Theory of Protostellar Accretion

This paper reviews current theoretical work on the various stages of accretion in protostars, and the relationship of these ideal stages to the spectral classes of observed protostellar systems. I discuss scaling relationships that have been obtained for models of pre-stellar cores as they evolve by ambipolar diffusion toward a central singularity, and expectations for the dynamical evolution as the cores collapse radially to form a rotating disk. I summarize work that suggests accretion in T Tauri systems may be limited by ionization to disk surface layers, and implications for the variation of the disk accretion rate with radius. Finally, I describe models for the asymptotic structure of free magnetocentrifugal winds, and show that the constant-density surfaces in these winds may be strongly collimated even when the streamlines are not.