Dynamical friction in an isentropic gas

TL;DR

This paper analytically investigates the structure of the flow and the dynamical friction force experienced by a gravitating object moving subsonically in an isentropic gas, highlighting how the force depends on the Mach number and adiabatic index.

Contribution

It provides a new analytical expression for dynamical friction in isentropic flows, extending understanding beyond isothermal cases and showing how the force varies with flow parameters.

Findings

Dynamical friction force decreases with increasing adiabatic index.

Wakes are less dense in isentropic flows compared to isothermal flows.

Derived explicit dependence of force on Mach number and adiabatic index.

Abstract

When a gravitating object moves across a given mass distribution, it creates an overdense wake behind it. Here, we performed an analytical study of the structure of the flow far from object when the flow is isentropic and the object moves subsonically within it. We show that the dynamical friction force is the main drag force on the object and by using a perturbation theory, we obtain the density, velocity and pressure of the perturbed flow far from the mass. We derive the expression of the dynamical friction force in an isentropic flow and show its dependence on the Mach number of the flow and on the adiabatic index. We find that the dynamical friction force becomes lower as the adiabatic index increases. We show analytically that the wakes are less dense in our isentropic case in comparison to the isothermal ones.