BEAMING

This project tackles the problem of the transport of angular momentum (AM) in stellar interiors, which is one of the most challenging issues faced today by stellar physics. Our poor understanding of the AM redistribution inside stars is a barrier to the modeling of stellar formation and evolution. The seismology of red giants has recently demonstrated that it can spectacularly contribute to making progress on this issue. The detection in the oscillation spectra of red giants of so-called mixed modes, which probe both the stellar core and the envelope, has made it possible to seismically measure their internal rotation. This has brought clear evidence that an additional mechanism of AM transport takes place in subgiants and red giants, the origin of which is unknown. In this project, we will thoroughly investigate whether this AM redistribution has a magnetic origin, which is one of the main scenarios that have been proposed. For this purpose, we will combine asteroseismology, spectropolarimetric observations, and state-of-the-art multidimensional MHD simulations. We will exploit the exquisite seismic data from the Kepler satellite to bring tight constraints on the time during the evolution and the timescale over which the episodes of AM redistribution occur. The cases of low-mass and intermediate-mass stars, which have qualitatively different evolution in the post-main-sequence, will both be addressed. By improving seismic inversion methods, we will also obtain constraints as localized as possible on the shape of the internal rotation profiles that result from this transport. These observations will yield key information to discriminate between the potential transport mechanisms. To test more particularly the hypothesis of a magnetically-induced transport of AM, we will seismically measure the rotation profiles of Kepler red giants for which an internal magnetic field can be detected and characterized (strength and topology). Using spectropolarimetric observations, we will identify among Kepler targets the descendants of so-called Ap stars, which harbor strong internal magnetic fields during the main sequence. We will also search for the seismic signature of internal magnetic fields in red giants, by attempting to measure the magnetic splitting of oscillation mode. Through numerical simulations, we will investigate the interaction between differential rotation and magnetic fields in red giants. We will first study the differential rotation produced by the core contraction and envelope expansion alone. By introducing magnetic fields, we will then determine the conditions under which different types of MHD instabilities could occur in the radiative interior of red giants and we will precisely estimate their efficiency at transporting AM. This will enable us to provide prescriptions for magnetically-induced AM transport with strong physical basis to be used in a new generation of 1D stellar evolution models. Direct comparisons will be performed between the predictions of these models and seismic inferences on the rotation of red giants. The Kepler targets for which we will have measured the topology and amplitude of the internal magnetic field will yield the most critical tests of the magnetic origin of AM transport in red giants.