The depletion of the red supergiant envelope radiative zone during common envelope evolution

TL;DR

This study uses stellar evolution simulations to show that during common envelope evolution in red supergiants, the radiative zone is depleted, challenging traditional models and supporting the alpha-formalism.

Contribution

It demonstrates that the envelope convective zone in RSGs disappears as the star shrinks, questioning the two-stage CEE model and validating the alpha-formalism.

Findings

Envelope convection disappears when RSG radius shrinks by an order of magnitude.

The inner boundary of the convective zone moves into the radiative zone during CEE.

The alpha-formalism remains the best phenomenological model for CEE.

Abstract

We conduct one-dimensional stellar evolution simulations of red supergiant (RSG) stars that mimic common envelope evolution (CEE) and find that the inner boundary of the envelope convective zone moves into the initial envelope radiative zone. The envelope convection practically disappears only when the RSG radius decreases by about an order of magnitude or more. The implication is that one cannot split the CEE into one stage during which the companion spirals-in inside the envelope convective zone and removes it, and a second slower phase when the companion orbits the initial envelope radiative zone and a stable mass transfer takes place. At best, this might take place when the orbital separation is about several solar radii. However, by that time other processes become important. We conclude that as of yet, the commonly used alpha-formalism that is based on energy considerations is the best phenomenological formalism.