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Accepted manuscript

Understanding the Drag Torque in Common Envelope Evolution

Published online by Cambridge University Press:  10 December 2025

Soumik Bhattacharyya
Affiliation:
National Institute of Science Education and Research, An OCC of Homi Bhabha National Institute, Bhubaneswar 752050, Odisha, India
Luke Chamandy
Affiliation:
National Institute of Science Education and Research, An OCC of Homi Bhabha National Institute, Bhubaneswar 752050, Odisha, India Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
Eric G. Blackman*
Affiliation:
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
Adam Frank
Affiliation:
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
Baowei Liu
Affiliation:
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA Center for Integrated Research Computing, University of Rochester, Rochester, NY 14627, USA
*
Author for correspondence: E. G. Blackman, Email: blackman@pas.rochester.edu.
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Abstract

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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.

Information

Type
Research Article
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2025. Published by Cambridge University Press on behalf of Astronomical Society of Australia