Orbital evolution of gas-driven inspirals with extreme mass-ratios: retrograde eccentric orbits

TL;DR

This study uses two-dimensional simulations to analyze the orbital evolution of low-mass bodies in gaseous disks, focusing on eccentric and retrograde orbits, and compares results with local dynamical friction predictions.

Contribution

It provides new insights into the torque and power on retrograde eccentric inspirals, validating local approximations for low mass ratios and specific disk conditions.

Findings

Power peaks at specific eccentricities depending on disk aspect ratio

Torque and power converge to local dynamical friction predictions as softening length decreases

Inspiral rates are largely independent of eccentricity for certain mass ratios and disk parameters

Abstract

Using two-dimensional simulations, we compute the torque and rate of work (power) on a low-mass gravitational body, with softening length $R_{\rm soft}$, embedded in a gaseous disk when its orbit is eccentric and retrograde with respect to the disk. We explore orbital eccentricities $e$ between $0$ and $0.6$. We find that the power has its maximum at $e\simeq 0.25(h/0.05)^{2/3}$, where $h$ is the aspect ratio of the disk. We show that the power and the torque converge to the values predicted in the local (non-resonant) approximation of the dynamical friction (DF) when $R_{\rm soft}$ tends to zero. For retrograde inspirals with mass ratios $\lesssim 5\times 10^{-4}$ embedded in disks with $h\geq 0.025$, our simulations suggest that (i) the rate of inspiral barely depends on the orbital eccentricity and (ii) the local approximation provides the value of this inspiral rate within a factor of $1.5$. The implications of the results for the orbital evolution of extreme mass-ratio inspirals are discussed.