On the interactions of turbulent convection and rotation in RGB stars

We have performed the first three-dimensional non-linear simulation of the turbulent convective envelope of a rotating 0.8 Msun RGB star using the ASH code. Adopting a global typical rotation rate of a tenth of the solar rate, we have analyzed the dynamical properties of the convection and the transport of angular momentum within the inner 50% in radius of the convective envelope. The convective patterns consist of a small number of large cell, associated with fast flows (about 3000 m/s) and large temperature fluctuations (about 300 K) in order to carry outward the large luminosity (L* = 400 Lsun) of the star. The interactions between convection and rotation give rise to a large radial differential rotation and a meridional circulation possessing one cell per hemisphere, the flow being poleward in both hemisphere. By analysing the redistribution of angular momentum, we find that the meridional circulation transports the angular momentum outward in the radial direction, and poleward in the latitudinal direction, and that the transport by Reynolds stresses acts in the opposite direction. From this 3-D simulation, we have derived an average radial rotation profile, that we will ultimately introduce back into 1-D stellar evolution code.