Observations of accretion and angular momentum regulation in young circumstellar disks and the implications for planetary formation

Accretion disks around young stars produce excess infrared continuum associated with the disk and excess optical and ultraviolet continua associated with the boundary layer or 'hot spot' as material falls from the disk onto the stellar photosphere. When we subtract the excess continuum and photospheric contributions to the total spectrum, we can obtain high-quality emission line profiles of the Balmer lines as well as permitted lines from other elements. These emission lines often exhibit redshifted absorption, indicative of infalling material. Remarkably, objects with large accretion rates tend to rotate slower than their counterparts that lack accretion disks. Hence, there must be some process, probably involving magnetic fields, that allows the star to accrete large amounts of material from the disk without increasing its rotational velocity. Young stars typically do not have optically thick inner disks that do not accrete. Hence, either planets form within accretion disks, or the timescale for planetary formation is considerably shorter than approx. 3 x 10(exp 6) yr, the duration of the classical T Tauri star phase of young stellar evolution.