The study of the global oscillations of stars, which is at the heart of asteroseismology, has enabled us to make significant progress in our understanding of stellar physics. In Sun-like stars, these oscillations are affected by the turbulent motions entailed by the convective instability in the envelope. In addition to modifying the resonant frequencies of the modes, turbulent convection is also responsible for their excitation, as well as part of their damping. As such, solar-like oscillations gives us access to crucial information about stellar convection, which constitutes one of the main obstacles towards better stellar modelling. However, in order to exploit the wealth of asteroseismic data at our disposal to better constrain the properties of stellar convection, it is necessary to theoretically model the relation between these properties and the asteroseismic observables (namely the amplitude of the modes, their lifetime, and the amount by which convection shifts their frequencies, referred to as surface effects).
In this talk, I will first focus on solar-like acoustic modes. Traditional approaches to study the effect of convection thereon are either based on parametric empirical formulations, or else on 3D simulations. These approaches show unavoidable limitations, among which the impossibility to realistically describe the full turbulent cascade, and especially the turbulent dissipation of kinetic energy. Here, I will present a new alternative theoretical framework designed to circumvent these limitations, based on Lagrangian stochastic models. I will demonstrate how, under reasonable assumptions, this sort of formalism can lead to simultaneous theoretical estimates for the amplitude, lifetime and surface effect of the acoustic modes, directly as a function of the turbulent fluctuations caused by convection, thus allowing to constrain turbulent convection models through the direct comparison of these estimates with observed mode properties.
The second part of this talk will focus on another kind of oscillations, namely the inertial modes recently observed on the surface of the Sun. These modes propagate under the action of rotation, through the Coriolis acceleration, and, for the most part, are predicted to be stable, meaning that they are likely also excited by turbulent convection, just like acoustic modes. In order to test that hypothesis, I will present a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes, which are described by means of a 2D linear wave equation, under the β-plane approximation. Based on the general agreement between the predicted and observed inertial mode amplitudes, I will show that the (linearly stable) solar inertial modes are indeed excited by turbulent convection. This formalism also shows that the power in high azimuthal order spectra is not easily separable into individual modes, thus complicating the interpretation of the observations.
Local contact: Stéphane MATHIS
Organization: Frédéric GALLIANO
