Prograde and Retrograde Gas Flow around Disk-embedded Companions: Dependence on Eccentricity, Mass and Disk Properties

TL;DR

This study uses 3D hydrodynamical simulations to explore how gas flow patterns around disk-embedded companions transition from prograde to retrograde as eccentricity increases, depending on companion and disk properties.

Contribution

It provides a unified empirical formula describing the critical eccentricity for flow reversal based on companion mass and disk aspect ratio, applicable across different regimes.

Findings

Flow transitions from prograde to retrograde at a critical eccentricity.

Critical eccentricity scales with disk aspect ratio for sub-thermal companions.

Critical eccentricity scales with (q/h)^{1/3} for super-thermal companions.

Abstract

We apply 3D hydrodynamical simulations to study the rotational aspect of gas flow patterns around eccentric companions embedded in an accretion disk around its primary host. We sample a wide range of companion mass ratio q and disk aspect ratio h, and confirm a generic transition from prograde (steady tidal interaction dominated) to retrograde (background Keplerian shear dominated) circum-companion flow when orbital eccentricity exceeds a critical value et. We find et \sim h for sub-thermal companions while et \sim (q/h)^1/3 for super-thermal companions, and propose an empirical formula to unify the two scenarios. Our results also suggest that et is insensitive to modest levels of turbulence, modeled in the form of a kinematic viscosity term. In the context of stellar-mass Black Holes (sBHs) embedded in AGN accretion disks, the bifurcation of their circum-stellar disk (CSD) rotation suggest the formation of a population of nearly anti-aligned sBHs, whose relevance to low spin gravitational waves (GW) events can be probed in more details with future population models of sBH evolution in AGN disks, making use of our quantitative scaling for et; In the context of circum-planetary disks (CPDs), our results suggest the possibility of forming retrograde satellites in-situ in retrograde CPDs around eccentric planets.