[DSRP Evidence](https://dsrpevidence.org/)

# Dynamical friction in stratified stellar envelopes

Damien Gagnier, 2026, Astronomy & Astrophysics — Astronomy

Patterns: [Systems](https://dsrpevidence.org/pattern/systems), [Relationships](https://dsrpevidence.org/pattern/relationships)

## In short

The finding shows that the force felt by an orbiting body (a part) depends on the global structure of the medium as a whole, not merely on the medium's local properties at the body's position, and that in multi-body systems one part's wake can substantially alter the force felt by another part.

## What they found (results)

Stratification was found to affect dynamical friction through the global structure of the acoustic wake rather than only through the local density, sound speed, and Mach number at the perturber's position, changing the radial force in amplitude and sign relative to homogeneous-medium predictions and yielding shorter inspiral times for giant-star envelope profiles; in double-perturber systems, a companion's wake substantially altered the radial force and reduced drag on the other body.

## What they set out to do (purpose)

To quantify how a radially stratified (rather than homogeneous) gaseous stellar envelope alters the dynamical friction felt by a gravitating object on a curved orbit within it, relevant to common-envelope and planetary-engulfment inspirals.

## Abstract

Dynamical friction prescriptions used for common-envelope and planetary engulfment inspirals often assume a homogeneous medium and/or rectilinear perturber motion. A gravitating object embedded in a giant-star envelope instead excites an orbit-scale wake while moving on a curved orbit through a finite, radially stratified medium. We quantified the linear barotropic acoustic wake and the associated gravitational back-reaction for low-mass perturbers on circular orbits. We formulated the barotropic acoustic response of a weak point perturber on a circular orbit in a hydrostatic, spherically stratified gaseous medium. The enthalpy perturbation was expanded into spherical harmonics and Fourier modes. The force is written as an adjacent-multipole sum, with coefficients computed from the retarded acoustic Green function. We applied the formulation to single perturbers in power-law density profiles and giant-star envelope models, as well as to double perturbers in power-law density backgrounds. We find that stratification affects dynamical friction through the global structure of the wake, not only through the local density, sound speed, and Mach number at the perturber position. The radial component is set by the low-order, orbit-scale wake and can strongly differ in amplitude and sign from the homogeneous-medium result. The azimuthal component is also modified by stratification, but in the supersonic regime it retains the Coulomb-logarithmic sensitivity of the homogeneous problem. In double-perturber systems, the companion wake can substantially change the radial force and reduce the azimuthal drag on a given component, but, unlike the perturber's own wake, it has no local Coulomb-logarithmic contribution. For the adopted giant-star envelope profiles, the azimuthal drag exerted by the stratified wake gives shorter inspiral times than uniform-medium prescriptions evaluated with the same local background quantities. The gravitational back-reaction of the linear barotropic wake in stratified stellar envelopes combines a global acoustic response with a cutoff-sensitive drag contribution. The formulation provides a flexible tool for computing embedded-perturber wakes in prescribed radial stratifications and is a first step toward computationally efficient, self-consistent models of common-envelope and planetary-engulfment inspirals.

These researchers were not testing DSRP. The finding is theirs; the correspondence to DSRP is drawn by this site.

[Source](https://doi.org/10.1051/0004-6361/202662064)