Obliquity Constraints on the Planetary-mass Companion HD 106906 b

TL;DR

This study investigates the angular momentum orientations of the planetary-mass companion HD 106906 b, its host binary, and debris disk, revealing a likely misalignment that informs its formation history.

Contribution

It provides the first measurement of the spin axis of HD 106906 b and analyzes the system's angular momentum architecture to infer its formation mechanism.

Findings

HD 106906 b's spin axis is nearly pole-on.

The debris disk is viewed nearly edge-on.

The system's vectors are likely misaligned, suggesting turbulent formation.

Abstract

We constrain the angular momentum architecture of HD 106906, a 13 $\pm$ 2 Myr old system in the ScoCen complex composed of a compact central binary, a widely separated planetary-mass tertiary HD 106906 b, and a debris disk nested between the binary and tertiary orbital planes. We measure the orientations of three vectors: the companion spin axis, companion orbit normal, and disk normal. Using near-IR high-resolution spectra from Gemini/IGRINS, we obtain a projected rotational velocity of $v\sin{i_p}$ = 9.5 $\pm$ 0.2 km/s for HD 106906 b. This measurement together with a published photometric rotation period implies the companion is viewed nearly pole-on, with a line-of-sight spin axis inclination of $i_p$ = 14 $\pm$ 4 degrees or 166 $\pm$ 4 degrees. By contrast, the debris disk is known to be viewed nearly edge-on. The likely misalignment of all three vectors suggests HD 106906 b formed by gravitational instability in a turbulent environment, either in a disk or cloud setting.