Michel Rieutord - Academia.edu (original) (raw)

Papers by Michel Rieutord

The Sun’s supergranulation refers to a physical pattern covering the surface of the quiet Sun wit... more The Sun’s supergranulation refers to a physical pattern covering the surface of the quiet Sun with a typical horizontal scale of approximately 30 000 kilometres and a lifetime of around 1.8 days. Its most noticeable observable signature is as a fluctuating velocity field of 360 m/s rms whose components are mostly horizontal. Supergranulation was discovered more than fifty years ago, however explaining why and how it originates still represents one of the main challenges of modern solar physics. A lot of work has been devoted to the subject over the years, but observational constraints, conceptual difficulties and numerical limitations have all concurred to prevent a detailed un-derstanding of the supergranulation phenomenon so far. With the advent of 21st century supercomputing resources and the availability of unprecedented high-resolution observations of the Sun, a stage at which key progress can be made has now been reached. A unify-ing strategy between observations and modelling...

arXiv (Cornell University), Jun 30, 2023

Context. Intermediate-mass stars are often fast rotators, and hence are centrifugally flattened a... more Context. Intermediate-mass stars are often fast rotators, and hence are centrifugally flattened and notably affected by gravity darkening. To analyse this kind of stars properly, one must resort to 2D models to compute the visible radiative flux and to take the geometrical effect of the star inclination into account. Aims. Assuming a given stellar age and chemical composition, our aim is to derive the mass and rotation rates of main sequence fast rotating stars, along with their inclination, from photometric quantities influenced by gravity darkening. Methods. We chose three observables that vary with mass, rotation, and inclination: the temperature derived by the infrared flux method T IRFM , the Strömgren c 1 index, and a second index c 2 built in the same way as the c 1 index, but sensitive to the UV side of the Balmer jump. These observables are computed from synthetic spectra produced with the PHOENIX code and rely on a 2D stellar structure from the ESTER code. These quantities are computed for a grid of models in the range 2 to 7 M ⊙ , and rotation rates from 30% to 80% of the critical rate. Then, for any triplet (T IRFM , c 1 , c 2), we try to retrieve the mass, rotation rate, and inclination using a Levenberg-Marquardt scheme, after a selection step to find the most suitable starting models. Results. Hare-and-hound tests showed that our algorithm can recover the mass, rotation rate, and inclination with a good accuracy. The difference between input and retrieved parameters is negligible for models lying on the grid and is less than a few percent otherwise. An application to the real case of Vega showed that the u filter is located in a spectral region where the modelled and observed spectra are discrepant, and led us to define a new filter. Using this new filter and subsequent index, the Vega parameters are also retrieved with satisfactory accuracy. Conclusions. This work opens the possibility to determine the fundamental parameters of rapidly rotating early-type stars from photometric space observations.

arXiv: Solar and Stellar Astrophysics, 2016

The internal rotation of low mass stars all along their evolution is of primary interest when stu... more The internal rotation of low mass stars all along their evolution is of primary interest when studying their rotational dynamics, internal mixing and magnetic fields generation. In this context, helio- and asteroseismology probe angular velocity gradients deep within solar type stars. Still the rotation of the close center of such stars on the main sequence is hardly detectable and the dynamical interactions of the radiative core with the surface convective envelope is not well understood. Among them, the influence of the differential rotation profile sustained by convection and applied as a boundary condition to the radiation zone may be very important leading to the formation of tachoclines. In the solar convective region, the equator is rotating faster than the pole while numerical simulations predict either a solar or an anti-solar rotation in other low mass stars envelopes depending on their convective Rossby number. In this work, we therefore build for the first time 2D steady...

arXiv: Solar and Stellar Astrophysics, 2017

Early-type stars generally tend to be fast rotators. In these stars, mode identification is very ... more Early-type stars generally tend to be fast rotators. In these stars, mode identification is very challenging as the effects of rotation are not well known. We consider here the example of alpha\alphaalpha Ophiuchi, for which dozens of oscillation frequencies have been measured. We model the star using the two-dimensional structure code ESTER, and we compute both adiabatic and non-adiabatic oscillations using the TOP code. Both calculations yield very complex spectra, and we used various diagnostic tools to try and identify the observed pulsations. While we have not reached a satisfactory mode-to-mode identification, this paper presents promising early results.

Journal of Fluid Mechanics, 2020

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

EPJ Web of Conferences, 2017

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

Astronomy & Astrophysics, 2018

Context. Gravity darkening (GD) and flattening are important consequences of stellar rotation. Th... more Context. Gravity darkening (GD) and flattening are important consequences of stellar rotation. The precise characterization of these effects across the Hertzsprung–Russell (H-R) diagram is crucial to a deeper understanding of stellar structure and evolution. Aims. We seek to characterize such important effects on Sargas (θ Scorpii), an evolved, fast-rotating, intermediate-mass (∼5 M⊙) star, located in a region of the H-R diagram where they have never been directly measured as far as we know. Methods. We use our numerical model CHARRON to analyze interferometric (VLTI/PIONIER) and spectroscopic (VLT/UVES) observations through a MCMC model-fitting procedure. The visibilities and closure phases from the PIONIER data are particularly sensitive to rotational flattening and GD. Adopting the Roche approximation, we investigate two GD models: (1) the β-model (Teff ∝ geff β), which includes the classical von Zeipel’s GD law, and (2) the ω-model, where the flux is assumed to be anti-parallel ...

Journal of Fluid Mechanics, 2018

We investigate the asymptotic properties of axisymmetric inertial modes propagating in a spherica... more We investigate the asymptotic properties of axisymmetric inertial modes propagating in a spherical shell when viscosity tends to zero. We identify three kinds of eigenmodes whose eigenvalues follow very different laws as the Ekman number$E$becomes very small. First are modes associated with attractors of characteristics that are made of thin shear layers closely following the periodic orbit traced by the characteristic attractor. Second are modes made of shear layers that connect the critical latitude singularities of the two hemispheres of the inner boundary of the spherical shell. Third are quasi-regular modes associated with the frequency of neutral periodic orbits of characteristics. We thoroughly analyse a subset of attractor modes for which numerical solutions point to an asymptotic law governing the eigenvalues. We show that three length scales proportional to$E^{1/6}$,$E^{1/4}$and$E^{1/3}$control the shape of the shear layers that are associated with these modes. These scale...

The ESTER project aims at building a stellar evolution code in two dimensions of space for the st... more The ESTER project aims at building a stellar evolution code in two dimensions of space for the study of effects of rotation. The numerical scheme is based on spectral methods with a spherical harmonic decomposition in the horizontal direction and a Chebyshev polynomial expansion in the vertical direction. Coordinates adapted to the centrifugally distorted shape are mapped to spherical coordinates. First tests on rotating polytropes are presented.

Physics of the Earth and Planetary Interiors, 2010

In this paper, we combine theoretical and experimental approaches to study the tidal instability ... more In this paper, we combine theoretical and experimental approaches to study the tidal instability in planetary liquid cores and stars. We demonstrate that numerous complex modes can be excited depending on the relative values of the orbital angular velocity˝o rbit and of the spinning angular velocity˝s pin , except in a stable range characterized by˝s pin /˝o rbit ∈ [−1;1/3]. Even if the tidal deformation is small, its subsequent instability-coming from a resonance process-may induce motions with large amplitude, which play a fundamental role at the planetary scale. This general conclusion is illustrated in the case of Jupiter's moon Io by a coupled model of synchronization, demonstrating the importance of energy dissipation by elliptical instability.

Nous avons etudie le spin-up par effet de maree dans les etoiles doubles serrees telles les varia... more Nous avons etudie le spin-up par effet de maree dans les etoiles doubles serrees telles les variables cataclysmiques, les sources X de faible masse et les couples ultra-serres de naines blanches, du a la perte de moment c1netique de ces systemes. Pour resoudre les problemes hydrodynamiques lies a cet effet, nous avons developpe une methode originale basee sur des developpements en harmoniques spheriques. Cette methode nous a permis de donner une solution analytique a toute une classe de problemes de la mecanique des fluides en rotation. Les resultats des applications aux cas des binaires cataclysmiques et des sources X de faible masse montrent que la dissipation visqueuse au sein du compagnon atteint 10-3 fois sa luminosite pendant l'evolution du systeme. Cependant, nous montrons aussi que les incertitudes sur certains parametres (telle la viscosite turbulente dans l'enveloppe convective) sont telles qu'il n'est pas encore exclu que cette source de chaleur soit tout ...

Astronomy & Astrophysics

Context. Altair is the fastest rotating star at less than 10 parsecs from the Sun. Its precise mo... more Context. Altair is the fastest rotating star at less than 10 parsecs from the Sun. Its precise modelling is a landmark for our understanding of stellar evolution with fast rotation, and all observational constraints are most welcome to better determine the fundamental parameters of this star. Aims. We wish to improve the seismic spectrum of Altair and confirm the δ-Scuti nature of this star. Methods. We used the photometric data collected by the Microvariability and Oscillations of STars (MOST) satellite in the form of a series of Fabry images to derive Altair light curves at four epochs, namely in 2007, 2011, 2012, and 2013. Results. We first confirm the presence of δ-Scuti oscillations in the light curves of Altair. We extend the precision of some eigenfrequencies and add new ones to the spectrum of Altair, which now has 15 detected eigenmodes. The rotation period, which is expected at ∼7h46min from models reproducing interferometric data, seems to appear in the 2012 data set, but...

EPJ Web of Conferences

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

EPJ Web of Conferences

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

The European Physical Journal C

We consider a binary system composed of a pulsar and a massive, fast rotating, highly distorted m... more We consider a binary system composed of a pulsar and a massive, fast rotating, highly distorted main sequence star of mass M, spin angular momentum S, dimensionless mass quadrupole moment J 2 , equatorial and polar radii R e , R p , flattening ν .

International Astronomical Union Colloquium

We present the current understanding of the dynamical aspects of the oscillations of rapidly rota... more We present the current understanding of the dynamical aspects of the oscillations of rapidly rotating stars.

Symposium - International Astronomical Union

In this review, we describe the physical processes driving the dynamical evolution of binary star... more In this review, we describe the physical processes driving the dynamical evolution of binary stars, namely the circularization of the orbit and the synchronization of their spin and orbital rotation. We also discuss the possible role of the elliptic instability which turns out to be an unavoidable ingredient of the evolution of binary stars.

Symposium - International Astronomical Union

Oscillation modes of rapidly rotating stars have not yet been calculated with precision, rotation... more Oscillation modes of rapidly rotating stars have not yet been calculated with precision, rotational effects being generally approximated by perturbation methods. We developed a mathematical formalism and a numerical method which fully account for the deformation of the star by the centrifugal force. The method has been first tested in the case of Maclaurin spheroids and then applied to uniformly rotating polytropic stars.

The Sun’s supergranulation refers to a physical pattern covering the surface of the quiet Sun wit... more The Sun’s supergranulation refers to a physical pattern covering the surface of the quiet Sun with a typical horizontal scale of approximately 30 000 kilometres and a lifetime of around 1.8 days. Its most noticeable observable signature is as a fluctuating velocity field of 360 m/s rms whose components are mostly horizontal. Supergranulation was discovered more than fifty years ago, however explaining why and how it originates still represents one of the main challenges of modern solar physics. A lot of work has been devoted to the subject over the years, but observational constraints, conceptual difficulties and numerical limitations have all concurred to prevent a detailed un-derstanding of the supergranulation phenomenon so far. With the advent of 21st century supercomputing resources and the availability of unprecedented high-resolution observations of the Sun, a stage at which key progress can be made has now been reached. A unify-ing strategy between observations and modelling...

arXiv (Cornell University), Jun 30, 2023

Context. Intermediate-mass stars are often fast rotators, and hence are centrifugally flattened a... more Context. Intermediate-mass stars are often fast rotators, and hence are centrifugally flattened and notably affected by gravity darkening. To analyse this kind of stars properly, one must resort to 2D models to compute the visible radiative flux and to take the geometrical effect of the star inclination into account. Aims. Assuming a given stellar age and chemical composition, our aim is to derive the mass and rotation rates of main sequence fast rotating stars, along with their inclination, from photometric quantities influenced by gravity darkening. Methods. We chose three observables that vary with mass, rotation, and inclination: the temperature derived by the infrared flux method T IRFM , the Strömgren c 1 index, and a second index c 2 built in the same way as the c 1 index, but sensitive to the UV side of the Balmer jump. These observables are computed from synthetic spectra produced with the PHOENIX code and rely on a 2D stellar structure from the ESTER code. These quantities are computed for a grid of models in the range 2 to 7 M ⊙ , and rotation rates from 30% to 80% of the critical rate. Then, for any triplet (T IRFM , c 1 , c 2), we try to retrieve the mass, rotation rate, and inclination using a Levenberg-Marquardt scheme, after a selection step to find the most suitable starting models. Results. Hare-and-hound tests showed that our algorithm can recover the mass, rotation rate, and inclination with a good accuracy. The difference between input and retrieved parameters is negligible for models lying on the grid and is less than a few percent otherwise. An application to the real case of Vega showed that the u filter is located in a spectral region where the modelled and observed spectra are discrepant, and led us to define a new filter. Using this new filter and subsequent index, the Vega parameters are also retrieved with satisfactory accuracy. Conclusions. This work opens the possibility to determine the fundamental parameters of rapidly rotating early-type stars from photometric space observations.

arXiv: Solar and Stellar Astrophysics, 2016

The internal rotation of low mass stars all along their evolution is of primary interest when stu... more The internal rotation of low mass stars all along their evolution is of primary interest when studying their rotational dynamics, internal mixing and magnetic fields generation. In this context, helio- and asteroseismology probe angular velocity gradients deep within solar type stars. Still the rotation of the close center of such stars on the main sequence is hardly detectable and the dynamical interactions of the radiative core with the surface convective envelope is not well understood. Among them, the influence of the differential rotation profile sustained by convection and applied as a boundary condition to the radiation zone may be very important leading to the formation of tachoclines. In the solar convective region, the equator is rotating faster than the pole while numerical simulations predict either a solar or an anti-solar rotation in other low mass stars envelopes depending on their convective Rossby number. In this work, we therefore build for the first time 2D steady...

arXiv: Solar and Stellar Astrophysics, 2017

Early-type stars generally tend to be fast rotators. In these stars, mode identification is very ... more Early-type stars generally tend to be fast rotators. In these stars, mode identification is very challenging as the effects of rotation are not well known. We consider here the example of alpha\alphaalpha Ophiuchi, for which dozens of oscillation frequencies have been measured. We model the star using the two-dimensional structure code ESTER, and we compute both adiabatic and non-adiabatic oscillations using the TOP code. Both calculations yield very complex spectra, and we used various diagnostic tools to try and identify the observed pulsations. While we have not reached a satisfactory mode-to-mode identification, this paper presents promising early results.

Journal of Fluid Mechanics, 2020

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

EPJ Web of Conferences, 2017

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

Astronomy & Astrophysics, 2018

Context. Gravity darkening (GD) and flattening are important consequences of stellar rotation. Th... more Context. Gravity darkening (GD) and flattening are important consequences of stellar rotation. The precise characterization of these effects across the Hertzsprung–Russell (H-R) diagram is crucial to a deeper understanding of stellar structure and evolution. Aims. We seek to characterize such important effects on Sargas (θ Scorpii), an evolved, fast-rotating, intermediate-mass (∼5 M⊙) star, located in a region of the H-R diagram where they have never been directly measured as far as we know. Methods. We use our numerical model CHARRON to analyze interferometric (VLTI/PIONIER) and spectroscopic (VLT/UVES) observations through a MCMC model-fitting procedure. The visibilities and closure phases from the PIONIER data are particularly sensitive to rotational flattening and GD. Adopting the Roche approximation, we investigate two GD models: (1) the β-model (Teff ∝ geff β), which includes the classical von Zeipel’s GD law, and (2) the ω-model, where the flux is assumed to be anti-parallel ...

Journal of Fluid Mechanics, 2018

We investigate the asymptotic properties of axisymmetric inertial modes propagating in a spherica... more We investigate the asymptotic properties of axisymmetric inertial modes propagating in a spherical shell when viscosity tends to zero. We identify three kinds of eigenmodes whose eigenvalues follow very different laws as the Ekman number$E$becomes very small. First are modes associated with attractors of characteristics that are made of thin shear layers closely following the periodic orbit traced by the characteristic attractor. Second are modes made of shear layers that connect the critical latitude singularities of the two hemispheres of the inner boundary of the spherical shell. Third are quasi-regular modes associated with the frequency of neutral periodic orbits of characteristics. We thoroughly analyse a subset of attractor modes for which numerical solutions point to an asymptotic law governing the eigenvalues. We show that three length scales proportional to$E^{1/6}$,$E^{1/4}$and$E^{1/3}$control the shape of the shear layers that are associated with these modes. These scale...

The ESTER project aims at building a stellar evolution code in two dimensions of space for the st... more The ESTER project aims at building a stellar evolution code in two dimensions of space for the study of effects of rotation. The numerical scheme is based on spectral methods with a spherical harmonic decomposition in the horizontal direction and a Chebyshev polynomial expansion in the vertical direction. Coordinates adapted to the centrifugally distorted shape are mapped to spherical coordinates. First tests on rotating polytropes are presented.

Physics of the Earth and Planetary Interiors, 2010

In this paper, we combine theoretical and experimental approaches to study the tidal instability ... more In this paper, we combine theoretical and experimental approaches to study the tidal instability in planetary liquid cores and stars. We demonstrate that numerous complex modes can be excited depending on the relative values of the orbital angular velocity˝o rbit and of the spinning angular velocity˝s pin , except in a stable range characterized by˝s pin /˝o rbit ∈ [−1;1/3]. Even if the tidal deformation is small, its subsequent instability-coming from a resonance process-may induce motions with large amplitude, which play a fundamental role at the planetary scale. This general conclusion is illustrated in the case of Jupiter's moon Io by a coupled model of synchronization, demonstrating the importance of energy dissipation by elliptical instability.

Nous avons etudie le spin-up par effet de maree dans les etoiles doubles serrees telles les varia... more Nous avons etudie le spin-up par effet de maree dans les etoiles doubles serrees telles les variables cataclysmiques, les sources X de faible masse et les couples ultra-serres de naines blanches, du a la perte de moment c1netique de ces systemes. Pour resoudre les problemes hydrodynamiques lies a cet effet, nous avons developpe une methode originale basee sur des developpements en harmoniques spheriques. Cette methode nous a permis de donner une solution analytique a toute une classe de problemes de la mecanique des fluides en rotation. Les resultats des applications aux cas des binaires cataclysmiques et des sources X de faible masse montrent que la dissipation visqueuse au sein du compagnon atteint 10-3 fois sa luminosite pendant l'evolution du systeme. Cependant, nous montrons aussi que les incertitudes sur certains parametres (telle la viscosite turbulente dans l'enveloppe convective) sont telles qu'il n'est pas encore exclu que cette source de chaleur soit tout ...

Astronomy & Astrophysics

Context. Altair is the fastest rotating star at less than 10 parsecs from the Sun. Its precise mo... more Context. Altair is the fastest rotating star at less than 10 parsecs from the Sun. Its precise modelling is a landmark for our understanding of stellar evolution with fast rotation, and all observational constraints are most welcome to better determine the fundamental parameters of this star. Aims. We wish to improve the seismic spectrum of Altair and confirm the δ-Scuti nature of this star. Methods. We used the photometric data collected by the Microvariability and Oscillations of STars (MOST) satellite in the form of a series of Fabry images to derive Altair light curves at four epochs, namely in 2007, 2011, 2012, and 2013. Results. We first confirm the presence of δ-Scuti oscillations in the light curves of Altair. We extend the precision of some eigenfrequencies and add new ones to the spectrum of Altair, which now has 15 detected eigenmodes. The rotation period, which is expected at ∼7h46min from models reproducing interferometric data, seems to appear in the 2012 data set, but...

EPJ Web of Conferences

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

EPJ Web of Conferences

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

The European Physical Journal C

We consider a binary system composed of a pulsar and a massive, fast rotating, highly distorted m... more We consider a binary system composed of a pulsar and a massive, fast rotating, highly distorted main sequence star of mass M, spin angular momentum S, dimensionless mass quadrupole moment J 2 , equatorial and polar radii R e , R p , flattening ν .

International Astronomical Union Colloquium

We present the current understanding of the dynamical aspects of the oscillations of rapidly rota... more We present the current understanding of the dynamical aspects of the oscillations of rapidly rotating stars.

Symposium - International Astronomical Union

In this review, we describe the physical processes driving the dynamical evolution of binary star... more In this review, we describe the physical processes driving the dynamical evolution of binary stars, namely the circularization of the orbit and the synchronization of their spin and orbital rotation. We also discuss the possible role of the elliptic instability which turns out to be an unavoidable ingredient of the evolution of binary stars.

Symposium - International Astronomical Union

Oscillation modes of rapidly rotating stars have not yet been calculated with precision, rotation... more Oscillation modes of rapidly rotating stars have not yet been calculated with precision, rotational effects being generally approximated by perturbation methods. We developed a mathematical formalism and a numerical method which fully account for the deformation of the star by the centrifugal force. The method has been first tested in the case of Maclaurin spheroids and then applied to uniformly rotating polytropic stars.