Flyby encounters exciting spiral arms in protoplanetary discs: implications for planet formation

Protoplanetary disks around young stars provide a unique laboratory for studying planet formation and planet-disc interactions. Several observed protoplanetary disks show prominent spiral arms, which could be a signpost for undetected planets (Grady et al. 1999, 2013). Moreover, stellar flybys can induce spiral features in circumstellar disks (Ménard et al. 2020). Studying the evolution and origin of spiral arms within protoplanetary disks will further shed light on how planetary systems are formed. To model a flyby of a massive object perturbing a protoplanetary disk, we use the 3-dimensional smoothed particle hydrodynamical (SPH) code phantom (Lodato & Price 2010; Price & Federrath 2010; Price et al. 2018). Recently, phantom has been well tested for parabolic flyby encounters interacting with gaseous and dusty disks (Cuello et al. 2019, 2020; Nealon et al. 2020; Ménard et al. 2020). We set up a suite of simulations modeling a gaseous protoplanetary disk around a central star, with various perturber masses ranging from 5M_Jup to ~1000M_Jup. The flyby excites a two-armed spiral structure in the disk. The lower mass perturber excites much weaker spirals than the higher-mass perturber. Long after the periastron passage of the perturber, the spirals begin to wind up on themselves since the perturber is not strongly interacting with the disk. The initial pattern speed of the spirals are closely set by the angular velocity of the perturber at periastron. The pattern speed of the spiral arms decreases with time. The pitch angle increases as the perturber are exciting spiral arms and then peaks roughly 2000 years after the closest approach. The pitch angle then begins to decrease as the spiral arms wind up. We find similar results when the perturber is on a moderately inclined orbit. From our hydrodynamical simulation results, flyby encounters can significantly affect the structure of protoplanetary disks, which has implications for planet formation. Dust grains will become trapped within spiral arms due to local gas pressure maxima. The dust trapping will promote gain growth and eventually planetesimal formation. Thies et al. (2010) demonstrated that flyby encounters might trigger gravitational instability, leading to the formation of brown dwarfs and giant planets in the outer regions of the disc. Hence, massive companions or planets can be formed at great distances from their host stars.