Thermohaline mixing and fossil magnetic fields in red giant stars

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 descendant

Angular momentum transport in the sun through meridian circulation

We investigated the transport of angularmomentum in the radiative interior of the Sun through the mecanism describ by ZAhn (1992), namely the meridian circulation driven by the solar wind and the turbulance caused by the shear instabilities due to the differential rotation

Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants

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 et al. (2006) 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. In the present work 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. We compute stellar models of various initial metallicities using the prescription by Ulrich (1972) (extended to the case of a non-perfect gas) for the turbulent diffusivity produced by the thermohaline instability in stellar radiation zones. Thermohaline mixing simultaneously accounts for the observed behaviour of the carbon isotopic ratio and of the abundances of Li, C and N on the upper part of the red giant branch. It reduces significantly the 3He production with respect to canonical evolution models as required by measurements of 3He/H in galactic HII regions. Thermohaline mixing is a fundamental physical process that must be included in stellar evolution modeling.Comment: Letter accepted for publication in A&A; for full resolution figures send request to [email protected]

Angular momentum transport by internal waves in the solar interior

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 (1981) and Garcia Lopez & Spruit (1991). 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 & Nicholson 1989). 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.Comment: 9 pages latex file using l-aa.sty, 2 postscript figures, accepted by A&

The equilibrium tide in viscoelastic parts of planets

International audienceEarth-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

Angular momentum extraction by gravity waves in the Sun

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, which are stochastically excited at the base of the convection zone of late type stars, can extract angular momentum from their radiative interior. The time-scale for this momentum extraction in a Sun-like star is of the order of 10^7 years. The process is particularly efficient in the central region, and it could produce there a slowly rotating core.Comment: 9 pages, 3 figues, accepted by Astrophysical Journal Letter, 26 June 200