Understanding the Drag Torque in Common Envelope Evolution

TL;DR

This paper develops a new 3D hydrodynamic model for the drag torque in common envelope evolution, revealing that the torque is mainly contributed by gas near the binary, which is roughly in corotation, and matches simplified models under certain conditions.

Contribution

It introduces a novel 3D simulation approach that accurately models the torque in common envelope evolution, improving upon idealized models by accounting for binarity and gas distribution.

Findings

Torque mainly from gas near the binary separation

Gas exhibits roughly corotational pattern

Simplified spheroid model reproduces torque evolution

Abstract

Common envelope (CE) evolution is largely governed by the drag torque applied on the in-spiralling stellar components by the envelope. Previous work has shown that idealized models of the torque based on a single body moving in rectilinear motion through an unperturbed atmosphere can be highly inaccurate. Progress requires new models for the torque that account for binarity. Toward this end we perform a new 3D global hydrodynamic CE simulation with the mass of the companion point particle set equal to the mass of the asymptotic giant branch star core particle to maximize symmetry and facilitate interpretation. First, we find that a region around the particles of a scale comparable to their separation contributes essentially all of the torque. Second, the density pattern of the torque-dominating gas and, to an extent, this gas itself, is roughly in corotation with the binary. Third, approximating the spatial distribution of the torquing gas as a uniform-density prolate spheroid whose major axis resides in the orbital plane and lags the line joining the binary components by a constant phase angle reproduces the torque evolution remarkably well, analogous to studies of binary supermassive black holes. Fourth, we compare the torque measured in the simulation with the predictions of a model that assumes two weak point-mass perturbers undergoing circular motion in a uniform background without gas self-gravity, and find remarkable agreement with our results if the background density is taken to be equal to a fixed fraction (~0.44) of the density at the spheroid surface. Overall, this work makes progress toward developing simple time-dependent models of the CE phase, for example by informing the development of drag force prescriptions for 1D spherically symmetric CE simulations, which could be used to explore the parameter space of luminous red novae or in binary population synthesis studies.