| Abstract |
Angular momentum transport by magnetic fields is a dominant physical process in star and disk formation processes. Dissipation of magnetic fields such as ambipolar diffusion and Ohmic dissipation during protostellar collapse is of critical importance because the ionization degree in star forming clouds is extremely low. These non-ideal magnetohydrodynamic effects extract magnetic flux and suppress angular momentum transport. In particular, the so-called magnetic braking catastrophe can be resolved with these effects. In this work, we perform three-dimensional radiation magnetohydrodynamic simulations including Ohmic dissipation and ambipolar diffusion. Without these effects, magnetic fields redistribute angular momentum so efficiently that no rotationally-supported circumstellar disk is formed even after formation of a second core. Ohmic dissipation works in a relatively high density region within a first core and suppresses angular momentum transport, and it enables formaiton of a small circumstellar disk around a second core. With both Ohmic dissipation and ambipolar diffusion, almost the entire first core becomes resistive and significant magnetic flux loss occurs. As a result, a circumstellar disk supported by rotation is formed even before the second collapse begins. Although the size of the disk remains still small, about 5 AU at the end of the first core phase, this disk will grow as gas accretion continues. |
