Seeing Things in Threes

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GW Ori is a system of three stars that are gravitationally bound. Aside from being a triple system, GW Ori also stands out for another reason — it harbors a circumtriple disk, which is a disk of gas and dust surrounding all three stars.

A Tricky Triple

ALMA_circumtriple_dust

The dust component of GW Ori’s disk as seen by ALMA, showing the three rings discussed in this study. The x- and y-axes of the plot are position offsets, with (0,0) being the position of GW Ori. The color of the rings indicate intensity of emission, with yellow being more intense than purple. The circle in the lower left corner shows the size of the beam used by ALMA to image the disk. [Bi et al. 2020]

GW Ori lives in a star cluster called Lambda Orionis, which appears near Betelgeuse on the sky. The inner stars of the system, GW Ori A and GW Ori B, orbit each other and are separated by about 1 astronomical unit (au). The third star, GW Ori C, revolves around its two companions at a distance of roughly 8 au. 

GW Ori’s circumtriple disk is enormous relative to the orbits of its stars. The dust component of the disk is about 400 au across, with the gas component spanning roughly 1,300 au. For scale, Neptune is only about 30 au from the Sun!

Models of GW Ori have suggested a gap in the disk between 25 and 55 au from its center. A recent study led by Jiaqing Bi (University of Victoria) attempted to test these models and probe the structure of GW Ori directly using observations by the Atacama Large Millimeter/submillimeter Array (ALMA).

Finding Rings in Radio

Bi and collaborators used ALMA observations taken at multiple frequencies to probe the gas and dust of the circumtriple disk. The dust component has characteristic emission that can be observed at 1.3 millimeters, while the gas can be studied using a particular transition of carbon monoxide.

gw-ori_rings_misalignment

A schematic showing the likely alignment of GW Ori’s dust rings. The diagram on the left is how the disk appears on the sky and the diagram on the right is how the disk would appear if seen edge-on. The red ellipse is the plane of the orbits of GW Ori AB and GW Ori C. The ring of white dots represents the gap between the inner dust ring and middle dust ring. The orientation axes for both diagrams are located in their respective lower left corners. [Bi et al. 2020]

The gas observations showed the expected disk rotation, and additional structure in the disk was immediately apparent in the dust observations. Bi and collaborators identified three dust rings in GW Ori’s disk at roughly 46, 188, and 338 au from its center, the innermost ring being the one that was suggested by past models. Additionally, an unexpected result was brought to fore — the dust rings may be very misaligned relative to one another!

Out of Balance But It’s Fine

Bi and collaborators found that the dust rings showed significant inclinations relative to the plane of the orbit of GW Ori A and B — specifically 11, 35, and 40 degrees starting from the innermost ring. The gas observations back this up, requiring a model that assumes some distortion from an undisturbed disk.

Additional analysis and simulations by Bi and collaborators suggest that the stars of GW Ori alone could not be responsible for this misalignment. The innermost ring also adds another puzzle to this system: in addition to being misaligned, it also has a non-zero eccentricity, meaning its center is different than those of the other rings.

A possible explanation could be additional companions to GW Ori, which are also carving out paths in the disk. This phenomenon has been observed by ALMA before in protoplanetary disks. If this is the case, it would be the first time circumtriple companions were detected. Only time will tell!

Citation

“GW Ori: Interactions between a Triple-star System and Its Circumtriple Disk in Action,” Jiaqing Bi et al 2020 ApJL 895 L18. doi:10.3847/2041-8213/ab8eb4