Orbital evolution of highly eccentric bodies embedded in a ringed accretion disc

TL;DR

This paper studies how highly eccentric, low-mass bodies evolve orbitally within ringed accretion discs, revealing mechanisms for orbit circularization, trapping, and migration influenced by disc interactions.

Contribution

It introduces a model for the orbital evolution of eccentric perturbers in ringed discs, highlighting the formation of population rings and migration traps.

Findings

Prograde perturbers circularize and converge toward the ring radius.

Perturbers tangent to the ring remain tangential and have high accretion rates.

Retrograde perturbers consistently migrate inward.

Abstract

Various processes can induce long-lived overdense rings and arcs in protoplanetary and AGN accretion discs, such as the accumulation of gas at the outer edge of the dead zone, or the infall of material. Using the local approximation of dynamical friction, we investigate the orbital evolution of a low-mass highly-eccentric point-mass accretor (perturber) embedded in an isothermal disc hosting a density ring. We specifically consider the regime in which the eccentricity exceeds four times the disc aspect ratio. For prograde perturbers, orbits that cross the ring progressively circularize while their semi-major axes converge toward the ring radius. As a result, perturbers accumulate, forming a population ring superimposed on the gaseous ring. The ring therefore acts as a migration trap for these eccentric orbits. We also find that prograde orbits tangent to the ring, either at apocentre or pericentre, remain tangential throughout their evolution; perturbers confined to these trajectories experience the highest accretion rates. In contrast, retrograde perturbers always migrate inward. Once the semi-major axis becomes smaller than the ring radius, the eccentricity grows, but not enough for the orbit to intersect the ring again. We also discuss how feedback effects, such as jet launching and thermal torques, could modify the effective forces acting on the perturbers.