Misaligned Rings in a Protoplanetary Disk

Independent research teams have used ALMA and the Very Large Telescope (VLT) to image a set of misaligned rings in a protoplanetary disk around the young, triple star system GW Orionis. This system includes an inner binary of 1 AU separation, and an outer star orbiting at 8 AU, with a different orbital plane. Young stars are surrounded by a circumstellar disk of gas and dust, within which planet formation can occur. Gravitational forces in multiple star systems can disrupt such a disk.

GW Orionis is a hierarchical triple system with a rare circumtriple disk. Bi et al present ALMA observations of 1.3 mm dust continuum and ¹²CO J = 2 − 1 molecular gas emission of the disk. For the first time, three dust rings are identified in the GW Ori disk at ~46, 188, and 338 AU, with estimated dust mass of 74, 168, and 245 Earth masses, respectively. The outermost ring is apparently the largest dust ring ever found in protoplanetary disks. The authors use visibility modeling of their dust continuum data to show that the disk has misaligned parts, and the innermost dust ring is eccentric. The disk misalignment is also suggested by the CO kinematics. These substructures are interpreted as evidence of ongoing dynamical interactions between the triple stars and the circumtriple disk.

Theoretical models predict that if a circumstellar disk is misaligned with the orbital plane of the stars, the disk should warp and break into precessing rings, a phenomenon known as disk tearing. Kraus et al. present ALMA and VLT observations of the triple-star system GW Orionis, and find evidence for such disk tearing. These authors imaged the GW Orionis system using submillimeter and near-infrared interferometry, which trace thermal dust emission, and using visible and near-infrared adaptive-optics imaging polarimetry at the VLT, which trace scattered light. These data allow enable Kraus et al. to constrain the dust distribution in the system. Their images show an eccentric ring that is misaligned with the orbital planes and the outer disk. The ring casts shadows on a strongly warped intermediate region of the disk. If planets can form within the warped disk, disk tearing could provide a mechanism for forming wide-separation planets on oblique orbits.

Figure caption: ALMA 230GHz (left) and VLT (center) images of the misaligned protoplanetary disk around the young triple star system GW Orionis (Bi et al. 2020, Kraus et al. 2020). (right) The 3D model of the GW Orionis system, in projection.

Publications:

Jiaquing Bi (University of Victoria) et al., GW Ori: Interactions between a Triple-star System and Its Circumtriple Disk in Action, Astronomical Journal, 159, 72 (4 August 2020).

Stefan Kraus (University of Exeter) et al., A triple-star system with a misaligned and warped circumstellar disk shaped by disk tearing, Science, 369, 1233 (4 September 2020).

NRAO Press Release: ALMA Discovers Misaligned Rings in Planet-Forming Disk Around Triple Stars