Dimming and Co Absorption toward the Aa Tau Protoplanetary Disk: an Infalling Flow Caused by Disk Instability?

AA Tau, a classical T Tauri star in the Taurus cloud, has been the subject of intensive photometric monitoring for more than two decades due to its quasi-cyclic variation in optical brightness. Beginning in 2011, AA Tau showed another peculiar variation—its median optical though near-IR flux dimmed significantly, a drop consistent with a 4-mag increase in visual extinction. It has stayed in the faint state since. Here we present 4.7 μm CO rovibrational spectra of AA Tau over eight epochs, covering an 11 yr time span, that reveal enhanced CO and CO absorption features in the Jlow ⩽ 13 transitions after the dimming. These newly appeared absorptions require molecular gas along the line of sight with T∼ 500 K and a column density of log (N CO 12 ) ∼18.5 cm, with line centers that show a constant 6 km s redshift. The properties of the molecular gas confirm an origin in the circumstellar material. We suggest that the dimming and absorption are caused by gas and dust lifted to large heights by a magnetic buoyancy instability. This material is now propagating inward, and on reaching the star within a few years will be observed as an accretion outburst.