading to a dynamical vision of the evolution of rotating stars from
their birth to their death.

---------------------------------------------------------
Title: Inhibition of thermohaline mixing by a magnetic field in
Ap star descendants: implications for the Galactic evolution of
3He
Authors: Charbonnel, C.; Zahn, J. -P.
2007A&A...476L..29C Altcode: 2007arXiv0711.3395C
Aims:To reconcile the measurements of ^3He/H in Galactic HII regions
with high values of ^3He in a couple of planetary nebulae, we propose
that thermohaline mixing is inhibited by a fossil magnetic field in
red giant stars that are descendants of Ap stars. Methods: We
examine the effect of a magnetic field on the salt-finger instability,
using a local analysis. Results: We obtain a threshold for
the magnetic field of 10^4{-}105 Gauss, above which it
inhibits thermohaline mixing in red giant stars located at or above
the bump. Fields of that order are expected in the descendants of
the Ap stars, taking into account the contraction of their core. Conclusions: We conclude that in a large fraction of the descendants
of Ap stars thermohaline mixing does not occur. As a consequence
these objects must produce ^3He as predicted by the standard theory
of stellar evolution and as observed in the planetary nebulae NGC
3242 and J320. The relative number of such stars with respect to
non-magnetic objects that undergo thermohaline mixing is consistent
with the statistical constraint coming from observations of the carbon
isotopic ratio in red giant stars. It also satisfies the Galactic
requirements for the evolution of the ^3He abundance.

---------------------------------------------------------
Title: On magnetic instabilities and dynamo action in stellar
radiation zones
Authors: Zahn, J. -P.; Brun, A. S.; Mathis, S.
2007A&A...474..145Z Altcode: 2007arXiv0707.3287Z
Context: We examine the MHD instabilities arising in the radiation
zone of a differentially rotating star, in which a poloidal field of
fossil origin is sheared into a toroidal field. Aims: We focus
on the non-axisymmetric instability that affects the toroidal magnetic
field in a rotating star, which was first studied by Pitts and Tayler
in the non-dissipative limit. If such an instability were able to mix
the stellar material, it could have an impact on the evolution of the
star. According to Spruit, it could also drive a dynamo. Methods:
We compare the numerical solutions built with the 3-dimensional ASH code
with the predictions drawn from an analytical study of the Pitts &amp;
Tayler instability. Results: The Pitts &amp; Tayler instability
is manifestly present in our simulations, with its conspicuous m=1
dependence in azimuth. But its analytic treatment used so far is too
simplified to be applied to the real stellar situation. Although the
instability generated field reaches an energy comparable to that of the
mean poloidal field, that field seems unaffected by the instability:
it undergoes Ohmic decline, and is neither eroded nor regenerated
by the instability. The toroidal field is produced by shearing
the poloidal field and it draws its energy from the differential
rotation. The small scale motions behave as Alfvén waves; they cause
negligible eddy-diffusivity and contribute little to the net transport
of angular momentum. Conclusions: In our simulations we observe
no sign of dynamo action, of either mean field or fluctuation type,
up to a magnetic Reynolds number of 10^5. However the Pitts &amp;
Tayler instability is sustained as long as the differential rotation
acting on the poloidal field is able to generate a toroidal field of
sufficient strength. But in the Sun such a poloidal field of fossil
origin is ruled out by the nearly uniform rotation of the deep interior.

---------------------------------------------------------
Title: Can a dynamo operate in stellar radiation zones?
Authors: Zahn, J. -P.; Brun, A. S.; Mathis, S.
2007sf2a.conf..566Z Altcode:
We examine the MHD instabilities arising in the radiation zone of a
differentially rotating star, in which a poloidal field of fossil origin
is sheared into a toroidal field. The numerical solutions built with the
3-dimensional ASH code are compared with the predictions drawn from an
analytical study of the Pitts &amp; Tayler instability. This instability
is manifestly present in our simulations, with its conspicuous m=1
dependence in azimuth. However, although the instability generated
field reaches an energy comparable to that of the mean poloidal field,
that field seems unaffected by the instability: it undergoes Ohmic
decline, and is neither eroded nor regenerated by the instability. The
instability is sustained as long as the differential rotation acting on
the poloidal field is able to generate a toroidal field of sufficient
strength but, up to a magnetic Reynolds number of 10^5, we observe
no sign of dynamo action, of either mean field or fluctuation type,
contrary to what was suggested by Spruit.

---------------------------------------------------------
Title: Transport and mixing by internal waves in stellar interiors:
effect of the Coriolis force
Authors: Mathis, S.; Zahn, J. -P.
2007arXiv0706.2446M Altcode:
We briefly recall the physical background of the transport of angular
momentum and the mixing of chemicals inside stellar radiation zones
and its importance for stellar evolution. Then, we describe its present
modeling, its successes and its weaknesses. Next, we introduce the new
theoretical developments that allow us to treat in a self-consistent
way the effect of the Coriolis force on the low-frequencies internal
waves and its consequences for the transport processes. This research is
aimed at improving the modeling of stellar interiors in the perspective
of future astero and helioseismology missions such as COROT and GOLF-NG.

---------------------------------------------------------
Title: Thermohaline mixing: a physical mechanism governing the
photospheric composition of low-mass giants
Authors: Charbonnel, C.; Zahn, J. -P.
2007A&A...467L..15C Altcode: 2007astro.ph..3302C
Aims:Numerous spectroscopic observations provide compelling evidence
for a non-canonical mixing process that modifies the surface abundances
of Li, C and N of low-mass red giants when they reach the bump in
the luminosity function. Eggleton and collaborators have proposed
that a molecular weight inversion created by the ^3He(^3He, 2p)^4He
reaction may be at the origin of this mixing, and relate it to the
Rayleigh-Taylor instability. We argue that one is actually dealing with
a double diffusive instability referred to as thermohaline convection
and we discuss its influence on the red giant branch. Methods:
We compute stellar models of various initial metallicities that include
thermohaline mixing, which is treated as a diffusive process based
on the prescription given originally by Ulrich for the turbulent
diffusivity produced by the thermohaline instability in stellar
radiation zones. Results: Thermohaline mixing simultaneously
accounts for the observed behaviour of the carbon isotopic ratio and
of the abundances of Li, C and N in the upper part of the red giant
branch. It significantly reduces the ^3He production with respect
to canonical evolution models as required by measurements of ^3He/H
in galactic HII regions. Conclusions: Thermohaline mixing
is a fundamental physical process that must be included in stellar
evolution modeling.

---------------------------------------------------------
Title: Concluding remarks
Authors: Zahn, Jean-Paul
2007IAUS..239..517Z Altcode: 2006astro.ph.12056Z
To be published in the proceedings of IAU Symposium 239 "Convection
in Astrophysics" (ed. F. Kupka, I. W. Roxburgh, K. L. Chan). Content:
1. From Nice to Prague, 2. The triumph of 3-D simulations, 3. How to
lower the cost - what else can be done? 4. A powerful tool to determine
surface abundances, 5. The effects of convection are not confined to
convection zones, 6. Towards a realistic model of the solar dynamo,
7. The moment of truth: facing observational tests, 8. On my wish-list.

---------------------------------------------------------
Title: On shear-induced turbulence in rotating stars
Authors: Mathis, S.; Palacios, A.; Zahn, J. -P.
2007A&A...462.1063M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Modeling Stellar Interiors with Rotational Mixing
Authors: Zahn, J. -P.
2007EAS....26...49Z Altcode:
Although it presently includes convective overshoot, microscopic
diffusion, gravitational settling and radiative acceleration,
the standard model of stellar structure is still unable to account
for various observational facts, and there is now a large consensus
that some extra mixing must occur in the radiation zones. To account
for such mixing, the minimalist approach consists in introducing a
parametrized turbulent diffusivity, and to adjust it so as to match
the observations. A better way is to strive to implement the physical
processes that may be responsible for this mixing, in particular
shear-induced turbulence and large-scale meridional circulation,
which both are linked with the differential rotation of the star. To
describe that rotational mixing, as we call it, one has thus to follow
the evolution of the internal rotation profile. By making some plausible
assumptions, its is possible to reduce the advection of angular momentum
through the 2-D circulation to a 1-D process, and that of the chemical
elements to a vertical diffusion. It is then possible to implement
these transports in a 1-D stellar evolution code. In massive stars,
angular momentum is transported mainly by the meridional circulation,
and there is good agreement between the observations and the predictions
based on the rotational mixing. This not so for solar-type stars where
another, more efficient mechanism is required to transport angular
momentum. A fossil magnetic field has been invoked, but recently it
has been shown that such a field would connect with the convection
zone, and imprint its differential rotation on the radiation zone,
which is not observed. The other candidate is the transport of angular
momentum by the internal gravity waves that are emitted at the base of
the convection zone; although their treatment is still somewhat crude,
it appears that they can explain both the quasi-uniform rotation of
the solar interior, and the depletion of lithium observed in such stars.

---------------------------------------------------------
Title: On hydrodynamic shear turbulence in spectrally stable
Keplerian disks
Authors: Lominadze, J. G.; Tevzadze, A. G.; Chagelishvili, G. D.;
Zahn, J. -P.; Chanishvili, R. G.
2007acag.conf..335L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamic models of the tachocline
Authors: Zahn, Jean-Paul
2007sota.conf...89Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Advances in Secular Magnetohydrodynamics of Stellar Interiors
Dedicated to Asteroseismic Spatial Missions
Authors: Mathis, S.; Eggenberger, P.; Decressin, T.; Palacios, A.;
Siess, L.; Charbonnel, C.; Turck-Chièze, S.; Zahn, J. -P.
2007EAS....26...65M Altcode: 2007astro.ph..3117M
With the first light of COROT, the preparation of KEPLER and the future
helioseismology spatial projects such as GOLF-NG, a coherent picture
of the evolution of rotating stars from their birth to their death is
needed. We describe here the modelling of the macroscopic transport of
angular momentum and matter in stellar interiors that we have undertaken
to reach this goal. First, we recall in detail the dynamical processes
that are driving these mechanisms in rotating stars and the theoretical
advances we have achieved. Then, we present our new results of numerical
simulations which allow us to follow in 2D the secular hydrodynamics
of rotating stars, assuming that anisotropic turbulence enforces a
shellular rotation law. Finally, we show how this work is leading to
a dynamical vision of the Hertzsprung-Russel diagram with the support
of asteroseismology and helioseismology, seismic observables giving
constraints on the modelling of the internal transport and mixing
processes. In conclusion, we present the different processes that
should be studied in the near future to improve our description of
stellar radiation zones.

---------------------------------------------------------
Title: Why Bothering to Measure Stellar Rotation with CoRoT?
Authors: Goupil, M. J.; Moya, A.; Suarez, J. C.; Lochard, J.; Barban,
C.; Dias Do Nascimento, J.; Dupret, M. A.; Samadi, R.; Baglin,
A.; Zahn, J. P.; Hubert, A. M.; Brun, S.; Boisnard, L.; Morel, P.;
Garrido, R.; Mathis, S.; Michel, E.; Renan de Medeiros, J.; Palacios,
A.; Lignières, F.; Rieutord, E. M.
2006ESASP1306...51G Altcode:
One important goal of the CoRoT experiment is to obtain information
about the internal rotation of stars, in particular the ratio of
central to surface rotation rates. This will provide constraints on
the modelling of transport mechanisms of angular momentum acting in
radiative (rotationally induced turbulent) and convective zones (plumes,
extension beyond convectively instable regions). Relations between
the surface rotation period and age, magnetic activity, mass loss and
other stellar characteristics can also be studied with a statistically
significant set of data as will be provided by Corot. We present various
theoretical efforts performed over the past years in order to develope
the theoretical tools which will enable us to study rotation with Corot.

---------------------------------------------------------
Title: Magnetic confinement of the solar tachocline
Authors: Brun, A. S.; Zahn, J. -P.
2006A&A...457..665B Altcode: 2006astro.ph.10069B
Context: .We study the physics of the solar tachocline (i.e. the thin
transition layer between differential rotation in the convection
zone and quasi uniform rotation in the radiative interior), and
related MHD instabilities. Aims: .We have performed 3D MHD
simulations of the solar radiative interior to check whether a fossil
magnetic field is able to prevent the spread of the tachocline. Methods: .Starting with a purely poloidal magnetic field and
a latitudinal shear meant to be imposed by the convection zone at
the top of the radiation zone, we have investigated the interactions
between magnetic fields, rotation and shear, using the spectral code
ASH on massively parallel supercomputers. Results: .In all
cases we have explored, the fossil field diffuses outward and ends
up connecting with the convection zone, whose differential rotation
is then imprinted at latitudes above ≈40° throughout the radiative
interior, according to Ferraro's law of isorotation. Rotation remains
uniform in the lower latitude region which is contained within closed
field lines. We find that the meridional flow cannot stop the inward
progression of the differential rotation. Further, we observe the
development of non-axisymmetric magnetohydrodynamic instabilities,
first due to the initial poloidal configuration of the fossil field,
and later to the toroidal field produced by shearing the poloidal field
through the differential rotation. We do not find dynamo action as
such in the radiative interior, since the mean poloidal field is not
regenerated. But the instability persists during the whole evolution,
while slowly decaying with the mean poloidal field; it is probably
sustained by small departures from isorotation. Conclusions:
.According to our numerical simulations, a fossil magnetic field cannot
prevent the radiative spread of the tachocline, and thus it is unable
to enforce uniform rotation in the radiation zone. Neither can the
observed thinness of that layer be invoked as a proof for such an
internal fossil magnetic field.

---------------------------------------------------------
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: Dynamical processes in stellar radiation zones
Authors: Mathis, S.; Decressin, T.; Palacios, A.; Siess, L.;
Charbonnel, C.; Turck-Chièze, S.; Zahn, J. -P.
2006IAUJD..17E..18M Altcode:
With the progress of asteroseismology, we need a coherent picture of
the evolution of rotating stars from their birth to their death. We
describe here the modelling of the macroscopic transport of matter
and angular momentum in stellar interiors that we have undertaken to
achieve this goal. First, we briefly recall the dynamical processes
that are driving these mechanisms in rotating stars. Then, we present
our new results of numerical simulations which allow us to follow in 2D
the secular hydrodynamics of rotating stars, assuming that anisotropic
turbulence enforces a cellular rotation law. We discuss the different
processes such as MHD instabilities that should be studied in the next
future to improve our description of dynamical processes in stellar
radiation zones. Finally, we show how our new results are leading
us to a dynamical vision of Hertzsprung-Russell diagram, in support
of asteroseismology.

---------------------------------------------------------
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: Tidal effects in extrasolar planetary systems
Authors: Mathis, S.; Zahn, J. -P.
2006sf2a.conf..487M Altcode:
With the discovery of new extrasolar planetary systems day after day,
we have to understand the physical processes which are driving the
dynamical evolution of such systems. We focus here on the tidal
processes acting between stars and their giant fluid planets. We
describe briefly our new theoretical results on the hydrodynamical
tides (equilibrium and dynamical tides). In particular, we present the
complete set of dynamical equations governing the tidal evolution which
we have derived in a consistent way and for the most general case:
eccentric orbits, non synchronized components, general inclination
between the orbital spin and those of components.

---------------------------------------------------------
Title: Dynamical processes in stellar radiation zones: secular
magnetohydrodynamics of rotating stars
Authors: Mathis, S.; Decressin, T.; Palacios, A.; Siess, L.;
Charbonnel, C.; Turck-Chièze, S.; Zahn, J. -P.
2006sf2a.conf..491M Altcode:
With the imminent launch of COROT and the preparation of new
helioseismology instruments such as GOLF-NG (cf. DynaMICS project), we
need a coherent picture of the evolution of rotating stars from their
birth to their death. We describe here the modeling of the macroscopic
transport of angular momentum and matter in stellar interiors that we
have undertaken to achieve this goal. First, we recall the dynamical
processes that are driving this transport in rotating stars and the
theoretical advances we have accomplished. Then, we present our new
results of numerical simulations which allow us to follow in 2D the
secular hydrodynamics of rotating stars, assuming that anisotropic
turbulence enforces a shellular rotation law. Finally, we show how
this work is leading to a dynamical vision of the Hertzsprung-Russel
diagram in support of asteroseismology and helioseismology.

---------------------------------------------------------
Title: Magnetic instabilities in stellar radiation zones
Authors: Brun, A. S.; Zahn, J. -P.
2006sf2a.conf..451B Altcode:
Using the 3-dimensional ASH code, we have studied numerically the
instabilities that occur in stellar radiation zones in presence of
large-scale magnetic fields and differential rotation. We confirm
that some configurations are linearly unstable, as predicted by
Tayler and collaborators, and we determine the saturation level of
the instability. However we found no sign of the dynamo mechanism
suggested recently by Spruit.

---------------------------------------------------------
Title: Energy flow in acoustic black holes
Authors: Choy, K.; Kruk, T.; Carrington, M. E.; Fugleberg, T.; Zahn,
J.; Kobes, R.; Kunstatter, G.; Pickering, D.
2006PhRvD..73j4011C Altcode: 2005gr.qc.....5163C
We present the results of an analysis of superradiant energy flow
due to scalar fields incident on an acoustic black hole. In addition
to providing independent confirmation of the recent results in
[E. Berti, V. Cardoso, and J. P. S. Lemos, Phys. Rev. D 70, 124006
(2004).PRVDAQ0556-282110.1103/PhysRevD.70.124006], we determine in
detail the profile of energy flow everywhere outside the horizon. We
confirm explicitly that in a suitable frame the energy flow is inward
at the horizon and outward at infinity, as expected on physical grounds.

---------------------------------------------------------
Title: Rotational mixing in stellar radiation zones: successes
and weaknesses
Authors: Zahn, J. -P.
2006EAS....19..147Z Altcode:
The common way to build stellar models is to treat the radiation
zones as inert, stable regions, in contrast with the well mixed
convection zones. This allows to account for the main features of
stellar evolution, such as the main sequence and the existence
of red giants. But there are many indications that some mixing
occurs within the radiation zones, and that it has an impact on the
later stages. Manuel Forestini was one the first to undertake the
implementation of such mixing in his stellar code, and thus to go
beyond the standard model. Here I will briefly describe the mixing
processes that are likely to operate in stellar radiation zones, as
we understand them today, and discuss how well they account for the
observed properties of stars.

---------------------------------------------------------
Title: Probing the internal magnetic field of slowly pulsating
B-stars through g modes
Authors: Hasan, S. S.; Zahn, J. -P.; Christensen-Dalsgaard, J.
2005A&A...444L..29H Altcode: 2005astro.ph.11472H
Context: . Aims: .We suggest that high-order g modes can be used
as a probe of the internal magnetic field of SPB (slowly pulsating
B) stars. The idea is based on earlier work by the authors hich
analytically investigated the effect of a vertical magnetic field on p
and g modes in a plane-parallel isothermal stratified atmosphere. It
was found that even a weak field can significantly shift the g-mode
frequencies - the effect increases with mode order. Methods:
.In the present study we adopt the classical perturbative approach
to estimate the internal field of a 4 solar mass SPB star by looking
at its effect on a low-degree (l=1) and high-order (n=20) g mode with
a period of about 1.5 d. Results: .We find that a polar field
strength of about 110 kG on the edge of the convective core is required
to produce a frequency shift of 1%. Frequency splittings of that order
have been observed in several SPB variables, in some cases clearly too
small to be ascribed to rotation. We suggest that they may be due to
a poloidal field with a strength of order 100 kG, buried in the deep
interior of the star. Conclusions: .

---------------------------------------------------------
Title: Transport and mixing by internal waves in stellar interiors:
effect of the Coriolis force
Authors: Mathis, S.; Zahn, J. -P.
2005sf2a.conf..319M Altcode:
We briefly recall the physical background of the transport of angular
momentum and the mixing of chemicals inside stellar radiation zones
and its importance for stellar evolution. Then, we describe its present
modeling, its successes and its weaknesses. Next, we introduce the new
theoretical developments that allow us to treat in a self-consistent
way the effect of the Coriolis force on the low-frequencies internal
waves and its consequences for the transport processes. This research is
aimed at improving the modeling of stellar interiors in the perspective
of future astero and helioseismology missions such as COROT and GOLF-NG.

---------------------------------------------------------
Title: What did we learn from the tidal evolution theory?
Authors: Zahn, J. -P.
2005ASPC..333..280Z Altcode:
These notes follow the powerpoint presentation which was prepared to
trigger the discussion. I tried my best to incorporate in the text the
comments, questions and answers made by the participants. I hope they
will forgive me for not quoting them individually.

---------------------------------------------------------
Title: Physical mechanisms for tidal dissipation
Authors: Zahn, J. -P.
2005ASPC..333....4Z Altcode:
A binary system tends to its state of minimum kinetic energy, for
given (and conserved) angular momentum : circular orbit, all spins
aligned, and rotation of the components synchronized with the orbital
motion. The pace at which this final state is achieved depends on the
physical processes which are responsible for the dissipation of the
kinetic energy. For stars (or planets) with an outer convection zone,
the dominant mechanism identified so far is the viscous dissipation
acting on the equilibrium tide. For stars with an outer radiation zone,
it is the radiative damping operating on the dynamical tide. I shall
review these physical processes, discuss the uncertainties of their
present treatment, present the latest developments, and compare the
theoretical predictions with the observed properties concerning the
orbital circularization of close binaries.

---------------------------------------------------------
Title: Equilibrium tides in differentially rotating stars
Authors: Mathis, S.; Zahn, J. -P.
2005ASPC..333..108M Altcode:
In this work, we present new theoretical advances in the modelization of
the equilibrium tide in the convective envelope of solar-type binary
stars. We simplify the problem by assuming that all the spins are
aligned but we take into account the two-dimensional character of the
rotation law in the convection zone. Following the method of Zahn (1966)
we first derive the adiabatic tide which is in phase with the perturbing
potential exerted by the companion. Next, we derive the dissipative tide
which is in quadrature with it, using the crude MLT eddy-viscosity to
model the action of turbulence on the large scale tidal flow. Finally,
we discuss the impact of those processes on the dynamical evolution of
binary systems. Note that our results may be applied as well to giant
planets and to exo-planets, since these possess also a convection zone.

---------------------------------------------------------
Title: Tidal Evolution and Oscillations in Binary Stars
Authors: Claret, A.; Giménez, A.; Zahn, J. -P.
2005ASPC..333.....C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Transport and mixing in the radiation zones of rotating
stars. II. Axisymmetric magnetic field
Authors: Mathis, S.; Zahn, J. -P.
2005A&A...440..653M Altcode: 2005astro.ph..6105M
The purpose of this paper is to improve the modeling of the mixing of
chemical elements that occurs in stellar radiation zones. In addition
to the classical rotational mixing considered in our previous paper,
which results of the combined action of the thermally-driven meridional
circulation and of the turbulence generated by the shear of differential
rotation, we include here the effect of an axisymmetric magnetic field
in a self-consistent way. We treat the advection of the field by the
meridional circulation, its Ohmic diffusion, and the production of its
toroidal component through the shear of differential rotation. The
Lorentz force is assumed not to exceed the centrifugal force; it
acts on the baroclinic balance and therefore on the meridional flow,
and it has a strong impact on the transport of angular momentum. All
variables and governing equations are expanded in spherical or spherical
vectorial functions, to arbitrary order: this yields a system of
partial differential equations in time and in the radial coordinate,
which is ready to be implemented in a stellar structure code.

---------------------------------------------------------
Title: Stability and turbulent transport in Taylor-Couette flow from
analysis of experimental data
Authors: Dubrulle, B.; Dauchot, O.; Daviaud, F.; Longaretti, P. -Y.;
Richard, D.; Zahn, J. -P.
2005PhFl...17i5103D Altcode: 2011arXiv1106.1276D
This paper provides discussion and prescription about stability and
transport in the Taylor-Couette experiment, a rotating shear flow with
shear perpendicular to the rotation axis. Such geometry frequently
occurs in geophysical or astrophysical context. The prescriptions
we obtain are the result of a detailed analysis of the experimental
data obtained in several studies of the transition to turbulence and
turbulent transport in Taylor-Couette flow. We first introduce a new
set of control parameters, based on dynamical rather than geometrical
considerations, so that they may be relevant to any rotating shear flows
in general and not only to Taylor-Couette flow. We then investigate
the transition thresholds in the supercritical and the subcritical
regime in order to extract their general dependencies on the control
parameters. The inspection of the mean profiles provides us with
some general hints on the turbulent to laminar shear ratio. Then the
examination of the torque data allows us to propose a decomposition
of the torque dependence on the control parameters in two terms,
one completely determined by measurements in the case where the outer
cylinder is at rest, the other one being a universal function deduced
here from experimental fits. As a result, we obtain a general expression
for the turbulent viscosity and compare it to existing prescriptions in
the literature. Finally, throughout the paper we discuss the influence
of additional effects such as stratification or magnetic fields.

---------------------------------------------------------
Title: Circularization in B-Bype Eclipsing Binaries in both Magellanic
Clouds
Authors: North, P.; Zahn, J. -P.
2005HiA....13..466N Altcode:
Making use of detached eclipsing binaries discovered by the OGLE and
MACHO teams in the SMC and in the LMC respectively we give the value of
the fractional radius above which cirularization occurs. This critical
radius is around 0.25 to 0.26 regardless of the mass surface gravity or
metallicity and is consistent with that found by Giuricin et al. (1984)
for galactic binaries. These empirical facts are shown to be consistent
with Zahn's (1975) theory of tidal dissipation. As a by-product of this
study we provide approximate stellar parameters of the average component
of 148 binaries in the SMC and of 354 binaries (some of which might
be non-detached) in the LMC under the assumption of equal components.

---------------------------------------------------------
Title: An hydrodynamic shear instability in stratified disks
Authors: Dubrulle, B.; Marié, L.; Normand, Ch.; Richard, D.; Hersant,
F.; Zahn, J. -P.
2005A&A...429....1D Altcode: 2004astro.ph.10224D
We discuss the possibility that astrophysical accretion disks
are dynamically unstable to non-axisymmetric disturbances with
characteristic scales much smaller than the vertical scale height. The
instability is studied using three methods: one based on the energy
integral, which allows the determination of a sufficient condition of
stability, one using a WKB approach, which allows the determination of
the necessary and sufficient condition for instability and a last one
by numerical solution. This linear instability occurs in any inviscid
stably stratified differential rotating fluid for rigid, stress-free or
periodic boundary conditions, provided the angular velocity Ω decreases
outwards with radius r. At not too small stratification, its growth rate
is a fraction of Ω. The influence of viscous dissipation and thermal
diffusivity on the instability is studied numerically, with emphasis
on the case when d ln Ω / d ln r =-3/2 (Keplerian case). Strong
stratification and large diffusivity are found to have a stabilizing
effect. The corresponding critical stratification and Reynolds number
for the onset of the instability in a typical disk are derived. We
propose that the spontaneous generation of these linear modes is the
source of turbulence in disks, especially in weakly ionized disks.

---------------------------------------------------------
Title: The Spite plateau: a puzzle and a challenge for the stellar
physicist
Authors: Zahn, Jean-Paul
2005IAUS..228...41Z Altcode:
In order to use the lithium abundance of the Spite plateau to constrain
the Big Bang Nucleosynthesis, one has to determine how much of the
original lithium has been destroyed by the various physical processes
that are known to operate in stellar radiation zones. These are briefly
reviewed, with emphasis on the mixing occurring in tachoclines and on
that generated indirectly by the transport of angular momentum through
internal gravity waves.

---------------------------------------------------------
Title: Processes competing with atomic diffusion: mass loss,
turbulence, rotation, etc.
Authors: Zahn, J. -P.
2005EAS....17..157Z Altcode:
Due to radiative acceleration and gravitational settling, stars
would display vast differences in their surface composition, if
these were not smoothed out, at least partly, by some competing
processes. Possible mechanisms have been suggested and examined by
Georges Michaud and his collaborators: mass loss, turbulent mixing
and meridional circulation. They will be discussed here in the light
of more recent developments.

---------------------------------------------------------
Title: Mixing in stellar radiation zones: new theoretical results
Authors: Mathis, S.; Zahn, J. -P.
2004sf2a.conf..543M Altcode: 2004sf2a.confE.289M
We briefly recall the physical background of the rotational mixing
occurring inside stellar radiation zones and its importance for stellar
evolution. We describe its present modelization, its successes and
its weaknesses. Next, we introduce the new theoretical results which
allow us to treat the hydrodynamical processes simultaneously in the
bulk of radiation zones and in the tachoclines, and we present our
new prescription for the horizontal turbulent transport which has
been derived from Couette-Taylor laboratory experiment. Finally,
we show how to introduce self-consistently the effect of an axi
or non-axisymmetric magnetic field, which may be responsible for
the angular momentum transport in low-mass stars (internal gravity
waves are another candidate). This research is aimed at improving the
modelization of stellar interiors in the perspective of future astero
and helioseismology missions such as COROT, EDDINGTON and GOLF-NG,
and of new powerful ground-based instruments as ESPADON.

---------------------------------------------------------
Title: Advances in the modelization of binary systems: revisiting
the equilibrium tide
Authors: Mathis, S.; Zahn, J. -P.
2004sf2a.conf..241M Altcode: 2004sf2a.confE.317M
In this work, we present new theoretical advances in the modelization
of the equilibrium tide in the convective envelope of solar-type
binary stars. We take into account the two-dimensional character of
the rotation law in the convection zone and the inclination of the
spin of each component with respect to the orbital one. Following the
method of Zahn (1966) we first derive the adiabatic tide which is in
phase with the perturbing potential exerted by the companion. Next,
we derive the dissipative tide which is in quadrature with it, using
the crude MLT eddy-viscosity to modelize the action of turbulence on
the large scale tidal flow. Finally, we discuss the impact of those
processes on the dynamical evolution of binary systems. Note that our
results may be applied as well to giant planets and to exo-planets,
since these possess also a convection zone.

---------------------------------------------------------
Title: Transport and mixing in the radiation zones of rotating
stars. I. Hydrodynamical processes
Authors: Mathis, S.; Zahn, J. -P.
2004A&A...425..229M Altcode: 2004astro.ph..6418M
The purpose of this paper is to improve the modelization of the
rotational mixing which occurs in stellar radiation zones, through
the combined action of the thermally driven meridional circulation
and of the turbulence generated by the shear of differential
rotation. The turbulence is assumed to be anisotropic, due to the
stratification, with stronger transport in the horizontal directions
than in the vertical. The main difference with the former treatments
by Zahn (\cite{Zahn1992}, A&amp;A, 265, 115) and Maeder &amp; Zahn
(\cite{Maeder1998}, A&amp;A, 334, 1000) is that we expand here
the departures from spherical symmetry to higher order, and include
explicitly the differential rotation in latitude, to first order. This
allows us to treat simultaneously the bulk of a radiation zone and its
tachocline(s). Moreover, we take fully into account the non-stationarity
of the problem, which will enable us to tackle the rapid phases
of evolution. The system of partial differential equations, which
govern the transport of angular momentum, heat and chemical elements,
is written in a form which makes it ready to implement in a stellar
evolution code. Here the effect of a magnetic field is deliberately
ignored; it will be included in forthcoming papers. Appendices
are only available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: On shear-induced turbulence in rotating stars
Authors: Mathis, S.; Palacios, A.; Zahn, J. -P.
2004A&A...425..243M Altcode: 2004astro.ph..3580M
We review various prescriptions which have been proposed for the
turbulent transport of matter and angular momentum in differentially
rotating stellar radiation zones. A new prescription is presented
for the horizontal transport associated with the anisotropic shear
turbulence which is produced by the differential rotation in latitude;
this “β-viscosity” is drawn from torque measurements in the classical
Couette-Taylor experiment (Richard &amp; Zahn \cite{Richard99}, A&amp;A,
347, 734). Its implementation in a stellar evolution code leads to
enhanced mixing, as illustrated by models of a rotating main-sequence
star of 1.5 solar mass.

---------------------------------------------------------
Title: Circularization in B-type eclipsing binaries in both Magellanic
Clouds
Authors: North, P.; Zahn, J. -P.
2004NewAR..48..741N Altcode:
By making use of detached eclipsing binaries with B-type components
discovered by the OGLE and MACHO teams in the SMC and in the LMC, we
give the value of the fractional radius above which circularization
occurs. This critical radius is around 0.24-0.26, regardless of the
mass, surface gravity or metallicity, and is consistent with that found
by Giuricin et al. [A&amp;A 134 (1984) 365] for galactic binaries. These
empirical facts are shown to be consistent with Zahn's [A&amp;A 41
(1975) 329] theory of tidal dissipation. As a by-product of this study,
we provide approximate stellar parameters of the average component of
148 binaries in the SMC and of up to 353 binaries (some of which might
be non-detached) in the LMC, under the assumption of equal components.

---------------------------------------------------------
Title: Summary and Concluding Remarks (Invited Review)
Authors: Zahn, J. -P.
2004IAUS..215..616Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Rotational mixing in stellar radiation zones
Authors: Zahn, J. -P.
2004EAS....13...63Z Altcode:
Contrary to what is assumed in building standard stellar models, all
stars undergo some mild mixing in their radiation zones. This mixing is
due mainly to a slow thermally driven meridional circulation, and shear
turbulence presumably contributes to it. It is the rotation of the star
which is responsible for these motions, and therefore the transport
of angular momentum must be described in time and space when modeling
stellar evolution. This rotational mixing plays an important role in
the evolution of massive stars; in particular it determines the late,
pre-supernova phases. We review the present state of the problem and
discuss briefly the open questions.

---------------------------------------------------------
Title: Evolution of Massive Stars, Mass Loss and Winds
Authors: Heydari-Malayeri, M.; Stee, Ph.; Zahn, J. -P.
2004EAS....13.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Foreword
Authors: Heydari-Malayeri, Mohammad; Stee, Philippe; Zahn, Jean-Paul
2004EAS....13D...3H Altcode:
This volume presents a large selection of the lectures which have
been given during two consecutive Summer Schools in Stellar Physics
(XII and XIII). The first of these schools took place in Aussois, a
ski resort in the Alps, on 13-18 October 2002, and it dealt with the
formation and the evolution of massive stars. The second was held at
Oléron, an island off the Atlantic shore, on 6-10 October 2003, and
it focussed on mass loss from stars and on stellar winds. The subjects
of these two schools are closely linked, since massive stars lose an
important fraction of their mass in the course of their evolution,
and that is why we merged the lecture notes in a single volume.In
spite of their relatively small number, massive stars play a key role
in several aspects. They are the principal sites of nucleosynthesis,
and because most of their matter is ejected by their winds and in
the final supernova explosion, they are responsible for the chemical
evolution of their host galaxy, and thus of the Universe. They are the
main energy providers to the interstellar medium, through their winds
and their UV radiation. Finally, since they are very luminous, they can
be detected in remote galaxies, delivering precious clues about star
formation.In recent years, one has become increasingly aware that their
evolution strongly depends on the mass they lose, and on the internal
mixing which is induced by rotation, as was emphasized by two world
experts of the subject: André Maeder in Aussois, and Georges Meynet in
Oléron. These effects must be taken into account when modeling these
stars, or else one misses completely the late stages of evolution.In
massive stars the mass loss occurs through radiation driven winds,
whose description has benefited tremendously from the development of
new techniques of high angular resolution, and from observation in
space. The physical processes involved are now much better understood,
and they are being reproduced in the laboratory. Who else than Stan
Owocki could we ask to give us such a complete picture of mass ejection,
by contrasting these winds blowing from hot stars with those, much
less powerful, emitted by solar-type stars?Beside these principal
lectures, several others were devoted to various specific topics:
the formation of massive stars, the late phases, rotational mixing,
the structure of the winds, the role of binarity, colliding winds,
wind diagnostics, even laboratory experiments reproducing radiative
shocks.We hope that the reader will find as much interest in these
proceedings as the participants of these schools did in listening to
the lectures. We wish to express our warm thanks to all lecturers for
the excellent quality of their talks and and of their proceedings,
and for their disponibility. We thank the Formation Permanente du
CNRS and the Programme National de Physique Stellaire for funding
these annual Schools on Stellar Physics, and to Frédéric Ménadier,
Monique Michel and Mireille Petit for taking care of the organisational
aspects. Mohammad Heydari-Malayeri, Philippe Stee and Jean-Paul Zahn

---------------------------------------------------------
Title: Direct NumeriSimulations of Penetrative Convection and
Generation of Internal Gravity Waves
Authors: Kiraga, M.; Jahn, K.; Stȩpień, K.; Zahn, J. -P.
2003AcA....53..321K Altcode:
Two-dimensional, direct numerisimulations are used to investigate
how thermal convection excites internal gravity waves in the stable
layer beneath a convectively unstable zone. The mechanienergy flux
carried downwards by the waves strongly depends on the viscosity
coefficient. This flux is compared with the energy flux predicted by
a simple parametric model of wave generation applied to two models of
convection dynamics: one based on the mixing-length treatment and the
other on a convective plume model. Numerisimulations always produce
substantially larger energy fluxes than the parametric models. This
difference may result from shortcomings of the parametric modeling
but also from the fact that our simulations are two-dimensional.

---------------------------------------------------------
Title: On hydrodynamic shear turbulence in stratified Keplerian disks:
Transient growth of small-scale 3D vortex mode perturbations
Authors: Tevzadze, A. G.; Chagelishvili, G. D.; Zahn, J. -P.;
Chanishvili, R. G.; Lominadze, J. G.
2003A&A...407..779T Altcode:
This is a sequel to Paper I (Chagelishvili et
al. \cite{Chagelishvili03}), where we presented the so-called bypass
concept for the onset of turbulence in shearing flows. According to this
concept, which was worked out during the last decade by the hydrodynamic
community for spectrally stable flows, vortical perturbations undergo
transient growth by extracting energy from the shear (a linear
process), thereby reaching an amplitude which is sufficient to allow
for non-linear interactions which, by positive feedback, sustain
turbulence. In Paper I we described this transient growth for 2D
perturbations in a Keplerian disk; we showed that their kinematics was
the same as in plane-parallel flow, and thus that they were not modified
by the presence of the Coriolis force. In the present paper, we pursue
our goal of applying the bypass scenario to astrophysical disks: we
investigate the linear dynamics of 3D small-scale vortical perturbations
for single spatial harmonics, in stably stratified, differentially
rotating disks, again in the framework of a nonmodal analysis. We find
that these 3D perturbations also undergo substantial transient growth,
and that they reach a peak amplitude that is comparable to that of
2D perturbations, as long as their vertical scale remains of the
order of the azimuthal scale. When the vertical wave-number exceeds
the azimuthal one, the amplification rate is reduced, but this may
be more than compensated to by the huge Reynolds number and the high
shear rate characterizing astrophysical Keplerian disks. Whereas in
2D the Coriolis force had no impact on the transient growth, in 3D
this force somewhat constricts the characteristics of the perturbation
dynamics in disk flows, and the initial transient growth is followed
by some reduction in amplitude. These differences are quantitative,
rather than of fundamental character. But the 3D case presents two
interesting novelties. In plane parallel flow, the perturbations do
not decay after their transient amplification, but their energy stays
on a plateau before being dissipated through viscous friction. More
importantly, especially for the astrophysicist, in disk flow the 3D
vortex mode perturbations excite density-spiral waves, whose energy
also settles on a plateau before viscous dissipation. These local vortex
mode perturbations fit naturally into the bypass concept of hydrodynamic
shear turbulence, which was first developed for plane-parallel flows. We
submit that these perturbations will also play an important role in
the onset and in the maintenance of turbulence in Keplerian disks.

---------------------------------------------------------
Title: Circularization in B-type eclipsing binariesin the Magellanic
Clouds
Authors: North, P.; Zahn, J. -P.
2003A&A...405..677N Altcode:
Making use of detached eclipsing binaries discovered by the OGLE and
MACHO teams in the SMC and in the LMC respectively, we give the value
of the fractional radius above which circularization occurs. We show
that this critical radius is consistent with that found for galactic
binaries by Giuricin et al. (1984) and does not, therefore, depend
strongly on metallicity. This conclusion is consistent with Zahn's
(1975) theory of tidal dissipation. The critical radius seems slightly
larger in the SMC than in the LMC, but this small difference is hardly
significant and should be confirmed by a larger sample. We also explore
empirically to what extent the critical radius might depend on stellar
mass and surface gravity. This paper utilizes public domain data
obtained by the OGLE Project described at http://sirius.astrouw.edu.pl/
~ ogle/

---------------------------------------------------------
Title: On hydrodynamic shear turbulence in Keplerian disks: Via
transient growth to bypass transition
Authors: Chagelishvili, G. D.; Zahn, J. -P.; Tevzadze, A. G.;
Lominadze, J. G.
2003A&A...402..401C Altcode: 2003astro.ph..2258C
This paper deals with the problem of hydrodynamic shear turbulence in
non-magnetized Keplerian disks. Several papers have appeared recently
on the subject, on possible linear instabilities which may be due
to the presence of a stable stratification, or caused by deviations
from cylindrical rotation. Here we wish to draw attention to another
route to hydrodynamic turbulence, which seems to be little known by
the astrophysical community, but which has been intensively discussed
among fluid dynamicists during the past decade. In this so-called bypass
concept for the onset of turbulence, perturbations undergo transient
growth and if they have initially a finite amplitude they may reach
an amplitude that is sufficiently large to allow positive feedback
through nonlinear interactions. This transient growth is linear in
nature, and thus it differs in principle from the well-known nonlinear
instability. We describe the type of perturbations that according
to this process are the most likely to lead to turbulence, namely
non-axisymmetric vortex mode perturbations in the two dimensional
limit. We show that the apparently inhibiting action of the Coriolis
force on the dynamics of such vortical perturbations is substantially
diminished due to the pressure perturbations, contrary to current
opinion. We stress the similarity of the turbulent processes in
Keplerian disks and in Cartesian flows and conclude that the prevalent
skepticism of the astrophysical community about the occurrence of
hydrodynamic shear turbulence in such disks is not founded.

---------------------------------------------------------
Title: Mixing in stellar radiation zones
Authors: Zahn, Jean-Paul
2003safd.book..205Z Altcode:
Stars undergo some mild mixing in their radiation zones, which is
due to a thermally driven large scale circulation, and presumably
to turbulence caused by shear instabilities. It is the rotation of
the star which is responsible for these motions, and therefore the
transport of angular momentum must be described in time and space when
modeling stellar evolution. We review the present state of the problem
and discuss briefly the open questions.

---------------------------------------------------------
Title: Orbital Circularization in the Magellanic Clouds
Authors: North, Pierre; Zahn, Jean-Paul
2003IAUJD..13E..33N Altcode:
Making use of detached eclipsing binaries discovered by the OGLE and
MACHO teams in the SMC and in the LMC respectively we give the value of
the fractional radius above which cirularization occurs. This critical
radius is around 0.25 to 0.26 regardless of the mass surface gravity or
metallicity and is consistent with that found by Giuricin et al. (1984)
for galactic binaries. These empirical facts are shown to be consistent
with Zahn's (1975) theory of tidal dissipation. As a by-product of this
study we provide approximate stellar parameters of the average component
of 148 binaries in the SMC and of 354 binaries (some of which might
be non-detached) in the LMC under the assumption of equal components.

---------------------------------------------------------
Title: Transport in stellar radiation zones with magnetic field
Authors: Mathis, S.; Zahn, J. -P.
2003sf2a.conf..601M Altcode: 2003sf2a.confE.250M
We examine the interaction between meridian circulation and turbulence
in rotating stars with an axisymetric magnetic field. In the same
way as Zahn (1992) and Spiegel and Zahn (1992), the turbulence is
assumed to be anisotropic, due to the stratification, with stronger
transport in the horizontal directions than in the vertical. We keep the
'shellular rotation' hypothesis, but we expand the differential rotation
in latitude to higher order, which allow us to treat simultaneously
the radiative interior and the tachocline(s). We derive the partial
differential equations which govern the transport of magnetic field,
temperature, angular momentum and chemical elements with taking into
account the non-stationarity of the problem, the mu-gradients, the
effect of horizontal turbulence in thermal imbalance and a general
equation of state like in Maeder and Zahn (1998). Finally, we apply the
beta-viscosity prescription which has been derived from Couette-Taylor
experiments (Richard and Zahn (1999)) to the problem of transport
in stellar interiors to obtain a new expression for the horizontal
component of the turbulent viscosity, nuh, and its companion
the horizontal diffusivity, Dh. The next step will be
to implement these new equations in existing stellar structure codes,
to model the evolution of rotating stars.

---------------------------------------------------------
Title: Jean-Louis Leroy: a pioneer
Authors: Zahn, J. -P.
2003EAS.....9....1Z Altcode:
This meeting was dedicated to Jean-Louis Leroy, who pioneered
polarimetry in French astronomy, and who is one of the leading experts
in the field. He applied this diagnostics tool to better understand
various astrophysical objects: the Sun, comets, stars, the interstellar
medium. Some of his major achievements will be recalled here.

---------------------------------------------------------
Title: Angular Momentum Extraction by Gravity Waves in the Sun
Authors: Talon, Suzanne; Kumar, Pawan; Zahn, Jean-Paul
2002ApJ...574L.175T Altcode: 2002astro.ph..6479T
We review the behavior of the oscillating shear layer produced by
gravity waves below the surface convection zone of the Sun. We show
that, under asymmetric filtering produced by this layer, gravity waves
of low spherical order that are stochastically excited at the base of
the convection zone of late-type stars can extract angular momentum from
their radiative interior. The timescale for this momentum extraction
in a Sun-like star is on the order of 107 yr. The process
is particularly efficient in the central region, and it could produce
there a slowly rotating core.

---------------------------------------------------------
Title: Seismic tests for solar models with tachocline mixing
Authors: Brun, A. S.; Antia, H. M.; Chitre, S. M.; Zahn, J. -P.
2002A&A...391..725B Altcode: 2002astro.ph..6180B
We have computed accurate 1-D solar models including both a macroscopic
mixing process in the solar tachocline as well as up-to-date
microscopic physical ingredients. Using sound speed and density
profiles inferred through primary inversion of the solar oscillation
frequencies coupled with the equation of thermal equilibrium, we
have extracted the temperature and hydrogen abundance profiles. These
inferred quantities place strong constraints on our theoretical models
in terms of the extent and strength of our macroscopic mixing, on the
photospheric heavy elements abundance, on the nuclear reaction rates
such as S11 and S34 and on the efficiency of
the microscopic diffusion. We find a good overall agreement between
the seismic Sun and our models if we introduce a macroscopic mixing
in the tachocline and allow for variation within their uncertainties
of the main physical ingredients. From our study we deduce that the
solar hydrogen abundance at the solar age is Xinv=0.732+/-
0.001 and that based on the 9Be photospheric depletion,
the maximum extent of mixing in the tachocline is 5% of the solar
radius. The nuclear reaction rate for the fundamental pp reaction is
found to be S11(0)=4.06+/- 0.07 10-25 MeV barns,
i.e., 1.5% higher than the present theoretical determination. The
predicted solar neutrino fluxes are discussed in the light of the new
SNO/SuperKamiokande results.

---------------------------------------------------------
Title: Foreword
Authors: Bouvier, Jérôme; Zahn, Jean-Paul
2002EAS.....3D...3B Altcode:
How do stars form in molecular clouds? When and how are planetary
systems assembled in circumstellar disks? How has the star formation
rate varied over cosmological timescales? In the last decade,
considerable progress has been made on these fundamental issues
linked to the star formation process and the physics of young stellar
objects and their circumstellar environment. Improved analytical
and numerical models have brought new insight into the phase of
cloud collapse and fragmentation as well as into the formation and
evolution of circumstellar disks that are prone to form planets. At
the same time, the maturation of observational techniques, such as
high angular resolution imaging, and the advent of powerful telescopes
on the ground and in space, have revealed new exciting aspects of the
formation and early evolution of stars from the solar neighborhood to
higly redshifted galaxies. The X^th Aussois School on Stellar Physics
took place on September 18-22, 2000, and was intended to draw an
up-to-date view of current results and concepts in the field of "Star
Formation and the Physics of Young Stars". The school is primarily
directed towards Ph.D. students in astrophysics and researchers from
neighbouring astrophysical fields but is also an opportunity for
those who work in this field to meet in a stimulating and yet relaxed
atmosphere. Nearly 60 participants attended the school and benefited
from week-long high-quality courses that depicted the current state
of understanding of the various aspects of star formation, from the
local to the cosmological scale: initial conditions in molecular
clouds, gravitational collapse, pre-main sequence evolution, theory
and observations of accretion disks and of the related mass ejection
phenomenon, high energy processes characterizing stellar youth,
observational studies of resolved young stellar populations in external
galaxies, and the cosmological history of star formation. We hope
that the reader will find in these Proceedings as much interest as
the audience had in listening to the talks and interacting with the
speakers, whom we would like to thank here for their disponibility
and the excellent quality of their conferences. We also thank all the
participants for the friendly atmosphere they brought to the school,
the Formation Permanente du CNRS and the Programme National de Physique
Stellaire for funding the annual Aussois School on Stellar Physics,
and particularly F. Bouillet, N. Thiéry, M. Michel and M. Petit for
taking care of all administrative and organisational aspects, as well
as the Director and employees of the Centre Paul Langevin for their
competence and hospitality; and, last but not least, Jean-Philippe
Beaulieu and Daniele Briot for taking and prodiving the pictures.

---------------------------------------------------------
Title: Convective Overshooting: Physical Properties and Seismic
Evidence (invited review)
Authors: Zahn, J. -P.
2002ASPC..259...58Z Altcode: 2002IAUCo.185...58Z; 2002rnpp.conf...58Z
No abstract at ADS

---------------------------------------------------------
Title: Star formation and the physics of young stars : Summer School
on Stellar Physics 10
Authors: Bouvier, Jerome; Zahn, Jean-Paul
2002EAS.....3.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Joint Discussion 5 - Mixing and diffusion in stars:
introduction and brief overview
Authors: Vandenberg, Don A.; Zahn, Jean-Paul
2002HiA....12..259V Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Concluding remarks [Mixing and diffusion in stars]
Authors: Zahn, Jean-Paul
2002HiA....12..307Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A powerful local shear instability in stratified disks
Authors: Richard, D.; Hersant, F.; Dauchot, O.; Daviaud, F.; Dubrulle,
B.; Zahn, J-P.
2001astro.ph.10056R Altcode:
In this paper, we show that astrophysical accretion disks are
dynamically unstable to non-axisymmetric disturbances. This instability
is present in any stably stratified anticyclonically sheared flow as
soon as the angular velocity decreases outwards. In the large Froude
number limit, the maximal growth rate is proportional to the angular
rotation velocity, and is independent of the stratification. In the low
Froude number limit, it decreases like the inverse of the Froude number,
thereby vanishing for unstratified, centrigugally stable flows. The
instability is not sensitive to disk boundaries. We discuss the possible
significance of our result, and its implications on the turbulent state
achieved by the disks. We conclude that this linear instability is one
of the best candidates for the source of turbulence in geometrically
thin disks, and that magnetic fields can be safely ignored when
studying their turbulent state. The relevance of the instability for
thick disks or nearly neutrally stratified disks remains to be explored.

---------------------------------------------------------
Title: Accretion discs models with the β-viscosity prescription
derived from laboratory experiments
Authors: Huré, J. -M.; Richard, D.; Zahn, J. -P.
2001A&A...367.1087H Altcode: 2000astro.ph.12262H
We examine under which conditions one may apply, to steady state
Keplerian accretion discs, the beta -viscosity prescription which
has been derived from rotating shear flow experiments (nu = beta
Omega R2, where Omega is the angular velocity at radius R
and beta is a constant of order 10-5; Richard &amp; Zahn
\cite{richard99}). Using a vertically averaged model, we show that this
law may be suitable for all three families of known systems: in young
stellar objects, evolved binary stars and Active Galactic Nuclei discs
(except in their outer gas pressure dominated regions where turbulence
becomes hypersonic). According to the standard criterion for viscous
stability, beta -discs are always stable throughout. Using realistic
opacities and equation of state, we demonstrate that these discs are
thermally unstable in the temperature domain where hydrogen recombines,
when they are optically thick, and this could lead to limit cycle
behavior. Radiation pressure dominated regions are thermally stable,
in contrast with alpha -discs. This results in a fully stable solution
for the innermost parts of AGN discs.

---------------------------------------------------------
Title: Shear turbulence beneath the solar tachocline
Authors: Schatzman, E.; Zahn, J. -P.; Morel, P.
2000A&A...364..876S Altcode: 2000astro.ph.10543E; 2000astro.ph.10543S
This paper presents a simple and efficient equation of state which
can be used for quick and accurate computing of the thermodynamic
functions of partly ionized and weakly coupled plasmas. Based on the
free energy minimization method, the improvement of the equation
of state includes a detailed account of the physical processes of
non-ideal effects consisting of electron degeneracy, Coulomb coupling
and pressure ionization. The treatment of Coulomb coupling combines
the results of the quantum exchange effect of degenerate electrons
at finite temperature, N-body semi-analytic theory and the extended
Debye-Hückel theory with hard-sphere correction. For the complicated
physical processes of pressure ionization an approximate model is
adopted. The nonideal corrections to equation of state are calculated
under solar interior conditions.

---------------------------------------------------------
Title: Hydrodynamic Instabilities
Authors: Zahn, J.
2000eaa..bookE2091Z Altcode:
Most of the universe is made of gas, over all scales ranging from the
intergalactic medium to stars and giant planets. This fluid rarely
remains in static equilibrium or in stationary motion—many forces
and processes conspire to render it unstable. When it is unstable,
it may oscillate more or less regularly around its equilibrium state,
as do variable stars. Alternatively, it may reach a state o...

---------------------------------------------------------
Title: Solar Interior
Authors: Zahn, J.
2000eaa..bookE2417Z Altcode:
The interior of the Sun is hidden from our sight, because it is opaque
to electromagnetic waves: the radiation we receive from it on Earth is
emitted in the outermost layers. Our knowledge of the solar interior is
based solely on theoretical models which are built with some assumptions
about the physical conditions and processes that are likely to prevail
there, and on helioseismology, a very pow...

---------------------------------------------------------
Title: Erratum: Standard Solar Models in the Light of New Helioseismic
Constraints. II. Mixing below the Convective Zone
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. P.
2000ApJ...536.1005B Altcode:
In the paper “Standard Solar Models in the Light of New
Helioseismic Constraints. II. Mixing below the Convective Zone”
by A. S. Brun, S. Turck-Chièze, and J. P. Zahn (525, 1032 [1999]),
several corrections are required: 1. The words “greater than”
just after equation (11) for the definition of rbcz
should be removed. 2. The beginning of first sentence of the next
paragraph should read: “With the latitudinal dependence of the angular
velocity at the base of the convection zone borrowed from Thompson et
al. (1996), Ωbcz/2π=456-72x2-42x4
nHz,” instead of “Ωbcz&gt;/2π=456-72x2-
42x4.” 3. In the footnote to Table 1, “Rbzc”
should be “Rbcz,” as it is appears for “Tbcz”
in the same footnote. 4. In Table 2, in the “Parameters” column,
“i0” should be “Z0,” as in Table 1. 5. In
Table 3, “Observaton” should be “Observation.” The Press sincerely
regrets these errors.

---------------------------------------------------------
Title: Tidal Dissipation
Authors: Zahn, Jean-Paul
2000astu.confE..36Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Turbulence in Differentially Rotating Flows: New Results from
a Couette-Taylor Experiment
Authors: Zahn, Jean-Paul; Richard, Denis
2000astu.progE..34Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Influence of the Tachocline on Solar Evolution.
Authors: Brun, A. S.; Zahn, J. -P.
2000NYASA.898..113B Altcode: 2000astro.ph..1510B
Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition from
differential rotation in the convection zone to almost uniform rotation
in the radiative interior, the tachocline. At present, this layer
extends only over a few percent of the solar radius and no definitive
explanations have been given for this thiness. Following Spiegel and
Zahn (1992, Astron. Astrophys.), who invoke anisotropic turbulence to
stop the spread of the tachocline deeper in the radiative zone as the
Sun evolves, we give some justifications for their hypothesis by taking
into account recent results on rotating shear instability (Richard and
Zahn 1999, Astron. Astrophys.). We study the impact of the macroscopic
motions present in this layer on the Sun's structure and evolution by
introducing a macroscopic diffusivity $D_T$ in updated solar models. We
find that a time dependent treatment of the tachocline significantly
improves the agreement between computed and observed surface chemical
species, such as the $^7$Li and modify the internal structure of the
Sun (Brun, Turck-Chièze and Zahn, 1999, in Astrophys. J.).

---------------------------------------------------------
Title: Plumes in Stellar Convection Zones.
Authors: Zahn, J. P.
2000NYASA.898...90Z Altcode: 1999astro.ph..9288Z
All numerical simulations of compressible convection reveal the presence
of strong downwards directed flows. Thanks to helioseismology, such
plumes have now been detected also at the top of the solar convection
zone, on super- granular scales. Their properties may be crudely
described by adopting Taylor's turbulent entrainment hypothesis,
whose validity is well established under various conditions. Using
this model, one finds that the strong density stratification does
not prevent the plumes from traversing the whole convection zone,
and that they carry upwards a net energy flux (Rieutord &amp; Zahn
1995). They penetrate to some extent in the adjacent stable region,
where they establish a nearly adiabatic stratification. These plumes
have a strong impact on the dynamics of stellar convection zones,
and they play probably a key role in the dynamo mechanism.

---------------------------------------------------------
Title: Preface (Advances in solar research at eclipses from ground
and from space)
Authors: Stavinschi, Magda; Zahn, Jean-Paul
2000ASIC..558D...7S Altcode: 2000asre.confD...7S
No abstract at ADS

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Title: Commission 35: Stellar Constitution: (Constitution Des Etoiles)
Authors: Zahn, J. -P.; VandenBerg, D.; Canal, R.; Chiosi, C.;
Dziembowski, W.; Guzik, J.; Meynet, G.; Michaud, G.; Renzini, A.;
Saio, H.; Tutukov, A.; Vauclair, G.
2000IAUTA..24..201Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Advances in solar research at eclipses from ground and
from space
Authors: Zahn, Jean-Paul; Stavinschi, Magda
2000ASIC..558.....Z Altcode: 2000asre.conf.....Z
No abstract at ADS

---------------------------------------------------------
Title: What we know about the solar interior
Authors: Zahn, J. -P.
2000ASIC..558..265Z Altcode: 2000asre.conf..265Z
Because the interior of the Sun is opaque to electromagnetic waves,
the radiation we receive from it on Earth is emitted in the outermost
layers: the photosphere. Therefore our knowledge of the solar interior
is based solely on theoretical models, which are built by making some
plausible assumptions about the physical conditions and processes
that are likely to prevail there. Fortunately, a powerful technique -
helioseismology - has been developed in the last twenty years, which
permits to probe directly the solar interior by means of acoustic waves,
and this had a tremendous impact on solar physics because it provides
tight observational constraints on our models. We shall illustrate
this here by a few examples of recent advances in modeling the Sun.

---------------------------------------------------------
Title: Mixing by internal waves beneath the convection zone-
comparison with analytical models
Authors: Kiraga, M.; Jahn, K.; Różyczka, M.; Stěpień, K.; Zahn,
J. -P.
2000IAUJD...5E..24K Altcode:
The results of 2D hydrodynamical simulations of penetrative convection
are presented. Amplitudes, frequencies, and the energy flux transported
by the internal gravity waves below the convection layer were determined
in several models. We find that these values depend strongly on the
value of the adopted viscosity coefficient. However, frequencies
and amplitudes of the waves in each computed model were higher than
predicted by parametric models of generation of waves, in which the
dynamics of the convection zone is based on the MLT, and it was lower
than in the `plume' model considered by Schatzman and Montalban.

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Title: Division IV: Stars: (Etoiles)
Authors: Cram, Lawrence; Barbuy, Beatriz; Gerbaldi, Michele; Lambert,
David; Pallavicini, Roberto; Zahn, Jean-Paul; Zinnecker, Hans
2000IAUTA..24..173C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Standard Solar Models in the Light of New Helioseismic
Constraints. II. Mixing below the Convective Zone
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. P.
1999ApJ...525.1032B Altcode: 1999astro.ph..6382B
In previous work, we have shown that recent updated standard solar
models cannot reproduce the radial profile of the sound speed at
the base of the convective zone and fail to predict the photospheric
lithium abundance. In parallel, helioseismology has shown that the
transition from differential rotation in the convective zone to almost
uniform rotation in the radiative solar interior occurs in a shallow
layer called the tachocline. This layer is presumably the seat of a
large-scale circulation and of turbulent motions. Here we introduce a
macroscopic transport term in the structure equations that is based on
a hydrodynamical description of the tachocline proposed by Spiegel &amp;
Zahn, and we calculate the mixing induced within this layer. We discuss
the influence of different parameters that represent the tachocline
thickness, the Brunt-Väisälä frequency at the base of the convective
zone, and the time dependence of this mixing process along the Sun's
evolution. We show that the introduction of such a process inhibits
the microscopic diffusion by about 25%. Starting from models including
a pre-main-sequence evolution, we obtain (1) a good agreement with
observed photospheric chemical abundance of light elements such as
3He, 4He, 7Li, and 9Be;
(2) a smooth composition gradient at the base of the convective zone;
and (3) a significant improvement of the sound-speed square difference
between the seismic Sun and the models in this transition region when
we allow the photospheric heavy-element abundance to adjust, within
the observational incertitude, as a result of the action of this mixing
process. The impact on neutrino predictions is also discussed.

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Title: Angular Momentum Redistribution by Waves in the Sun
Authors: Kumar, Pawan; Talon, Suzanne; Zahn, Jean-Paul
1999ApJ...520..859K Altcode: 1999astro.ph..2309K
We calculate the angular momentum transport by gravito-inertial-Alfvén
waves and show that, so long as prograde and retrograde gravity waves
are excited to roughly the same amplitude, the sign of angular momentum
deposit in the radiative interior of the Sun is such as to lead to an
exponential growth of any existing small radial gradient of rotation
velocity just below the convection zone. This leads to formation of
a strong thin shear layer (of thickness about 0.3% Rsolar)
near the top of the radiative zone of the Sun on a timescale of order
20 yr. When the magnitude of differential rotation across this layer
reaches about 0.1 μHz, the layer becomes unstable to shear instability
and undergoes mixing, and the excess angular momentum deposited in the
layer is returned to the convection zone. The strong shear in this
layer generates a toroidal magnetic field which is also deposited
in the convection zone when the layer becomes unstable. This could
possibly start a new magnetic activity cycle seen at the surface.

---------------------------------------------------------
Title: Star Formation in Accretion Disks Around Massive Black Holes
and Pregalactic Enrichment
Authors: Collin, Suzy; Zahn, Jean-Paul
1999Ap&SS.265..501C Altcode: 1999astro.ph..1085C
Broad Absorption Lines (BALs) prove the existence of a high velocity
outflowing gas with metallicities larger than solar in the central
few parsecs of high redshift quasars. At the same distance from the
black hole, accretion disks in quasars and Active Galactic Nuclei
(AGN) are locally gravitationally unstable, and clumps must form with
a size of the order of the scale height of the disk. This is hardly a
coincidence, and we have tried to link these two facts. We have assumed
that the unstable clumps give rise to protostars, which become massive
stars after a rapid stage of accretion, and explode as supernovae,
producing strong outflows perpendicular to the disk and inducing outward
transfer of angular momentum in the plane of the disk. As a consequence
a self-regulated disk made of gas and stars where supernovae sustain
the inflow mass rate required by the AGN is a viable solution in this
region of the disk. This model could explain the BALs, and could also
account for a pregalactic enrichment of the intergalactic medium and
of the Galaxy, if massive black holes formed early in the Universe.

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Title: Turbulence in differentially rotating flows. What can be
learned from the Couette-Taylor experiment
Authors: Richard, Denis; Zahn, Jean-Paul
1999A&A...347..734R Altcode: 1999astro.ph..3374R
The turbulent transport of angular momentum plays an important role
in many astrophysical objects, but its modelization is still far from
satisfactory. We discuss here what can be learned from laboratory
experiments. We analyze the results obtained by Wendt (1933) and Taylor
(1936) on the classical Couette-Taylor flow, in the case where angular
momentum increases with distance from the rotation axis, which is the
most interesting for astrophysical applications. We show that when the
gap between the coaxial cylinders is wide enough, the criterion for
the onset of the finite amplitude instability can be expressed in terms
of a gradient Reynolds number. Based on Wendt's results, we argue that
turbulence may be sustained by differential rotation when the angular
velocity decreases outward, as in keplerian flows. From the rotation
profiles and the torque measurements we deduce a prescription for
the turbulent viscosity which is independent of gap width; with some
caution it may be applied to stellar interiors and to accretion disks.

---------------------------------------------------------
Title: Star formation and evolution in accretion disks around massive
black holes.
Authors: Collin, Suzy; Zahn, Jean-Paul
1999A&A...344..433C Altcode:
We develop an exploratory model for the outer, gravitationally
unstable regions of accretion disks around massive black holes. We
consider black holes of mass 10(6) to 10(10) Msun, and
primeval or solar abundances. In a first step we study star formation
and evolution in a purely gaseous marginally unstable disk, and we
show that unstable fragments should collapse rapidly and give rise
to compact objects (planets or protostars), which then accrete at a
high rate and in less than 10(6) years acquire a mass of a few tens of
Msun, according to a mechanism first proposed by Artymowicz
et al. (1993). When these stars explode as supernovae, the supernova
shells break out of the disk, producing strong outflows. We show that
the gaseous disk is able to support a large number of massive stars and
supernovae while staying relatively homogeneous. An interesting aspect
is that the residual neutron stars can undergo other accretion phases,
leading to other (presumably powerful) supernova explosions. In a second
step we assume that the regions at the periphery of the disk provide a
quasi stationary mass inflow during the lifetime of quasars or of their
progenitors, i.e. ~ 10(8) yrs, and that the whole mass transport is
ensured by the supernovae, which induce a transfer of angular momentum
towards the exterior, as shown by the numerical simulations of Rozyczka
et al. (1995). Assuming that the star formation rate is proportional
to the growth rate of the gravitational instability, we solve the
disk structure and determine the gas and the stellar densities,
the heating being provided mainly by the stars themselves. We find
self-consistent solutions in which the gas is maintained in a state
very close to gravitational instability, in a ring located between
0.1 and 10 pc for a black hole mass of 10(6) M_sun, and between 1 and
100 pc for a black hole mass of 10(8) M_sun or larger, whatever the
abundances, and for relatively low accretion rates (&lt;= 10% of the
critical accretion rate). For larger accretion rates the number of stars
becomes so large that they inhibit any further star formation, and/or
the rate of supernovae is so high that they distroy the homogeneity
and the marginal stability of the disk. We postpone the study of this
case. Several consequences of this model can be envisioned, besides the
fact that it proposes a solution to the problem of the mass transport in
the intermediate region of the disk where global instabilities do not
work. As a first consequence, it could explain the high velocity metal
enriched outflows implied by the presence of the broad absorption lines
in quasars. As a second consequence it could account for a pregalactic
enrichment of the intergalactic medium, if black holes formed early
in the Universe. Finally it could provide a triggering mechanism for
starbursts in the central regions of galaxies. A check of the model
would be to detect a supernova exploding within a few parsecs from
the center of an AGN, an observation which can be performed in the
near future.

---------------------------------------------------------
Title: Turbulence and differentially rotating flows
Authors: Zahn, Jean-Paul
1999bha..progE..51Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Beyond the Mixing-Length Treatment: Why and How?
Authors: Zahn, J. -P.
1999ASPC..173..121Z Altcode: 1999sstt.conf..121Z
No abstract at ADS

---------------------------------------------------------
Title: Accretion Disks and Star Formation
Authors: Collin, Suzy; Zahn, J. P.
1999IAUS..194..246C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamical Simulations of Penetrative Convection and
Generation of Internal Gravity Waves
Authors: Kiraga, M.; Zahn, J. -P.; Stȩpień , K.; Jahn, K.; Rózyczka,
M.; Muthsam, H. J.
1999ASPC..173..269K Altcode: 1999sstt.conf..269K
No abstract at ADS

---------------------------------------------------------
Title: Mixing Below the Solar Convective Zone
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. -P.
1999ASPC..173..293B Altcode: 1999sstt.conf..293B
No abstract at ADS

---------------------------------------------------------
Title: Star Formation in Accretion Disks Around Massive Black Holes
and Pregalactic Enrichment
Authors: Collin, S.; Zahn, J. -P.
1999gecd.conf..501C Altcode:
High redshift quasars are known to harbour in their very center (~one
parsec) high velocity outflows of gas with a metallicity larger than
solar (observed as the so-called "Broad Absorption Lines"). We propose
that this enrichment takes place in the accretion disk fueling the
black holes. In the gravitationally unstable part of the disk, between
0.01 and 1 pc, we show that stars can form and grow by accretion,
then evolve to the stage of supernovae and explode in the disk. The
gaseous fraction of the disk is maintained in a state of marginal
instability by the heating of the stars, and supernovae provide
the transfer of momentum required for a large accretion rate. This
mechanism is particularly efficient for disks made of primordial gas,
and it works as well for small (growing) black holes as for powerful
quasars. In the course of the process, the overabundant gas flows out of
the disk, enriching in heavy elements the forming host galaxy and the
intergalactic medium. This mechanism explains the enrichment and the
outflows in quasars, but we propose that it could also be the starting
point of heavy element formation in the Universe, provided that massive
black holes now present in the centres of galaxies formed early in
the past. It can explain without the need to appeal to a pregalactic
"Population III", the abundances observed in the oldest stars of the
Galactic halo, the abundances in the intergalactic medium (i.e. the
L-α forest clouds), and the abundances in the damped L-α and metal
systems which are supposed to be tracers of massive galaxies.

---------------------------------------------------------
Title: Stellar evolution with rotation. III. Meridional circulation
with MU -gradients and non-stationarity
Authors: Maeder, Andre; Zahn, Jean-Paul
1998A&A...334.1000M Altcode:
Models of stellar evolution with rotation are very much needed in
order to interpret recent stellar properties, in particular for massive
stars. For that we proceed to a detailed investigation of some critical
physical processes in rotating stars: 1. The equation expressing the
transport of angular momentum by circulation and diffusion is revised
to also account for expansion and contraction in non-stationary
models. 2. We examine the current expressions for the velocity U(r)
of the meridional circulation, also taking into account mu -gradients
and horizontal turbulence. We show that there are no “mu -currents”
but just meridional currents which can be sizeably influenced by
the gradients of mean molecular weight mu . A proper account of the
mu -gradients may reduce U(r) by one or two orders of magnitude with
respect to current expressions. 3. While the usual expressions for the
meridional circulation would predict an infinite velocity at the edge
of a radiative and semiconvective zone and an inverted circulation in
a semiconvective zone, the present developments give a continuity of
the solutions for the circulation. 4. The approximation of a stationary
circulation, which is no longer valid after the main-sequence phase,
is also removed and the case of a general equation of state is
considered. We notice that in the stationary regime the horizontal
fluctuations of mu represent some fixed fraction of the vertical mu
-gradient. To first order, this fraction is not dependent on rotation,
because the building of horizontal fluctuations by the circulation is
compensated by the smoothing due to horizontal turbulence.

---------------------------------------------------------
Title: Macroscopic Transport Large-scale advection, turbulent
diffusion, wave transport
Authors: Zahn, Jean-Paul
1998SSRv...85...79Z Altcode:
While the solar convection zone is very well mixed by its turbulent
motions, chemical composition gradients build up in the radiative
interior due to microscopic diffusion and settling, and to nuclear
burning. Standard models, which ignore any type of macroscopic
transport, cannot explain the depletion of lithium in solar-type stars,
as they evolve; neither do they account for the observed profile of
molecular weight at the base of the solar convection zone.

---------------------------------------------------------
Title: Turbulent Transport in Stellar Interiors
Authors: Michaud, G.; Zahn, J. -P.
1998ThCFD..11..183M Altcode:
Differential rotation is probably the major cause of turbulence in
stably stratified stellar interiors. The boundary of the superficial
solar convection zone plays a critical role for both the large scale
circulation and the differential rotation. The turbulence arises from
the barotropic instability in a vertically stratified medium and is
expected to be anisotropic. It tends to suppress one of its causes,
namely differential rotation in latitude. It offers an explanation
for the thinness of the solar tachocline, the boundary layer beneath
the convection zone where solar seismology shows that rotation varies
from differential above to apparently uniform below. The anisotropy of
turbulence also strongly reduces the efficiency of vertical particle
transport. We show that for an anisotropy A of horizontal to vertical
velocities, the vertical diffusivity is a factor A2 less
than the horizontal diffusivity. Transport by meridional circulation
is also reduced, as well as the efficiency of a composition gradient in
suppressing meridional circulation. These effects of anisotropy explain
the very small upper limit that observations of the concentration
of chemical elements impose to vertical transport in stars. However
the recent results of helioseismology, that the solar core rotates at
nearly the same rate as the whole radiative zone, cannot currently be
explained by anisotropic turbulent transport. It suggests the need for
an additional transport process such as a magnetic torquing or gravity
waves. Furthermore, near the base of the convection zone, magnetic
instabilities could provide an alternate mechanism to mix angular
momentum preferentially in latitude compared with radial mixing. The
quality of the helioseismology data is improving very rapidly. It holds
the promise to determine, within the next few years, the velocity field
within the Sun to great accuracy. This should allow us to distinguish
between the various hydrodynamical and hydromagnetic models.

---------------------------------------------------------
Title: Macroscopic Transport
Authors: Zahn, J. -P.
1998sce..conf...79Z Altcode:
No abstract at ADS

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Title: Gravity Waves and the Evolution of the Solar Rotation Profile
Authors: Talon, Suzanne; Zahn, Jean-Paul
1998ASPC..154..806T Altcode: 1998csss...10..806T
We present here numerical results for the evolution of angular
momentum in the Sun, including transport by gravity waves as well as
by meridional circulation and shear turbulence which are present in a
differentially rotating star. We show that quasi-solid body rotation
may be achieved in a timescale shorter than the age of the Sun.

---------------------------------------------------------
Title: Macroscopic Processes in the Solar Interior
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. P.
1998ESASP.418..439B Altcode: 1998astro.ph..7090B; 1998soho....6..439B
With the recent results of heliseismology aboard SOHO, the solar models
are more and more constrained (Brun, Turck-Chièze et Morel 1998)
. New physical processes, mainly connected to macroscopic motions, must
be introduced to understand these news observations. In this poster,
we present solar models with such macroscopic motions, as turbulent
pressure in the outer layers, mixing due to the tachocline (Spiegel and
Zahn 1992), and some mixing in the core (Morel and Schatzman 1996). From
our results, we could say that: (1) Mixing in the core is unlikely (δ
c2/c2 &gt; 2%) (2) Turbulent pressure improves
the absolute value of the acoustic modes frequencies (~5 μ Hz at 4 mHz)
(3) And mixing in a tachocline of thickness of 0.05 plus or minus 0.03
Rodot (Corbard et al. 1997) looks promising.

---------------------------------------------------------
Title: Towards a hydrodynamical model predicting the observed solar
rotation profile
Authors: Talon, Suzanne; Zahn, Jean-Paul
1998A&A...329..315T Altcode:
We present here the results of a numerical study performed to
investigate the role of gravity waves in the evolution of the solar
rotation profile. We show that, together with meridional circulation
and shear turbulence, they may enforce solid body rotation in the solar
interior. The transient phase is characterized by an outer portion
of the radiative zone rotating at the velocity of the convection
zone and an inner portion in strong differential rotation with
depth. The gravity waves slow down the outer portion, whereas it is
the meridional circulation which dominates the transport of momentum
in the interior. With our treatment of the excitation and damping of
these waves, solid body rotation is achieved on a timescale of a few
giga-years, depending on the initial velocity of the model.

---------------------------------------------------------
Title: Rotation and angular momentum transport
Authors: Zahn, J. -P.
1997ASSL..225..187Z Altcode: 1997scor.proc..187Z
The internal rotation law of the Sun depends both on the rate at which
angular momentum has been lost by the wind, and on the efficiency of
the processes which extract that angular momentum from the radiative
interior. The transport by the meridian circulation and the associated
hydrodynamical instabilities is insufficient to yield the flat profile
which is deduced from helioseismology, and therefore other processes
must be operating. One possibility is magnetic torquing, but some
observations are hard to reconcile with it. Another is the transport
through the internal waves emitted by the convection zone and this
mechanism seems the most promising: according to recent estimates,
it should operate on a timescale of a about $10^7$ years, i.e. three
orders of magnitude less than the present spin-down time.

---------------------------------------------------------
Title: Angular momentum transport by internal waves in the solar
interior.
Authors: Zahn, J. -P.; Talon, S.; Matias, J.
1997A&A...322..320Z Altcode: 1996astro.ph.11189Z
The internal gravity waves of low frequency which are emitted at the
base of the solar convection zone are able to extract angular momentum
from the radiative interior. We evaluate this transport with some
simplifying assumptions: we ignore the Coriolis force, approximate the
spectrum of turbulent convection by the Kolmogorov law, and couple this
turbulence to the internal waves through their pressure fluctuations,
following Press (1981ApJ...245..286P) and Garcia Lopez &amp; Spruit
(1991ApJ...377..268G). The local frequency of an internal wave varies
with depth in a differentially rotating star, and it can vanish at some
location, thus leading to enhanced damping (Goldreich &amp; Nicholson
1989ApJ...342.1079G). It is this dissipation mechanism only that we
take into account in the exchange of momentum between waves and stellar
rotation. The flux of angular momentum is then an implicit function of
depth, involving the local rotation rate and an integral representing
the cumulative effect of radiative dissipation. We find that the
efficiency of this transport process is rather high: it operates on
a timescale of 10^7^ years, and is probably responsible for the flat
rotation profile which has been detected through helioseismology.

---------------------------------------------------------
Title: Rotational mixing in early-type stars: the main-sequence
evolution of a 9Msun_ star.
Authors: Talon, S.; Zahn, J. -P.; Maeder, A.; Meynet, G.
1997A&A...322..209T Altcode: 1996astro.ph.11131T
We describe the main-sequence evolution of a rotating 9Msun_
star. Its interior rotation profile is determined by the redistribution
of angular momentum through the meridian circulation and through
the shear turbulence generated by the differential rotation; the
possible effect of internal waves is neglected. We examine the
mixing of chemicals produced by the same internal motions. Our
modelization is based on the set of equations established by Zahn
(1992A&amp;A...265..115Z) and completed in Matias, Talon &amp; Zahn
(1996, preprint). Our calculations show that the amount of mixing
associated with a typical rotation velocity of ~100km/s yields stellar
models whose global parameters are very similar to those obtained
with the moderate overshooting (d/H_P_=~0.2) which has been invoked
until now to fit the observations. Fast rotation (~300km/s) leads
to significant changes of the C/N and O/N surface ratios, but the
abundance of He is barely increased. The modifications of the internal
composition profile due to such rotational mixing will certainly affect
the post-main-sequence evolution.

---------------------------------------------------------
Title: Anisotropic diffusion and shear instabilities.
Authors: Talon, S.; Zahn, J. -P.
1997A&A...317..749T Altcode: 1996astro.ph..9010T
We examine the role of anisotropic turbulence on the shear instabilities
in a stratified flow. Such turbulence is expected to occur in the
radiative interiors of stars, due to their differential rotation and
their strong stratification, and the turbulent transport associated
with it will be much stronger in the horizontal than in the vertical
direction. It will thus weaken the restoring force which is caused by
the gradient of mean molecular weight (μ). We find that the critical
shear which is able to overcome the μ-gradient is substantially
reduced by this anisotropic turbulence, and we derive an estimate for
the resulting turbulent diffusivity in the vertical direction.

---------------------------------------------------------
Title: Stellar rotation and mixing
Authors: Zahn, J. P.
1997IAUS..181..175Z Altcode:
No abstract at ADS

---------------------------------------------------------
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: Chaotic Dynamics of the Solar Cycle
Authors: Spiegel, Edward A.; Zahn, Jean-Paul
1996cuny.rept.....S Altcode:
The cyclic variation of solar activity is both irregular and
intermittent. We have sought to isolate and illuminate the physical
mechanisms of this behavior and to provide a mathematical description
of it. Our work has brought out three ingredients of the solar cycle
that we believe to be central to its operation. (1) The seat of the
solar cycle is in a shear layer just below the solar convection zone. We
have investigated the structure of this layer (which we call the solar
tachocline) in some detail. (2) The spatio-temporal development of the
solar cycle is represented by the propagation of robust solitary waves
which are affected by dissipation and instability. We have studied the
structure and interactions of such waves, which we call solitoids. (3)
On top of the simple propagative behavior of the solar solitoids there
are intermissions during which the number of sunspots remains quite
small. We attribute these intermissions (such as the Maunder minimum)
to a form of interaction between the convection zone and the tachocline
which is characteristic of a process that we have developed and that
we call on/off intermittency. These three ingredients make up some of
the key features of the solar cycle and may be expected to play a role
in future simulations of the solar cycle.

---------------------------------------------------------
Title: Corot: a Space Project Devoted to the Study of Convection
and Rotation in Stars
Authors: Catala, C.; Auvergne, M.; Baglin, A.; Bonneau, F.; Magnan,
A.; Vuillemin, A.; Goupil, M. J.; Michel, E.; Boumier, P.; Dzitko,
H.; Gabriel, A.; Gautier, D.; Lemaire, P.; Mangeney, A.; Mosser, B.;
Turck-Chiéze, S.; Zahn, J. P.
1995ESASP.376b.549C Altcode: 1995soho....2..549C; 1995help.confP.549C
No abstract at ADS

---------------------------------------------------------
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: Convective Penetration
Authors: Zahn, J. P.
1995LIACo..32..111Z Altcode: 1995sews.book..111Z
No abstract at ADS

---------------------------------------------------------
Title: Lithium depletion in late-type stars through wind-driven
mixing.
Authors: Zahn, J. -P.
1995MmSAI..66..485Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Lithium Depletion in the Sun
Authors: Morel, P.; Provost, J.; Berthomieu, G.; Matias, J.; Zahn,
J. P.
1995LIACo..32..395M Altcode: 1995sews.book..395M
No abstract at ADS

---------------------------------------------------------
Title: Transport processes in stellar interiors
Authors: Zahn, Jean-Paul
1995LNP...458..153Z Altcode:
We first recall Evry Schatzman’s pioneering contributions to the
theory of transport of matter and angular momentum in the radiative
interior of stars. He suggested that this transport could be of
turbulent nature and he linked it to the internal rotation state. We
then review the major breakthroughs which have been accomplished
during the eighties, mainly in the observational domain. Finally,
we briefly describe some recent achievements on the theoretical side,
and allude to the work in progress.

---------------------------------------------------------
Title: Lithium depletion in late type stars through wind-driven mixing
Authors: Zahn, J. -P.
1995HiA....10..461Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Le monde des etoiles
Authors: Benest, Daniel; Froeschle, Claude; Gouguenheim, Lucienne;
Loulergue, Michelle; Rozelot, Jean-Pierre; Waelkens, Christoffel;
Zahn, Jean-Paul
1995lmde.book.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: COROT: A Proposal to Study Stellar Convection and Internal
Rotation
Authors: Catala, C.; Mangeney, A.; Gautier, D.; Auvergne, M.; Baglin,
A.; Goupil, M. J.; Michel, E.; Zahn, J. P.; Magnan, A.; Vuillemin,
A.; Boumier, P.; Gabriel, A.; Lemaire, P.; Turck-Chieze, S.; Dzitko,
H.; Mosser, B.; Bonneau, F.
1995ASPC...76..426C Altcode: 1995gong.conf..426C
No abstract at ADS

---------------------------------------------------------
Title: Convective Penetration in the Sun in Presence of Microscopic
Diffusion
Authors: Provost, J.; Morel, P.; Berthomieu, G.; Zahn, J. P.
1995LIACo..32..201P Altcode: 1995sews.book..201P
No abstract at ADS

---------------------------------------------------------
Title: A case study of the quadruple system ξ Ursae Majoris: its
activity and lithium depletion.
Authors: Cayrel de Strobel, G.; Cayrel, R.; Friel, E.; Zahn, J. -P.;
Bentolila, C.
1994A&A...291..505C Altcode:
The quadruple system ξ UMa is an interesting test case for the
numerous problems encountered when studying the evolutionary status,
chromospheric activity and lithium depletion of solar-type stars. We
have carried out a detailed analysis of the major components A and B
of ξ UMa from high resolution, high signal-to-noise CFHT spectra. We
determine accurate temperatures for each of the principal components,
neither disturbed by their invisible, much less massive companion,
and find that the two differ in temperature by 300 K, one being
somewhat hotter than the Sun, the other cooler. The metal abundances
derived from iron and a few other elements are identical for the two
stars, and the system is moderately metal-deficient, by a factor of
two with respect to the Sun. We concur with previous studies that
lithium is fairly abundant in A whereas it is below detection in B:
we measured log N(Li)=2.33 in A and place a more stringent upper
limit log N(Li)&lt;0.8 for B. We discuss this unusual combination of
lithium abundances and the position of the stars in the HR diagram
in the light of the complex binary nature of the two components, and
of current theories of stellar evolution and of lithium depletion. We
suggest that B has kept a high level of activity because its rotation
is tidally locked with the orbital motion, and that it has therefore
lost about ten times more matter than a single star of the same mass,
which explains its strong lithium depletion.

---------------------------------------------------------
Title: Rotation and lithium depletion in late-type binaries
Authors: Zahn, J. -P.
1994A&A...288..829Z Altcode:
Late-type stars possess two firmly established properties: both their
rotation speed and their lithium abundance decrease with age. Quite
naturally, it has been suggested that the lithium depletion is
related with the loss of angular momentum. Such a causal link may
just be postulated in the evolutionary calculations, and the free
parameters calibrated with the observations (Endal &amp; Sofia 1978;
Pinsonneault et al. 1989). But it can also be justified on theoretical
reasons, with the meridian circulation playing the major role in
the transport of matter and angular momentum (Zahn 1992a). In the
present paper, we seek an observational confirmation of this link
in the behavior of close binaries. In addition to the torque exerted
on them by the stellar wind, which is responsible for the spin-down
of single stars, binary stars experience a tidal torque, which tends
to synchronize their rotation with the orbital motion. As a result,
they exchange lesser amounts of angular momentum than single stars of
the same mass and age, and therefore they should retain more of their
original lithium. Based on the current tidal theory (Zahn 1989), we
calculate the dynamical evolution of close binaries, and infer from it
the differences in lithium abundance one should expect between them
and single stars. Underdepletion should occur in systems which were
tidally locked on the ZAMS, i.e. for orbital periods below =~8 days
for solar-type stars of Population I, and 6 days for halo stars. Our
theoretical predictions are in good agreement with the latest available
data (Soderblom et al. 1993; Thorburn et al. 1993; Spite et al. 1994),
and we take this as a proof for the existence of a physical link
between the depletion of lithium and the loss of angular momentum. We
conclude that the abundance of lithium measured in old disk and halo
stars by Spite &amp; Spite (1982) is less than the original one, and
cannot be taken as such to constrain the models of the primordial
nucleosynthesis. Another implication is that magnetic torquing has
played little role in the radiative interior of late-type stars,
and that the core of these stars is probably rotating faster than
their surface.

---------------------------------------------------------
Title: Penetration below a Convection Zone
Authors: Hurlburt, Neal E.; Toomre, Juri; Massaguer, Josep M.; Zahn,
Jean-Paul
1994ApJ...421..245H Altcode:
Two-dimensional numerical simulations are used to investigate how fully
compressible nonlinear convection penetrates into a stably stratified
zone beneath a stellar convection zone. Estimates are obtained of the
extent of penetration as the relative stability S of the stable to
the unstable zone is varied over a broad range. The model deals with
a perfect gas possessing a constant dynamic viscosity. The dynamics
is dominated by downward-directed plumes which can extend far into
the stable material and which can lead to the excitation of a broad
spectrum of internal gravity waves in the lower stable zone. The
convection is highly time dependent, with the close coupling between
the lateral swaying of the plumes and the internal gravity waves they
generate serving to modulate the strength of the convection. The depth
of penetration delta, determined by the position where the time-averaged
kinetic flux has its first zero in the stable layer, is controlled by
a balance between the kinetic energy carried into the stable layer by
the plumes and the buoyancy braking they experience there. A passive
scalar is introduced into the unstable layer to evaluate the transport
of chemical species downward. Such a tracer is effectively mixed
within a few convective overturning times down to a depth of delta
within the stable layer. Analytical estimates based on simple scaling
laws are used to interpret the variation of delta with S, showing that
it first involves an interval of adiabatic penetration if the local
Peclet number of the convection exceeds unity, followed by a further
thermal adjustment layer, the depths of each interval scaling in turn
as S-1 and S-1/4. These estimates are in accord
with the penetration results from the simulations.

---------------------------------------------------------
Title: Mixing processes and stellar evolution.
Authors: Zahn, J. -P.
1994ems..conf..285Z Altcode:
The author reviews some of the mixing processes that may influence the
evolution of massive stars, such as penetrative convection, and puts his
emphasis on those which occur in the radiative envelope. There the main
transport mechanisms are a thermally driven meridian circulation, which
departs significantly from the classical Eddington-Sweet description,
together with turbulent motions generated by the differential
rotation. This rotation-induced mixing will surround the convective
core of such stars with a region of decreasing helium content, which may
prevent semi-convection from ever appearing; the extent of this region
depends sensitively on the rotation rate. The results are compared with
those obtained earlier by Mestel (1953). Work is in progress to verify
whether stars which rotate sufficiently fast may be thoroughly mixed.

---------------------------------------------------------
Title: Structure and evolution of massive stars: mixing processes
and stellar evolution
Authors: Zahn, J. -P.
1993SSRv...66..285Z Altcode:
We review some of the mixing processes that may influence the evolution
of massive stars, such as penetrative convection, and put our emphasis
on those which occur in the radiative envelope. There the main transport
mechanisms are a thermally driven meridian circulation, which departs
significantly from the classical Eddington-Sweet description, together
with turbulent motions generated by the differential rotation. This
rotation-induced mixing will surround the convective core of such
stars with a region of decreasing helium content, which may prevent
semi-convection from ever appearing; the extent of this region depends
sensitively on the rotation rate. The results are compared with those
obtained earlier by Mestel (1953). Work is in progress to verify
whether stars which rotate sufficiently fast may be thoroughly mixed.

---------------------------------------------------------
Title: Turbulence in stellar interiors.
Authors: Zahn, Jean-Paul
1993ASPC...40..236Z Altcode: 1993ist..proc..236Z; 1993IAUCo.137..236Z
This review focusses on the most recent work which has been achieved
concerning turbulence in stellar interiors. Among all possible causes
for such turbulence, the most powerful is certainly the convective
instability in unstably stratified regions, but little was known until
now beyond the fact that thermal convection is capable of establishing
an almost adiabatic stratification.

---------------------------------------------------------
Title: Astrophysical Fluid Dynamics. Les Houches Session LXVII
Authors: Zahn, J. -P.; Zinn-Justin, J.
1993asfd.conf.....Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Astrophysical fluid dynamics.
Authors: Zahn, J. -P.; Zinn-Justin, J.
1993afd..conf.....Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Instabilities and turbulence in rotating stars.
Authors: Zahn, J. -P.
1993afd..conf..561Z Altcode:
Contents: 1. Rotating objects. 2. Baroclinic instabilities. 3. Shear
instabilities. 4. Thermal imbalance and meridional circulation in
rotating stars. 5. Turbulence in rotating stars.

---------------------------------------------------------
Title: Seismological constraints on convective penetration in the Sun.
Authors: Berthomieu, G.; Morel, P.; Provost, J.; Zahn, J. -P.
1993ASPC...40...60B Altcode: 1993IAUCo.137...60B; 1993ist..proc...60B
Penetrative convection is expected below stellar convection zones, where
it should achieve a nearly adiabatic stratification. A theoretical
prediction of the penetration depth has been recently made by
Zahn (1991) which includes an arbitrary parameter ζ depending
on the properties of the convective motions. The authors use the
helioseismological constraints to calibrate the value of this parameter.

---------------------------------------------------------
Title: The solar tachocline.
Authors: Spiegel, E. A.; Zahn, J. -P.
1992A&A...265..106S Altcode:
Acoustic sounding of the Sun reveals that the variation of angular
velocity with latitude is independent of depth in the convection
zone. By contrast, deep within the radiative zone, the rotation appears
to be rigid. The transition between the two rotation laws occurs in a
thin, unresolved layer that the authors here call the tachocline. This
paper is an examination of the structure and previous evolution of this
layer. It is assumed that the stress exerted by the convection zone
is prescribed, much as oceanographers take the wind stress on the sea
surface as given. It is concluded that the helioseismic observations are
best rationalized by a scenario in which, after an initial adjustment or
spindown period, the subconvective rotation settles into a quasisteady
state with a turbulent boundary layer. In the tachocline, the advection
of angular momentum is controlled by horizontal turbulence. If this
turbulence is intense enough, the tachocline is thin and is unresolved.

---------------------------------------------------------
Title: Circulation and turbulence in rotating stars.
Authors: Zahn, J. -P.
1992A&A...265..115Z Altcode:
The author examines the interaction between meridian circulation
and turbulence in rotating, non-magnetic stars. The turbulence is
assumed to be anisotropic, with stronger transport in the horizontal
directions than in the vertical, thereby enforcing a rotation rate
which depends only on depth, to first approximation. This conjecture
is supported by the interior rotation of the Sun, which is now being
revealed through acoustic sounding. The main result is that both the
meridian circulation and the turbulence are determined by the loss of
angular momentum, which the author ascribes here solely to a stellar
wind. When there is no wind, the meridian flow is very weak, and it
can even vanish in slow rotators. When the wind is active, it drives
the circulation in order to transport angular momentum to the surface,
but the advection of chemical species by this flow is partly inhibited
because of the horizontal turbulence.

---------------------------------------------------------
Title: Rotation-induced mixing and lithium depletion in galactic
clusters.
Authors: Charbonnel, C.; Vauclair, S.; Zahn, J. -P.
1992A&A...255..191C Altcode:
Results are presented of computations of lithium depletion in a
rotating star due to turbulence induced by nuclear burning and by
rotation-induced mixing, taking into account the deceleration of the
rotation as the star evolves on the main sequence (but treating the
interior as if it were rotating uniformly). Numerical simulations
are performed for a range of stellar masses, and their results are
compared with the Li and Be abundances observed in the Hyades. It is
found that the theoretical predictions agree reasonably well with
the observations. However, some discrepancies remain, leading to
a conclusion that it is necessary to take into account the mixing
associated with the differential rotation which builds up inside the
star while it is spinning down on the main sequence.

---------------------------------------------------------
Title: Convective Penetration in the Sun
Authors: Berthomieu, G.; Morel, P.; Provost, J.; Zahn, J. -P.
1992ASPC...26..158B Altcode: 1992csss....7..158B
No abstract at ADS

---------------------------------------------------------
Title: Effect of horizontal turbulent diffusion on transport by
meridional circulation.
Authors: Chaboyer, B.; Zahn, J. -P.
1992A&A...253..173C Altcode:
The present examination of horizontal turbulent motion effects of
chemical transport by a stellar radiation zone's meridional circulation
notes that such diffusion, when sufficiently strong, inhibits the
advection of a chemical element through homogenization of horizontal
layers; the vertical transport then reduces to a diffusion process,
and the differential rotation is smoother out in latitude. The presence
of horizontal turbulence may explain the discrepancy between chemical
transport and angular momentum transport as observed in the sun. The
advection of angular momentum is formulated as a one-dimensional
problem.

---------------------------------------------------------
Title: The Turbulent Tachycline
Authors: Spiegel, E. A.; Zahn, J. -P.
1992ASPC...26..188S Altcode: 1992csss....7..188S
No abstract at ADS

---------------------------------------------------------
Title: Present state of the tidal theory.
Authors: Zahn, J. -P.
1992btsf.work..253Z Altcode: 1992bats.proc..253Z
Contents: 1. Introduction. 2. The mechanism of tidal breaking. 3. The
physical causes of tidal friction. 4. Refining the theory to meet the
observations. 5. Concluding remarks.

---------------------------------------------------------
Title: Book-Review - Rotation and Mixing in Stellar Interiors
Authors: Goupil, M. J.; Zahn, J. P.; de Jager, C.
1992SSRv...59..410G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Convective penetration in stellar interiors.
Authors: Zahn, J. -P.
1991A&A...252..179Z Altcode:
Evidence for convective penetration from a variety of sources is used
to deduce a simple model which establishes a relation between the
subadiabatic extent of a convective region and the velocity of the
penetrating motions. It is found that the subadiabatic penetration at
the bottom of a convective envelope is of the order of a pressure scale
height, and that above a convective core it amounts to a substantial
fraction of the core radius. It is shown that the requirements
for nearly adiabatic penetration are met deep enough in a stellar
interior and that the departures from adiabaticity are confined to
a thin boundary layer. This allows the use of Roxburgh's integral
constraint to predict the actual size of a convective core.

---------------------------------------------------------
Title: Convection and Turbulence in Stars
Authors: Zahn, Jean-Paul
1991LNP...373..355Z Altcode: 1991lsse.conf..355Z
No abstract at ADS

---------------------------------------------------------
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: On the Nature of Disk Viscosity.
Authors: Zahn, J. P.
1991sepa.conf...87Z Altcode: 1991IAUCo.129...87Z
No abstract at ADS

---------------------------------------------------------
Title: Convective Penetration into Stellar Radiation Zones
Authors: Zahn, Jean-Paul
1991LNP...388..225Z Altcode: 1991ctsm.conf..225Z
We recall the evidence for convective penetration exhibited by
geophysical fluids, laboratory experiments and computer simulations. A
simple model is deduced from these observations, which serves to
establish a relation between the subadiabatic extent of a convective
region and the velocity of the penetrating motions. Assuming that this
velocity obeys the usual scaling of thermal convection, we find that the
subadiabatic penetration at the bottom of a convective envelope is of
the order of a pressure scale-height, and that above a convective core
it amounts to a substantial fraction of the core radius. We show that
the requirements for nearly adiabatic penetration are met deep enough
in a stellar interior, and that the departures from adiabaticity are
confined to a thin boundary layer. This allows the use of Roxburgh's
integral constraint to predict the actual size of a convective core.

---------------------------------------------------------
Title: Theory of Transport Processes
Authors: Zahn, Jean-Paul
1990ASSL..159..425Z Altcode: 1990insu.conf..425Z; 1990IAUCo.121..425Z
No abstract at ADS

---------------------------------------------------------
Title: Turbulent shear flow and rotation
Authors: Zahn, J. P.
1990nwus.book..291Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Conclusion
Authors: Zahn, Jean-Paul
1990LNP...366..183Z Altcode: 1990rmsi.conf..183Z
No abstract at ADS

---------------------------------------------------------
Title: Turbulent Transport in Stellar Radiation Zones: Causes
and Effects
Authors: Zahn, Jean-Paul
1990LNP...366..141Z Altcode: 1990rmsi.conf..141Z
We briefly review the instabilities which are the most likely to
generate turbulent transport of chemicals and of angular momentum
in radiative stellar interiors, namely the shear instabilities
due to differential rotation. Estimates are given for the turbulent
diffusivity, and it is examined how a meridional circulation can cause
such differential rotation.

---------------------------------------------------------
Title: Rotation and Mixing in Stellar Interiors
Authors: Goupil, Marie-Jo; Zahn, Jean-Paul
1990LNP...366.....G Altcode: 1990rmsi.conf.....G
No abstract at ADS

---------------------------------------------------------
Title: Tidal evolution of close binary stars. II. Orbital
circularization oflate-type binaries.
Authors: Zahn, J. -P.; Bouchet, L.
1989A&A...223..112Z Altcode:
The premain-sequence evolution of close binary systems with masses
ranging from 0.5 to 1.25 solar masses is described. It is demonstrated
that most of the orbital circularization occurs during the Hayashi
phase, and that the subsequent decrease in eccentricity on the
main-sequence is negligible. The theoretical cutoff period, which
separates the circular from the eccentric systems, lies between 7.2 and
8.5 days, depending on the masses and on the assumptions made about the
initial conditions; it agrees well with the observed period of about
8 days. This cutoff period is a function of the initial radius of the
stars on the Hayashi track. In these calculations, the values predicted
by the star formation theory of Stahler et al. (1980) are used; hence
the good agreement mentioned above provides a new validation of that
theory. It is also shown that systems around that cutoff period reach
the zero-age main-sequence with their components rotating faster than
the orbital rate.

---------------------------------------------------------
Title: Tidal evolution of close binary stars. I - Revisiting the
theory of the equilibrium tide
Authors: Zahn, J. -P.
1989A&A...220..112Z Altcode:
The theory of the equilibrium tide in stars that possess a convective
envelope is reexamined critically, taking recent developments into
account and treating thermal convection in the most consistent way
within the mixing-length approach. The weak points are identified and
discussed, in particular, the reduction of the turbulent viscosity when
the tidal period becomes shorter than the convective turnover time. An
improved version is derived for the secular equations governing the
dynamical evolution of close binaries of such type.

---------------------------------------------------------
Title: Open problems in modeling the solar and stellar environment.
Authors: Zahn, Jean-Paul
1989mse..proc..119Z Altcode:
Contents: 1. Basic physical properties of the stellar
environment. 2. Instabilities. 3. Accretion disks and collimated
flows. 4. The stars. 5. Conclusion.

---------------------------------------------------------
Title: Turbulent transport in the radiative zone of a rotating star
Authors: Zahn, J. -P.
1987ASSL..137..201Z Altcode: 1987isav.symp..201Z
Hydrodynamical instabilities that occur in the radiation zone of a
differentially rotating star may produce the mild turbulent mixing
which is required to explain various observations. The author discusses
a consistent scheme in which the energy which sustains the turbulent
motions originates in the thermal imbalance caused by the rotation. An
approximate expression is derived for the vertical diffusion coefficient
and the consequences of this process are compared with those of the
classical meridional circulation.

---------------------------------------------------------
Title: Solar and Stellar Convection
Authors: Zahn, J. -P.
1987LNP...292...55Z Altcode: 1987ssp..conf...55Z
The author examines the progress made during the last decade in
our knownledge of stellar convection, first in the theory and using
numerical simulation, and second through observations, from ground
and space. Special emphasis is put on those results which allow to
tackle the basic questions: (1) Is the same mechanism responsible
for the different types and scales of motions that are currently
identified in the solar convection zone: granulation, mesogranulation,
supergranulation, large eddies? (2) On which scale(s) is energy injected
into the convective motions? For what reason?

---------------------------------------------------------
Title: Instabilities due to convection and rotation
Authors: Zahn, J. -P.
1987ASSL..136..143Z Altcode: 1987ilet.work..143Z
This paper examines whether some of the instabilities observed in
early-type luminous stars can be caused by thermal convection or
are due to the rotation of those stars. The effects of instabilities
associated with convection on the behavior of the stellar surface layers
are reviewed. The usefulness of numerical simulations in studying such
effects is addressed. Baroclinic instabilities and shear instabilities
caused by rotation are discussed, and the possible mechanisms for
maintaining differential rotation are considered.

---------------------------------------------------------
Title: Hydrodynamics in Stars
Authors: Zahn, J. -P.
1986MitAG..67..163Z Altcode:
Fluid dynamics plays a central role in astrophysics, even though it has
been given, until now, insufficient recognition. The purpose of this
paper is to contribute a little to remedy this situation by presenting
some of the hydrodynamical problems occurring in an important class
of objects, the stars.

---------------------------------------------------------
Title: Astronomie Flammarion, Vols. 1+2.
Authors: Rösch, J.; Wlérick, G.; Guibert, J.; Bonnet, R.; Vedrenne,
G.; Celnikier, L.; Morando, B.; Boischot, A.; Brahic, A.; Kandel,
R.; Dumont, S.; Praderie, F.; Magnan, C.; Gerbaldi, M.; Zahn, J. -P.;
Gouguenheim, L.; Bottinelli, L.; Heidmann, N.; Heidmann, J.; Véron,
P.; Nottale, L.; Pecker, J. -C.; Golay, M.
1985afv..book.....R Altcode:
Contents: L'astronomie à l'œil nu (J. Rösch). L'astronomie
optique; de Galilée à la fin du XXe siècle
(G. Wlérick). Télescopes et lunettes du XXe siècle
(G. Wlérick). La radioastronomie (J. Guibert). L'astronomie
spatiale (R. Bonnet). L'astronomie des hautes énergies
(G. Vedrenne). L'exploration in situ dumilieu astronomique
(L. Celnikier). Le système solaire: généralités (B. Morando). Les
planètes telluriques (L. Celnikier). Astéroïdes, comètes et
météorites (L. Celnikier). Jupiter (A. Boischot). Le monde de Saturne
(A. Brahic). Les confins du système solaire (Uranus, Neptune et Pluton)
(A. Brahic). Le Soleil, les planètes, la Terre (R. Kandel). Le Soleil,
étoile typique (S. Dumont). Les spectres stellaires: la connaissance
des étoiles (F. Praderie). L'interaction des étoiles et du milieu
environnant (C. Magnan). La Galaxie: étoiles et matière interstellaire
(M. Gerbaldi). L'évolution des étoiles (J.-P. Zahn). Les facteurs de
l'évolution galactique (L. Gouguenheim). Forme et classification des
galaxies (L. Bottinelli, N. Heidmann). Amas et superamas de galaxies
(J. Heidmann). Les quasars (P. Véron). Évolution des galaxies
(L. Gouguenheim). Les faits cosmologiques (L. Bottinelli). La cosmologie
de la grandeexplosion (L. Nottale). Le débat cosmologique (L. Nottale).

---------------------------------------------------------
Title: Penetrative cellular convection in a stratified atmosphere
Authors: Massaguer, J. M.; Latour, J.; Toomre, J.; Zahn, J. -P.
1984A&A...140....1M Altcode:
In the present investigation of penetrative convection within a simple
compressible model, the middle one of the three layers of differing
stratification prior to the onset of convection is a convectively
unstable polytrope bounded above and below by two stably stratified
polytropes. One- and two-mode steady solutions with hexagonal planforms
have been studied for Rayleigh numbers up to aobut 1000 times critical,
and for a range of Prandtl numbers, horizontal wavenumbers, and
stratifications. These indicate that the penetration into the lower
stable layer by downward plumes is substantially larger in a stratified
medium than in a Boussinesq fluid, and produces an extended region
of adiabatic stratification. The strong asymmetry between upward and
downward penetration in compressible media has major implications for
the mixing of stable regions above and below stellar convection zones.

---------------------------------------------------------
Title: Introductory report : Stability of Rotation Laws
Authors: Zahn, J. P.
1984LIACo..25..407Z Altcode: 1984tpss.conf..407Z; 1984trss.conf..407Z
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Astrophysical Processes in Upper Main Sequence
Stars
Authors: Cox, A. N.; Vauclair, S.; Zahn, J. P.
1984S&T....67Q.526C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Astrophysical Processes in Upper Main Sequence
Stars - Saas-Fee 13TH - 1983MAR
Authors: Cox, A.; Vauclair, S.; Zahn, J. P.
1984Mercu..13...93C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Tidal Effects in Close Binary Stars
Authors: Zahn, J. P.
1984IAUS..105..379Z Altcode:
This review addresses the question of what can be learned about the
stellar interiors from the study of the tidal effects in close binary
stars.

---------------------------------------------------------
Title: Un télescope pour les amateurs au Pic-du-Midi.
Authors: Zahn, J. -P.
1983LAstr..97...27Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Les instabilités dues à la rotation.
Authors: Zahn, J. -P.
1983ihaa.conf..389Z Altcode:
Contents: Instabilité de cisaillement. Instabilité
barocline. Instabilités multidiffusives. Comparaison des différentes
instabilités. Esquisse d'une solution.

---------------------------------------------------------
Title: Nonlinear Anelastic Modal Theory for Solar Convection
Authors: Latour, J.; Toomre, J.; Zahn, J. -P.
1983SoPh...82..387L Altcode: 1983IAUCo..66..387L
Preliminary solar envelope models have been computed using the
single-mode anelastic equations as a description of turbulent
convection. This approach provides estimates for the variation
with depth of the largest convective cellular flows, akin to giant
cells, with horizontal sizes comparable to the total depth of the
convection zone. These modal nonlinear treatments are capable of
describing compressible motions occurring over many density scale
heights. Single-mode anelastic solutions have been constructed for a
solar envelope whose mean stratification is nearly adiabatic over most
of its vertical extent because of the enthalpy (or convective) flux
explicitly carried by the big cell; a sub-grid scale representation of
turbulent heat transport is incorporated into the treatment near the
surface. The single-mode equations admit two solutions for the same
horizontal wavelength, and these are distinguished by the sense of
the vertical velocity at the center of the three-dimensional cell. It
is striking that the upward directed flows experience large pressure
effects when they penetrate into regions where the vertical scale height
has become small compared to their horizontal scale. The fluctuating
pressure can modify the density fluctuations so that the sense of the
buoyancy force is changed, with buoyancy braking actually achieved near
the top of the convection zone. The pressure and buoyancy work in the
shallow but unstable H+ and He+ ionization regions
can serve to decelerate the vertical motions and deflect them laterally,
leading to strong horizontal shearing motions. It appears that such
dynamical processes may explain why the amplitudes of flows related to
the largest scales of convection are so feeble in the solar atmosphere.

---------------------------------------------------------
Title: Circulation méridienne dans les étoiles.
Authors: Zahn, J. -P.
1983ihaa.conf..409Z Altcode:
Contents: Le déséquilibre radiatif dans une étoile en rotation. Les
premiers travaux. La solution des Tassoul.

---------------------------------------------------------
Title: Astrophysical process in upper main sequence stars
Authors: Cox, A. N.; Vauclair, S.; Zahn, J. P.
1983apum.conf.....C Altcode: 1983SAAS...13.....C; 1983QB799.C69......
No abstract at ADS

---------------------------------------------------------
Title: Instability and Mixing Processes in Upper Main Sequence Stars
Authors: Zahn, J. -P.
1983apum.conf..253Z Altcode: 1983SAAS...13..253Z
Introduction Generalities The basic equations Global time scales
The Brunt-Väisälä frequency The Rayleigh frequency Application
to rotating stars The Eddington-Sweet Circulation The early
investigations The Tassoul's solution Ekman Pumping, Spin-Down The
Ekman layer Ekman pumping and spin-down time Penetrative Convection
Mixing length treatment Hydrodynamical calculations Double Diffusive
Instabilities. Semi-Convection Shear Flow Instability Generalities
Startified shear flows Effect of rotation The Taylor experiment
Baroclinic Instability The basic mechanism The instability condition
Application to stars Multidiffusive Instabilities Involving Angular
Momentum Goldreich-Schubert-Fricke instability Triply diffusive
instabilities Trubulence and Mixing Comparison of the instabilities
Three-dimensional turbulence Two-dimensional turbulence Turbulent mixing
Sketch of a Possible Solution General outline Quantitative requirements
Efficiency of the turbulence generation Vertical diffusivity and
mixing References

---------------------------------------------------------
Title: Nonlinear modal analysis of penetrative convection
Authors: Zahn, J. -P.; Toomre, J.; Latour, J.
1982GApFD..22..159Z Altcode:
It is pointed out that thermal convection in many astrophysical and
geophysical settings occurs in an unstable layer bounded above and
below by regions which are stably stratified. The convective motions
may extend a substantial distance into the adjacent stable zones. If
the motions have little direct effect upon the mean stratification
of the stable zone, then they are usually referred to as convective
overshooting. The primary objective of the present investigation is
to study the dynamics of overshooting at very large Rayleigh numbers,
mainly with stellar applications in mind. Numerically this is only
feasible if severe simplifications are made in the description of
what are likely to be turbulent motions. The approach employed in the
investigation utilizes nonlinear modal equations in which the vertical
and temporal structure of the convection is described accurately at
the expense of the horizontal structure. A summary of the salient
properties of penetrative convection is provided on the basis of the
results of the conducted studies.

---------------------------------------------------------
Title: Book reviews
Authors: Ne'Eman, Y.; Zahn, J. -P.; Habing, H. J.; Wittenberg, H.;
Zwaan, C.; Murray, C. A.; de Jager, Cornelis; Kresák, L.
1982SSRv...33..459N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Hydrodynamic Instabilities and the Transition
to Turbulence
Authors: Swinney, H. L.; Gollub, J. P.; Zahn, J. P.
1982SSRv...33R.459S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stellar convection theory. III - Dynamical coupling of the
two convection zones in A-type stars by penetrative motions
Authors: Latour, J.; Toomre, J.; Zahn, J. -P.
1981ApJ...248.1081L Altcode:
The thermal convection occurring over many density scale heights in an
A-type star outer envelope, encompassing both the hydrogen and helium
convectively unstable zones, is examined by means of anelastic modal
equations. The single-mode anelastic equations for such compressible
convection display strong overshooting of the motions into adjacent
radiative zones, which would preclude diffusive separation of
elements in the supposedly quiescent region between the two unstable
zones. In addition, the anelastic solutions reveal that the two zones
of convective instability are dynamically coupled by the overshooting
motions. The two solutions that the nonlinear single-mode equations
admit for the same horizontal wavelength are distinguished by the
sense of the vertical velocity at the center of the three-dimensional
cell. It is suggested that strong horizontal shear flows should be
present just below the surface of the star, and that the large-scale
motions extending into the stable atmosphere would appear mainly as
horizontal flows.

---------------------------------------------------------
Title: Nonlinear Anelastic Models of Solar Convection
Authors: Toomre, J.; Latour, J.; Zahn, J. -P.
1981BAAS...13Q.907T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Workshop on pulsating B stars. Proceedings of a workshop held
at Nice Observatory, June 1 - 5, 1981. (Centenaire de l'Observatoire
de Nice 1881 - 1981).
Authors: Auvergne, M.; Baglin, A.; Ducatel, D.; Le Contel, J. -M.;
Morel, P. J.; Sareyan, J. -P.; Valtier, J. -C.; Sterken, C.; Zahn,
J. -P.
1981wopb.book.....A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Cent ans d'astronomie à l'Observatoire de Nice 1881 - 1981.
Authors: Clorennec, A.; Feldman, R.; Zahn, J. -P.; Willemse, H.
1981cad..book.....C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Convection zones in sun and stars.
Authors: Zahn, J. P.
1981ASIC...68..123Z Altcode: 1981spss.conf..123Z
Aspects of the current understanding of the nature, description and
observational properties of solar and stellar convection zones are
discussed. The origin of a convective zone as a result of the increase
in the luminosity/mass ratio of a star in its center and increases
in the opacity in the outer layers of a star is discussed, and the
standard mixing length treatment of the ensuing envelope convection
is presented. The expected effects of convective motions are examined
in order to demonstrate the inadequacy of the standard approach in
accounting for the generation of kinetic energy and the transport of
angular momentum and magnetic field, and alternative hydrodynamic
approaches are described. Observations of solar oscillations and
supergranulation and stellar atmospheres, rotation rates and orbital
eccentricities which have a bearing on the properties and existence
of convection zones are also noted.

---------------------------------------------------------
Title: Pulsating Stars. (Book Reviews: Theory of Stellar Pulsation)
Authors: Zahn, Jean-Paul
1980Sci...210.1119Z Altcode: 1980Sci...210.1119C
No abstract at ADS

---------------------------------------------------------
Title: Anelastic Modal Theory Applied to the Solar Convection Zone
Authors: Toomre, J.; Latour, J.; Zahn, J. P.
1980BAAS...12..895T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Cellular convection in a stratified atmosphere
Authors: Massaguer, J. M.; Zahn, J. -P.
1980A&A....87..315M Altcode:
The properties of thermal convection in an atmosphere spanning
several density scale heights are examined. Both the conductivity
and the shear viscosity are assumed to be constant. The anelastic
approximation is used, together with a modal expansion procedure
in the horizontal directions; this expansion is severely truncated,
keeping one or two modes. Only steady solutions are built, for Rayleigh
numbers up to 100,000 times critical and Prandtl numbers of 1, 0.01
and 0.0001. The most striking properties of the solutions is that they
display an inversion of the buoyancy force in the upper part of the
domain, and that the pressure fluctuations play an essential role in
controlling the dynamics. Both the effects are ignored in the current
mixing-length approach.

---------------------------------------------------------
Title: Stellar convection theory
Authors: Zahn, J. -P.
1980LNP...114....1Z Altcode: 1980sttu.coll....1Z; 1980IAUCo..51....1Z
Stellar turbulence due to thermal convection is considered, as well as
the modelling of convective transport and specifically, the study of the
advection of a passive scalar, ignoring molecular diffusion. Attention
is given to the introduction of the diffusion approximation in applying
the mixing length approach to stellar convection. Solving the original
fluid dynamical equations is discussed in terms of hydrodynamical
approaches, which include working either in the Fourier space or
the physical space or utilizing the modal expansion procedure. In
addition, considering the effect of strong density stratification,
the anelastic approximation is discussed, with attention given to
pressure and buoyancy forces, and pressure and temperature fluctuations.

---------------------------------------------------------
Title: Compressible Convection in the Outer Envelope of A-type Stars
Authors: Toomre, J.; Latour, J.; Zahn, J. -P.
1978BAAS...10..677T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the boundary conditions imposed by a stratified fluid
Authors: Latour, J.; Zahn, J. -P.
1978GApFD..10..311L Altcode:
Through generalization of the results obtained by Stix (1970) and
by Whitehead (1971), the paper presents the homogeneous boundary
conditions that are imposed by a fluid of stable but otherwise arbitrary
stratification. These penetrating conditions are strictly valid only
in the limit of steady linearized flow. However, under some conditions
they can also be applied to nonlinear problems. An example of linear
penetrative convection is treated in order to illustrate the use of
these boundary conditions.

---------------------------------------------------------
Title: Défense du site de l'observatoire.
Authors: Zahn, J. -P.; Baglin, A.
1978BONic..13...25Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Tidal friction in close binary systems.
Authors: Zahn, J. -P.
1977A&A....57..383Z Altcode: 2009A&A...500..121Z
We examine various physical mechanisms which may produce tidal friction
in close binary stars. We find that the most efficient in stars with
convective envelopes is turbulent viscosity retarding the equilibrium
tide, and in stars with radiative envelopes the action of radiative
damping on the dynamical tide. Theoretical predictions based on
these dissipative processes are in good agreement with the rotational
velocities and orbital eccentricities observed in close binaries. The
results are applied to the X-ray binaries Her X-1 and Cen X-3.

---------------------------------------------------------
Title: Anelastic Stellar Convection Theory Applied to A-type Stars.
Authors: Toomre, J.; Latour, J.; Zahn, J. -P.
1977BAAS....9..337T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Penetrative convection in stars
Authors: Zahn, J. -P.
1977LNP....71..225Z Altcode: 1977IAUCo..38..225Z; 1977stco.coll..225Z; 1977psc..conf..225Z
Methods developed to describe stellar convection are reviewed, and
their suitability for describing the penetration of convective motions
into stable surroundings is considered. Phenomenological approaches
based on local and nonlocal mixing-length treatments are examined
along with procedures based on the concept of thermals and direct
approaches for treating Boussinesq convection as well as convection
in a stratified medium. Possible observational tests of the various
methods are outlined.

---------------------------------------------------------
Title: Problems of stellar convection
Authors: Spiegel, E. A.; Zahn, J. -P.
1977LNP....71.....S Altcode: 1977IAUCo..38.....S; 1977stco.coll.....S
The papers deal with various aspects of stellar convection theory
ranging from mixing-length formalisms to computations based on full
equations. The major subjects encompass mixing-length theory, linear
theory, observational aspects of convection, numerical solutions
to stellar-convection problems, effects of rotation and magnetic
fields, penetration of convection, special types of convection,
waves, and turbulence. Specific topics include the current state of
mixing-length theory, dynamical instabilities in stars, observations
bearing on convection, the evolutionary pattern of exploding
granules, numerical methods in convection theory, convection in
rotating stars, and convective dynamos. Other contributions examine
penetrative convection in stars, convective overshooting in the solar
photosphere, thermosolutal convection, Urca convection, photoconvection,
convection in the helium flash, and the roles of determinism and chaos
in turbulence.

---------------------------------------------------------
Title: Stellar convection theory. II. Single-mode study of the second
convection zone in an A-type star.
Authors: Toomre, J.; Zahn, J. -P.; Latour, J.; Spiegel, E. A.
1976ApJ...207..545T Altcode:
The anelastic modal equations presented in Paper I are considered
in their simplest version: only one mode is retained in the
representation of the fluctuating dynamic and thermodynamic variables
of convection theory. These single-mode equations are used to examine
the structure of the second convection zone of an A-type star. Two-
and three-dimensional numerical solutions are obtained for a range of
parameters in the theory, and a simple analysis is provided for their
interpretation. The principal results are for three-dimensional motions,
since these are most likely to be relevant to stellar convection. Such
motions produce a convective heat flux several orders of magnitude
greater than predicted by standard mixing-length theory for the same
situation; we find that convection carries up to 6 percent of the
total flux. The most significant astrophysical implication of our
results is that they suggest strong overshooting into the adjacent
radiative zones. We anticipate that mixing will extend to the overlying
hydrogen convection zone. This would rule out some interpretations of
metallic-line stars which invoke diffusive element separation between
the two convection zones. Subject headings: convection - stars:
interiors - stars: metallic-line

---------------------------------------------------------
Title: Stellar convection theory. I. The anelastic modal equations.
Authors: Latour, J.; Spiegel, E. A.; Toomre, J.; Zahn, J. -P.
1976ApJ...207..233L Altcode:
Methods are developed for dealing with the various dynamical
problems that arise because of convective zones in stars. A system
of equations for stellar convection is derived from the full
equations of compressible fluid dynamics with the aid of two major
approximations. The first of these is the anelastic approximation,
which involves both the filtering out of acoustic waves and a suitable
linearization of the fluctuating thermodynamic variables. The second
one approximates the horizontal structure of convection by expanding
the motion in a set of horizontal cellular platforms and severely
truncating the expansion. The resulting system of partial differential
equations, referred to as the anelastic modal equations, is outlined
along with suggested boundary conditions and techniques for solving
the equations. Ways of assessing the overall validity of the present
treatment are discussed.

---------------------------------------------------------
Title: The Second Convection Zone in an A-type Star
Authors: Latour, J.; Spiegel, E. A.; Toomre, J.; Zahn, J. P.
1975BAAS....7..526L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The dynamical tide in close binaries.
Authors: Zahn, J. -P.
1975A&A....41..329Z Altcode:
The non-adiabatic oscillations of a star, driven by an outer rotating
gravitational field, have been studied by the use of matched asymptotic
expansions. The interior and envelope solutions in this procedure are
derived in Sections 2 and 3. The results apply to stars which have
a convective core and a radiative envelope, and they are discussed
in Section 4. We find that the resonances of the free gravity modes
are damped by radiative dissipation, which operates in a relatively
thin region below the surface of the star. Due to that dissipation,
some properties of the dynamical tide have observable consequences in
close binary systems: (1) A torque is applied to a binary component;
this serves to make it corotate with its companion in a time which can
be short compared to its nuclear life. (2) Before that synchronization
is achieved, the brightness distribution over the surface of the star
is in general phase shifted relative to the external driving potential.

---------------------------------------------------------
Title: Differential Rotation and Turbulence in Stars
Authors: Zahn, J. P.
1975MSRSL...8...31Z Altcode: 1975phs..conf...31Z
No abstract at ADS

---------------------------------------------------------
Title: Rotational Instabilities and Stellar Evolution
Authors: Zahn, J. -P.
1974IAUS...59..185Z Altcode:
This review deals with the local instabilities arising when the effects
of rotation are taken into account in the evolution of a nonmagnetic
star. The Rayleigh and shear instabilities will be examined under the
conditions prevailing in radiative zones where the effect of density
stratification, thermal diffusion, viscosity and varying chemical
composition must be taken into account. The possible consequences on
the evolution of a star are finally outlined.

---------------------------------------------------------
Title: Pleading for Hydrodynamics
Authors: Zahn, J. P.
1972oss..conf..371Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Sur la rotation des étoiles doubles serées.
Authors: Zahn, J. -P.
1972CRASB.274.1443Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Instabilities of Differential Rotation
Authors: Spiegel, E. A.; Zahn, J. -P.
1970CoASP...2..178S Altcode: 1970ComAp...2..178S
No abstract at ADS

---------------------------------------------------------
Title: Forced Oscillations in Close Binaries. The Adiabatic
Approximation
Authors: Zahn, J. P.
1970A&A.....4..452Z Altcode:
The forced oscfflations of a star are discussed, due to a periodic
varying gravitational field. The following approximations are made:
the star is not rotating, the oscfflations are adiabatic and small
enough to allow linearization. Olver's method is employed to derive a
second-order asymptotic expression for the eigenfunctions. The results
predict large amplitudes for the tides near the surface, where however
adiabacy is a pcor approximation.

---------------------------------------------------------
Title: Appel aux observateurs
Authors: Zahn, J. -P.
1969LAstr..83..114Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Possible Effects of the Resonance in Close Binaries on their
Mass Exchanges
Authors: Zahn, J. -P.
1969ASSL...13..267Z Altcode: 1969mlfs.conf..267Z
No abstract at ADS

---------------------------------------------------------
Title: On Possible Non-Radial Pulsations of Baker's One-Zone Model
Authors: Zahn, J. P.
1968ApL.....1..209Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Internal structure and evolutionary changes in binaries:
Variation des éléments orbitaux sous l'influence des marées
Authors: Zahn, J. P.
1967oeds.conf..108Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Internal structure and evolutionary changes in binaries:
Causes possibles d'évolution homogène des binaires serrées
Authors: Zahn, J. P.
1967oeds.conf..124Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Les éruptions solaires
Authors: Zahn, J. -P.
1966LAstr..80..359Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Les marées dans une étoile double serrée (suite)
Authors: Zahn, J. P.
1966AnAp...29..489Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Les marées dans une étoile double serrée (suite et fin)
Authors: Zahn, J. P.
1966AnAp...29..565Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Les marées dans une étoile double serrée
Authors: Zahn, J. P.
1966AnAp...29..313Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Éclipse de Lune du 19 décembre 1964
Authors: Marin, M.; Zahn, J. -P.
1965LAstr..79..326M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: L'éclipse de Lune des 24 et 25 juin 1964
Authors: Marin, M.; Zahn, J. -P.
1965LAstr..79...14M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: L'âge des étoiles
Authors: Zahn, J. -P.
1964LAstr..78..457Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Préparons-nous à observer la prochaine éclipse de Lune
Authors: Coupinot, G.; Zahn, J. -P.
1964LAstr..78..148C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Cinq jeunes suivent un stage à l'Observatoire de Nice
Authors: Zahn, J. -P.
1964LAstr..78...96Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Appel aux Jeunes
Authors: Zahn, J. -P.
1964LAstr..78...64Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Le magnétisme stellaire
Authors: Zahn, J. P.
1963LAstr..77..313Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Autres observations de l'éclipse partielle de Lune des 15-16
juillet 1954
Authors: Hannorat, M.; Zahn, J. -P.
1954LAstr..68..384H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: L'éclipse partielle de Soleil du 30 juin 1954
Authors: Pollachi, L. G.; Porret, R.; Weber, R.; Dumser, L.; Kern,
H.; Zahn, J. -P.; Badina, V. -E.
1954LAstr..68..372P Altcode:
No abstract at ADS