The Physics of Disk Winds

The last decade of research on star formation has established two important facts, namely that most if not all young stellar objects (henceforth YSO’s) are associated with both outflows and disks. The obvious question is whether or not these two phenomena are physically related. The outflow data clearly show that radiation cannot be the driving agent for the outflows, so that hydromagnetic drives that employ rotation and strong magnetic fields are favoured. Since both the central YSO and surrounding accretion disk are natural reservoirs in this regard, the theoretical and observational debate has increasingly turned to discriminating between these two possibilities. If outflows are stellar, then interaction between the wind and a surrounding flaring disk may redirect it into bipolar outflow. If outflows are disk winds however, the connection between outflows and disks is more profound. In the latter case, the outflow is intrinsically bipolar and disk accretion rates and wind mass loss rates are fundamentally connected since the disk angular momentum is carried off by the wind. The mechanical energy of the outflow derives from the gravitational binding energy released by the accretion flow of material in the disk. Moreover, evolution of the disk, a widely held notion, would imply that outflows must also evolve with time.