The Interaction of Stellar Objects within a Common Envelope

TL;DR

This study uses detailed 3D hydrodynamic simulations to analyze the complex interactions between stellar objects within a common envelope, revealing the dominance of gravitational drag over hydrodynamic forces and challenging existing simplified models.

Contribution

It provides new insights into the relative importance of gravitational and hydrodynamic forces during stellar inspiral in common envelopes, highlighting limitations of previous simplified prescriptions.

Findings

Gravitational drag dominates over hydrodynamic drag during inspiral.

Simple gravitational capture radius models underestimate dissipation rates.

Mass accretion rates fluctuate but are generally lower than predicted by simple models.

Abstract

We use high-resolution, three-dimensional hydrodynamic simulations to study the hydrodynamic and gravitational interaction between stellar companions embedded within a differentially rotating common envelope. Specifically, we evaluate the contributions of the nonaxisymmetric gravitational tides and ram pressure forces to the drag force and, hence, to the dissipation rate and the mass accumulated onto the stellar companion. We find that the gravitational drag dominates the hydrodynamic drag during the inspiral phase, leading to the result that a simple prescription based on a gravitational capture radius formalism significantly underestimates the dissipation rate and overestimates the inspiral decay timescale. Although the rate of mass accretion fluctuates significantly, we observe a secular trend leading to an effective rate of mass accretion which is significantly less than the rate based on a gravitational capture radius. The implications of these results are discussed within the context of accretion of compact objects in the common-envelope phase.