Gaseous Dynamical Friction on Hyperbolic Scatterings

TL;DR

This paper investigates how gaseous media influence hyperbolic encounters between equal-mass objects, revealing complex wake structures and diverse orbital evolutions, with implications for understanding dynamical friction in astrophysical contexts.

Contribution

It provides a detailed analysis of gaseous dynamical friction on hyperbolic orbits using linear perturbation theory, highlighting new classes of orbital behavior and differences from previous models.

Findings

Gas dissipates orbital energy, reducing semi-major axes.

Orbital angular momentum can increase or decrease.

Eccentricity is generally damped, favoring supersonic captures.

Abstract

We present a study of equal-mass hyperbolic encounters, embedded in a uniform gaseous medium. Using linear perturbation theory, we calculate the density wakes excited by these perturbers and compute the resulting forces exerted on them by the gas. We compute the changes to orbital energy, orbital angular momentum and apsidal precession across a wide range of eccentrities and pericenter Mach numbers. We identify six distinct classes of hyperbolic orbits, differing through their wake structure and subsequent orbital evolution. We find the gas to always dissipate orbital energy, leading to smaller semi-major axes and higher pericenter Mach numbers. The orbital angular momentum can either increase or decrease, whereas we typically find the orbital eccentricity to be damped, promoting supersonic gas-captures. Additionally, we find that the force exerted by the gas is not strictly frictional -- particularly for asymptotically subsonic trajectories. Therefore, despite the orbit-integrated changes to orbital parameters being similar to those predicted by the \cite{O99} prescription, the time evolution of the density wakes and the instantaneous forces exerted on the perturbers are significantly different.