The powerful outflows from pre-main-sequence stars are now understood as a general byproduct of disk accretion. The relation between mass accretion rates and mass loss rates now spans several orders of magnitude by connecting the T Tauri stars with the FU Ori objects. The bipolar nature of these outflows, which begin in the earliest stages of star formation, clearly points to a disk origin. With mass ejection rates of order 10% of the disk accretion rates, outflows represent perhaps as much as half of the energy released by disk accretion.

Neither thermal nor radiation pressures are able to drive the observed rates of mass loss. The inescapable conclusion is that these jets and winds are produced by magnetic acceleration; models show that magnetic fields rotating with the disk naturally produce the necessary collimation along the rotation axis. The precise manner in which this acceleration and collimation takes place is uncertain because the magnetic field structure in the inner disk is not known.

Magnetic fields also play an important role in accretion onto pre-main-sequence stars. The magnetic fields of T Tauri stars are apparently strong enough to hold off disks from the stellar surface; the accreting gas deviates from the disk plane as it falls in along the stellar magnetic field lines, eventually shocking at the stellar surface and producing the observed hot continuum radiation (Figure 8.1).