Abstract

Contributed Talk - Splinter CloseBinaries   (MW-0250)

Dynamical friction in stratified stellar envelopes

Damien Gagnier
University of Heidelberg, ARI/ZAH and HITS

The inspiral of a gravitating object inside a giant-star envelope is driven by the exchange of orbital energy and angular momentum with the surrounding gas. This exchange is often mediated by the gaseous wake raised by the perturber, whose gravitational back-reaction exerts a drag force commonly referred to as dynamical friction. This force influences the inspiral rate, the envelope response, and the fate of the embedded object, making it a key mechanism in common-envelope evolution and planetary engulfment. Dynamical-friction prescriptions provide useful closures for stellar-evolution calculations, but their applicability to inspiral inside stellar envelopes remains limited. Homogeneous-medium prescriptions, such as those of Ostriker (1999) and Kim & Kim (2007), neglect the radial stratification of the medium, while prescriptions based on rectilinear motion also omit the radial force toward the centre of motion that arises from orbital curvature. A spiralling-in perturber instead excites a global wake while moving on a curved orbit through a finite, radially stratified medium. The resulting force can therefore depend on the envelope structure away from the perturber, not only on the density, sound speed, and Mach number evaluated at its position. In this talk, I will present a formulation of the linear acoustic response of a point perturber on a circular orbit in a hydrostatic, spherically stratified gaseous medium, and of the associated dynamical-friction force. I will show applications to spiralling-in perturbers in giant-star envelope models, and discuss the assumptions and limitations of the approach. Combined with local (magneto-)hydrodynamical simulations resolving the nonlinear near-field flow (e.g., MacLeod et al. 2017; De et al. 2020; Gagnier et al. 2026), these calculations provide a promising step toward computationally efficient models of common-envelope and planetary-engulfment inspirals.