Gaseous wakes and dynamical friction: mass-losing and mass-gaining perturbers

TL;DR

This paper extends the classical analysis of dynamical friction to perturbers with changing mass in gaseous media, revealing significant differences in wake structure and force estimates, especially for mass-changing objects.

Contribution

It introduces a generalized linear analysis of dynamical friction accounting for time-varying mass, highlighting the impact on wake structure and force calculations.

Findings

Wake structure differs significantly for variable-mass perturbers.

Instantaneous-mass approximation can underestimate or overestimate drag force by over 50%.

Mass history influences the density wake and dynamical friction force.

Abstract

An extended gravitational object embedded in a parent system comprised of gas and collisionless particles may undergo both dynamical friction (DF) and mass loss by tidal forces. If the object is compact enough, it can increase its mass through accretion of material from the surrounding medium. We extend the classical linear analysis of DF on a constant-mass body in a gaseous medium to the case where its mass changes with time. We show that the structure of the wake may differ significantly from the constant-mass case. For instance, the front-back symmetry of density about subsonic constant-mass perturbers is broken down for variable-mass perturbers. The density wake keeps a memory of the past mass history of the perturber. For dissolving perturbers, the density field is more dense than expected using the instantaneous mass of the perturber in the classical formula. As a consequence, the instantaneous-mass approximation underestimates the drag force for mass-losing perturbers and overestimates it for mass-gaining perturbers. We present cases in which the percentage error in the drag force using the instantaneous-mass approximation is greater than 50%.