Abstract
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 (∼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 ± 0.°3 (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 13CO (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.
Highlights
Protoplanetary disks are expected to undergo dramatic morphological changes concurrent with the processes of planet formation
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
As seen in the figure, AA Tau hosts a system of nested dust rings with an apparent inclination of ∼59◦, which we confirm through visibility modeling
Summary
Protoplanetary disks are expected to undergo dramatic morphological changes concurrent with the processes of planet formation. Multiple dust rings have been imaged in the disks around HL Tau, TW Hya, HD 163296, and HD 169142 (ALMA Partnership et al 2015; Andrews et al 2016; Isella et al 2016; Fedele et al 2017), and visibility modeling has suggested one other candidate system, DM Tau (Zhang et al 2016) These rings, and the gaps between them, may trace planet formation at its earliest stages (e.g. Flock et al 2015; Ruge et al 2016), other explanations have been proposed A possible interpretation of these optical light variations is an extended non-axisymmetric feature (such as a disk warp or protoplanet) passing in front of the star at a distance of >8 AU, assuming a distance of 145 pc to AA Tau (Bouvier et al 2013; Rodriguez et al 2015) This naturally suggests an investigation of the interplay between fine-scale structures in the outer disk and the inner disk morphology.
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