A Multi-Ringed, Modestly-Inclined Protoplanetary Disk around AA Tau

Abstract

AA Tau is the archetype for a class of stars with a peculiar periodic photometric variability thought to be related to a warped inner disk structure with a nearly edge-on viewing geometry. We present high resolution ($\sim$0.2") ALMA observations of the 0.87 and 1.3~mm dust continuum emission from the disk around AA Tau. These data reveal an evenly spaced three-ringed emission structure, with distinct peaks at 0.34", 0.66", and 0.99", all viewed at a modest inclination of 59.1$^{\circ}\pm$0.3$^{\circ}$ (decidedly not edge-on). In addition to this ringed substructure, we find non-axisymmetric features including a `bridge' of emission that connects opposite sides of the innermost ring. We speculate on the nature of this `bridge' in light of accompanying observations of HCO$^+$ and $^{13}$CO (J=3--2) line emission. The HCO$^+$ emission is bright interior to the innermost dust ring, with a projected velocity field that appears rotated with respect to the resolved disk geometry, indicating the presence of a warp or inward radial flow. We suggest that the continuum bridge and HCO$^+$ line kinematics could originate from gap-crossing accretion streams, which may be responsible for the long-duration dimming of optical light from AA Tau.