Dynamics of Protoplanetary Disks

Protoplanetary disks are quasi-steady structures whose evolution and dispersal determine the environment for planet formation. I review the theory of protoplanetary disk evolution and its connection to observations. Substantial progress has been made in elucidating the physics of potential angular momentum transport processes-including self-gravity, magnetorotational instability, baroclinic instabilities, and magnetic braking-and in developing testable models for disk dispersal via photoevaporation. The relative importance of these processes depends upon the initial mass, size, and magnetization of the disk, and subsequently on its opacity, ionization state, and external irradiation. Disk dynamics is therefore coupled to star formation, pre-main-sequence stellar evolution, and dust coagulation during the early stages of planet formation and may vary dramatically from star to star. The importance of validating theoretical models is emphasized, with the key observations being those that probe disk structure on the scales between 1 AU and 10 AU, where theory is most uncertain.