Institute for Advanced Study Astrophysics Seminar

Magnetization plays an important role in modern theories of the star-formation process. Slippage of neutrals relative to ions tied to magnetic fields in molecular cloud cores leads to a gravomagneto catastrophe in which the central regions formally acquire an infinite density concentration in finite time. In the presence of rotation, inside-out collapse follows to form a star plus disk, but only if the resultant MHD is non-ideal at high densities. The trapping of a finite amount of magnetic flux in the system leads to a global form of magnetorotational instability in the disk that transfers mass inwards and angular momentum outwards. Predictions are made for the resulting distribution of magnetization that are compatible with empirical measurements of the field in protoplanetary disks, but the data set is sparse. The magnetic field automatically achieves a configuration favorable for forming a disk wind, except that the disk rotation is sub-Keplerian by an amount that makes thermal launch difficult. Lightly loaded, magnetocentrifugally driven, disk winds do not possess physical characteristics that look like observed jets in young stellar objects. Such jets are much better explained as the X-winds that result when the accretion disk interacts with a strongly magnetized central star. The theory of the resultant funnel flows onto the star is generalized to arbitrary superpositions of magnetic multipoles, with the concept of trapped flux at the X-point tested against observations.