The Surprising Physics of Boundary Layer Accretion
The boundary layer is where the disk meets the central object. I present simulations and theory of thin disk accretion onto a central object through a boundary layer. Such a scenario occurs if the central object has a material inner boundary (i.e. is not a black hole), and the magnetic field is weak enough that it does not disrupt the disk and channel matter onto the magnetic poles. Examples of such systems include cataclysmic variables in outburst, FU Orionis stars, and weakly magnetized neutron stars accreting at a high rate.
For a Keplerian disk in steady state, half of the energy is dissipated in the boundary layer. However, the boundary layer can be "missing" in observations, suggesting the dissipation rate is lower than expected. My talk will focus on angular momentum transport in the boundary layer. In particular, I show that angular momentum is transported via waves excited by shear instabilities in the boundary layer. Because waves are nonlocal, they cannot be parametrized by an alpha viscosity prescription. Angular momentum transport by waves can reduce the spinup rate of the star compared to the expected viscous steady state value. It also potentially provides a solution to the missing boundary layer problem.