A revised energy formalism for common-envelope evolution: repercussions for planetary engulfment and the formation of neutron star binaries

TL;DR

This paper introduces a revised energy formalism for common-envelope evolution, revealing that more energy is transferred during the process than previously thought, impacting models of binary star formation and planetary engulfment.

Contribution

The authors derive a new energy formalism that accounts for decreasing enclosed mass, leading to more accurate predictions of common-envelope outcomes.

Findings

The energy transfer during common-envelope evolution is larger than previously estimated.

The companion mass needed to eject the envelope is reduced by up to 50%.

Outer envelope energy deposition can be up to 7 times higher.

Abstract

Common-envelope evolution is a stage in binary system evolution in which a giant star engulfs a companion. The standard energy formalism is an analytical framework to estimate the amount of energy transferred from the companion's shrinking orbit into the envelope of the star that engulfed it. We show analytically that this energy transfer is larger than predicted by the standard formalism. As the orbit of the companion shrinks, the mass it encloses becomes smaller, and the companion is less bound than if the enclosed mass had remained constant. Therefore, more energy must be transferred to the envelope for the orbit to shrink further. We derive a revised energy formalism that accounts for this effect, and discuss its consequences in two contexts: the formation of neutron star binaries, and the engulfment of planets and brown dwarfs by their host stars. The companion mass required to eject the stellar envelope is smaller by up to $50\%$, leading to differences in common-envelope evolution outcomes. The energy deposition in the outer envelope of the star, which is related to the transient luminosity and duration, is up to a factor of $\approx7$ higher. Common-envelope efficiency values above unity, as defined in the literature, are thus not necessarily unphysical, and result at least partly from an incomplete description of the energy deposition. The revised energy formalism presented here can improve our understanding of stellar merger and common-envelope observations and simulations.