Differential rotation decay in the radiative envelopes of CP stars

Abstract

Stars of spectral classes A and late B are almost entirely radiative. CP stars are a slowly rotating subgroup of these stars. It is possible that they possessed long-lived accretion disks in their T Tauri phase. Magnetic coupling of disk and star leads to rotational braking at the surface of the star. Microscopic viscosities are extremely small and will not be able to reduce the rotation rate of the core of the star. We investigate the question of whether the magneto-rotational instability can provide turbulent angular momentum transport. We discuss whether differential rotation is present in CP stars. Numerical MHD simulations of thick stellar shells are performed. An initial differential rotation law is subject to the influence of a magnetic field. This configuration does indeed give rise to a magneto-rotational instability. The emerging flows and magnetic fields efficiently transport angular momentum outwards. Weak dependence on the magnetic Prandtl number (∼10-2 in stars) is found from the simulations. First tests with stratified shells show different flows, but the same efficiency of angular-momentum transport. Since the estimated time-scale of decay of differential rotation is 107-108 yr, and comparable to the life-time of A stars, we find the braking of the core to be an ongoing process in many CP stars. The evolution of the surface rotation of CP stars with age will be an observational challenge, and of considerable value in verifying these simulations.