Merger by Migration at the Final Phase of Common Envelope Evolution

TL;DR

This paper challenges the traditional CE -prescription, proposing that in massive envelopes, the final orbital separation results from migration and merger processes similar to planet migration, rather than simple energy formalism predictions.

Contribution

It introduces a new perspective on common envelope evolution, emphasizing migration and merger mechanisms over the standard energy formalism for massive stars.

Findings

Merger is a common outcome for certain giant star binaries.

The CE -prescription fails to predict final separations in massive envelopes.

Migration processes resemble planet migration in protoplanetary disks.

Abstract

I find the common envelope (CE) energy formalism, the CE \alpha-prescription, to be inadequate to predict the final orbital separation of the CE evolution in massive envelopes. I find that when the orbital separation decreases to ~10 times the final orbital separation predicted by the CE \alpha-prescription, the companion has not enough mass in its vicinity to carry away its angular momentum. The core-secondary binary system must get rid of its angular momentum by interacting with mass further out. The binary system interacts gravitationally with a rapidly-rotating flat envelope, in a situation that resembles planet-migration in protoplanetary disks. The envelope convection of the giant carries energy and angular momentum outward. The basic assumption of the CE \alpha-prescription, that the binary system's gravitational energy goes to unbind the envelope, breaks down. Based on that, I claim that merger is a common outcome of the CE evolution of AGB and red super-giants stars with an envelope to secondary mass ratio of (M_env/M_2)>~5. I discuss some other puzzling observations that might be explained by the migration and merger processes.