The Role of Magnetic Fields in the Protostellar Accretion Phase

We summarize recent work addressing the role of magnetic fields in the process of star formation and disk accretion. After a short review of the basic observational results, we concentrate on the efficiency of magnetic braking during cloud collapse and its consequences on the formation of centrifugally supported disks around young stars. Then, we relate this issue to the well-known magnetic flux problem of star formation, and we show that the introduction of non-ideal MHD effects is a necessary step toward the development of self-consistent models for the collapse of molecular clouds and the formation of disks. Finally, we discuss the structure and evolution of magnetized accretion disks around young stars that have dragged their magnetic field in the phase of gravitational collapse and we focus on two main diffusive processes: viscous stresses that redistribute mass and angular momentum, and the resistive diffusion of mass across magnetic field lines.