Orbital Evolution of Gas-driven Inspirals with Extreme Mass Ratios: Retrograde Eccentric Orbits

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.