Formation of misaligned second-generation discs through flyby encounters

TL;DR

This study uses simulations to show that flyby encounters can create second-generation protoplanetary discs tilted at roughly twice the flyby inclination, revealing how such interactions influence disc orientation.

Contribution

It demonstrates through simulations that inclined flybys can produce tilted second-generation discs at predictable angles, a novel insight into disc formation during stellar encounters.

Findings

Second-generation discs form at about twice the flyby tilt.

The tilt relationship is consistent across various flyby parameters.

Presence of a pre-existing disc around the perturber affects the tilt outcome.

Abstract

Observations reveal protoplanetary discs being perturbed by flyby candidates. We simulate a scenario where an unbound perturber, i.e., a flyby, undergoes an inclined grazing encounter, capturing material and forming a second-generation protoplanetary disc. We run $N$--body and three-dimensional hydrodynamical simulations of a parabolic flyby grazing a particle disc and a gas-rich protoplanetary disc, respectively. In both our $N$--body and hydrodynamic simulations, we find that the captured, second-generation disc forms at a tilt twice the initial flyby tilt. This relationship is robust to variations in the flyby's tilt, position angle, periastron, and mass. We extend this concept by also simulating the case where the flyby has a disc of material prior to the encounter but we do not find the same trend. An inclined disc with respect to the primary disc around a misaligned flyby is tilted by a few degrees, remaining close to its initial disc tilt. Therefore, if a disc is present around the flyby before the encounter, the disc may not tilt up to twice the perturber tilt depending on the balance between the angular momentum of the circumsecondary disc and captured particles. In the case where the perturber has no initial disc, analyzing the orientation of these second-generation discs can give information about the orbital properties of the flyby encounter.