The Dynamics of Infall and Accretion Shocks in the Outer Disk

High-spatial-resolution observations of disks around young stars suggest planetary systems begin forming early, during the protostellar phase (≤1 Myr) when stars accrete most of their mass. We investigate this era using a theoretical and modeling framework we call the shock twist-angle Keplerian (STAK) disk. Gas infalling from the protostellar envelope enters the disk on passing through a shock, where angular momentum is conserved but energy is dissipated. While the pre-shock gas follows free-fall parabolic trajectories, the post-shock gas is on lower-energy, elliptical orbits. We construct synthetic observations and find that the deviations from circular Keplerian orbits are detectable in Doppler-shifted molecular spectral lines using interferometers such as ALMA. The STAK disk departs in a distinctive way from circular Keplerian models: the intensity and velocity-moment maps are asymmetric, having an inner twist with respect to the disk structure traced by the dust continuum. We examine archival ALMA data for the class 0/I protostar L1527 and find the C18O velocity moment map has features resembling the STAK model. We encourage observers to search for the twist-angle kinematic signature of protostars' envelope-disk shocks, using spectral line observations with sub-km-s^−1 spectral resolution and sufficient angular resolution to fully resolve the disk.