Why is it difficult to form rotationally supported disks during the protostellar collapse of magnetized dense cores? Zhi-Yun Li University of Virginia, Charlottesville, USA
The formation of rotationally supported disks (RSDs) is a crucial event during the early epoch of star formation. It is generally expected that disks would form automatically out of the collapse of rotating cores because of angular momentum conservation. In the presence of the observed level of magnetic fields, this outcome is no longer guaranteed, because of magnetic braking. Indeed, in the simplest case of ideal MHD limit, both analytic work and numerical simulations showed that the RSD formation is completely suppressed by excessive magnetic braking, leading to the so-called ``magnetic braking catastrophe.'' We have shown recently that, at least in two dimensions (assuming axisymmetry), non-ideal MHD effects (including the Ohmic dissipation, ambipolar diffusion and Hall effect, computed using a simplified chemical network and taking into account of dust grains) do not weaken the magnetic braking enough to enable RSDs to form under typical cloud conditions. Nevertheless, RSDs are observed around at least more evolved young stellar objects and have to form sooner or later. I will discuss possible resolutions to this conundrum, including non-axisymmetic magnetic interchange instabilities in 3D (for which we have preliminary results), enhanced magnetic diffusivities (perhaps due to turbulence), and outflow stripping of the slowly rotating protostellar envelope (which acts as a brake on the rapidly rotating disk), and comment on the apparently discrepant results in the literature on this important topic for early epoch of star formation.
Collaborators: Ruben Krasnopolsky Sienny Shang, ASIAA, Taiwan	Key publication