Dimming and CO absorption toward the AA Tau protoplanetary disk: An infalling flow caused by disk instability?

TL;DR

This study presents long-term spectroscopic observations of AA Tau, revealing increased CO absorption during a dimming event, suggesting that magnetic buoyancy instability lifts circumstellar material inward, potentially leading to future accretion outbursts.

Contribution

It provides the first detailed spectroscopic evidence linking disk instability and infalling gas to observed dimming in AA Tau.

Findings

Enhanced CO absorption features appeared after dimming.

The molecular gas has a temperature of around 500 K.

The gas shows a consistent redshift of 6 km/s.

Abstract

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 4.7um CO rovibrational spectra of AA Tau over eight epochs, covering an eleven-year time span, that reveal enhanced 12CO and 13CO absorption features in the $J_{\rm low}\leqslant$13 transitions after the dimming. These newly appeared absorptions require molecular gas along the line of sight with T~500 K and a column density of log (N12CO)~18.5 cm^{-2}, with line centers that show a constant 6 km s$^{-1}$ 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.