Energetics of star-disc encounters in the non-linear regime

TL;DR

This paper studies how a star's accretion disc responds energetically to close encounters with passing objects, emphasizing the importance of non-linear effects in most cases.

Contribution

It provides a detailed analysis of energy and angular momentum transfer during star-disc fly-bys using a non-linear approach, highlighting the significance of unbound disc material.

Findings

Energy transfer exceeds the binding energy of outer disc material by up to a factor of five.

Most close encounters result in disc material becoming unbound, indicating destructive interactions.

Resonance effects can cause the disc to lose energy and angular momentum to the perturber in more distant encounters.

Abstract

We investigate the response of a circumstellar accretion disc to the fly-by of a perturbing mass on a parabolic orbit. The energy and angular momentum transferred during the encounter are calculated using a reduced three-body method. In almost all close encounters the energy and angular momentum transfer is dominated by disc material becoming unbound from the system, with the contributions from close disc particle -- star encounters being significant. For more distant encounters with some prograde element to the motion the disc material loses energy and angular momentum to the perturber's orbit through a resonance feature. The magnitude of the energy transfer calculated in our simulations is greater than that of the binding energy of material exterior to periastron by a factor of two in the prograde case, and up to a factor of five in the case of the retrograde encounter. The destructive nature of the encounters indicates that a non-linear treatment is essential in all but the most distant encounters.