Suzanne Talon

Influence of Gravity Waves on the Internal Rotation and Li Abundance of Solar-Type Stars

The Suns rotation profile and lithium content have been difficult to understand in the context of conventional models of stellar evolution. Classical hydrodynamic models predict that the solar interior must rotate highly differentially, in disagreement with observations. It has recently been shown that internal waves produced by convection in solar-type stars produce an asymmetric, shear layer oscillation, similar to Earths quasi-biennial oscillation, that leads to efficient angular momentum redistribution from the core to the envelope. We present results of a model that successfully reproduces both the rotation profile and the surface abundance of lithium in solar-type stars of various ages.

Hydrodynamical stellar models including rotation, internal gravity waves, and atomic diffusion - I. Formalism and tests on Pop I dwarfs

In this paper, we develop a formalism in order to incorporate the contribution of internal gravity waves to the transport of angular momentum and chemicals over long time-scales in stars. We show that the development of a double peaked shear layer acts as a filter for waves, and how the asymmetry of this filter produces momentum extraction from the core when it is rotating faster than the surface. Using only this filtered flux, it is possible to follow the contribution of internal waves over long (evolutionary) time-scales. We then present the evolution of the internal rotation profile using this formalism for stars which are spun down via magnetic torquing. We show that waves tend to slow down the core, creating a “slow” front that may then propagate from the core to the surface. Further spin down of the surface leads to the formation of a new front. Finally we show how this momentum transport reduces rotational mixing in a 1.2 M⊙, Z = 0.02 model, leading to a surface lithium abundance in agreement with observations in the Hyades.In this paper, we develop a formalism, in order to incorporate the contribution of internal gravity waves to the transport of angular momentum and chemicals over long time-scales in stars. We show that the development of a double-peaked shear layer acts as a filter for waves, and also how the asymmetry of this filter produces momentum extraction from the core when it is rotating faster than the surface. Using only this filtered flux, it is possible to follow the contribution of internal waves over long (evolutionary) time-scales. 
We then present the evolution of the internal rotation profile using this formalism for stars which are spun down via magnetic torquing. We show that waves tend to slow down the core, creating a “slow” front that may then propagate from the core to the surface. Further spin down of the surface leads to the formation of a new front. Finally we show how this momentum transport reduces rotational mixing in a

Rotational mixing in low-mass stars - I Effect of the

1.2~M_\odot

{\mu}

,

-gradients in main sequence and subgiant Pop I stars

Z=0.02

Angular Momentum Extraction by Gravity Waves in the Sun

model, leading to a surface lithium abundance in agreement with observations in the Hyades.

Angular Momentum Redistribution by Waves in the Sun

We present a first set of results concerning stellar evolution of rotating low-mass stars. Our models include fully consistent transport of angular momentum and chemicals due to the combined action of rotation induced mixing (according to Maeder & Zahn 1998) and element segregation. The analysis of the eects of local variations of molecular weight due to the meridional circulation on the transport of angular momentum and chemicals are under the scope of this study. We apply this mechanism to low mass main sequence and subgiant stars of population I. We show that the so-called-currents are of major importance in setting the shape of the rotation profile, specially near the core. Furthermore, as shown by Talon & Charbonnel (1998) and Charbonnel & Talon (1999) using models without-currents, we confirm that rotation-induced mixing in stars braked via magnetic torquing can explain the blue side of the Li dip, as well as the low Li abundances observed in subgiants even when-currents are taken into account. We emphasize that variations are not to be neglected when treating rotation-induced mixing, and that they could be of great importance for latter evolutionary stages.

Seismic diagnostics for transport of angular momentum in stars. I. Rotational splittings from the pre-main sequence to the red-giant branch.

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.

Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars

We calculate the angular momentum transport by gravito-inertial-Alfven 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% R☉) 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.

Angular momentum transport by internal gravity waves I - Pop I main sequence stars

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 Ms 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.