Dynamics of the envelope of a rapidly rotating star or giant planet in gravitational contraction

Abstract

We wish to understand the processes that control the fluid flows of a gravitationally contracting and rotating star or giant planet. We consider a spherical shell containing an incompressible fluid that is slowly absorbed by the core so as to mimick gravitational contraction. We also consider the effects of a stable stratification that may also modify the dynamics of a pre-main sequence star of intermediate mass. This simple model reveals the importance of both the Stewartson layer attached to the core and the boundary conditions met by the fluid at the surface of the object. In the case of a pre-main sequence star of intermediate mass where the envelope is stably stratified, shortly after the birth line, the spin-up flow driven by contraction overwhelms the baroclinic flow that would take place otherwise.This model also shows that for a contracting envelope, a self-similar flow of growing amplitude controls the dynamics. It suggests that initial conditions on the birth line are most probably forgotten. Finally, the model shows that the near (Stewartson) layer that lies on the tangent cylinder of the core is likely a key feature of the dynamics that is missing in 1D models.This layer can explain the core and envelope rotational coupling that is required to explain the slow rotation of cores in giant and subgiants stars.