A Near-coplanar Stellar Flyby of the Planet Host Star HD 106906

We present an investigation into the kinematics of HD 106906 using the newly released Gaia DR2 catalog to search for close encounters with other members of the Scorpius-Centaurus (Sco-Cen) association. HD 106906 is an eccentric spectroscopic binary that hosts both a large asymmetric debris disk extending out to at least 500 au and a directly imaged planetary-mass companion at a projected separation of 738 au. The cause of the asymmetry in the debris disk and the unusually wide separation of the planet is not currently known. Using a combination of Gaia DR2 astrometry and ground-based radial velocities, we explore the hypothesis that a close encounter with another cluster member within the last 15 Myr is responsible for the present configuration of the system. Out of 461 stars analyzed, we identified two candidate perturbers that had a median closest approach distance within 1 pc of HD 106906: HIP 59716 at D-CA = 0.65(-0.40)(+0.93) pc (t(CA )= -3.49(-1.76)(+0.90) Myr) and HIP 59721 at D-CA = 0.71(-0.11)(+0.18) pc (t(CA) = -2.18(-0.04)(+0.54) Myr), with the two stars likely forming a wide physical binary. The trajectories of both stars relative to HD 106906 are almost coplanar with the inner disk (Delta theta = 5 degrees.4 +/- 1.7 and 4 degrees.2(-1.1)(+0.9)). These two stars are the best candidates of the currently known members of Sco-Cen for having a dynamically important close encounter with HD 106906, which may have stabilized the orbit of HD 106906 b in the scenario where the planet formed in the inner system and attained high eccentricity by interaction with the central binary.