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 ^(12)CO and ^(13)CO absorption features in the J_low ≤ 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) ~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.
© 2015 The American Astronomical Society. Received 2015 January 9; accepted 2015 March 18; published 2015 May 20. We thank Jerome Bouvier and Konstantin Grankin for sharing their AA Tau photometric data, and the anonymous referee for helpful comments. K.Z., N.C. and G.A.B. gratefully acknowledge support from the NSF AAG and NASA Origins of Solar Systems programs. C.S. acknowledges the financial support of the NOAO Leo Goldberg Fellowship program. N.J.T.'s contributions were made at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA and with support from Origins of Solar Systems Program grant 13-OSS13-0114. J.M.C. acknowledges support from NSF award AST-1140063. The spectra presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The VLT data presented were acquired under program ID 179.C-0151. Finally, the authors wish to acknowledge the significant cultural role of the summit of Mauna Kea.
Published - 0004-637X_805_1_55.pdf
Submitted - 1503.06359v1.pdf