A Circumbinary Disk in the Final Stages of Common Envelope and the Core-Degenerate Scenario for Type Ia Supernovae

TL;DR

This paper investigates the final stages of common envelope evolution, proposing that fallback material forms a circumbinary disk which promotes white dwarf mergers, potentially leading to Type Ia supernovae via the core-degenerate scenario.

Contribution

It introduces the core-degenerate scenario where fallback material forms a circumbinary disk, facilitating white dwarf mergers and supernova explosions, a novel pathway in Type Ia supernova progenitor models.

Findings

A significant fraction of ejected mass remains bound after CE

Fallback material forms a circumbinary disk around the remnant binary

Circumbinary disk interaction reduces orbital separation, promoting mergers

Abstract

We study the final stages of the common envelope (CE) evolution and find that a substantial fraction of the ejected mass does not reach the escape velocity. To reach this conclusion we use a self-similar solution under simplifying assumptions. Most of the gravitational energy of a companion white dwarf (WD) is released in the envelope of a massive asymptotic giant branch (AGB) or the red giant branch (RGB) star in a very short time. This rapid energy release forms a blast wave in the envelope. We follow the blast wave propagation from the center of the AGB outwards, and show that ~1-10 per cent of the ejected envelope remains bound to the remnant binary system. We suggest that due to angular momentum conservation and further interaction with the binary system, the fallback material forms a circumbinary disk around the post-AGB Core and the companion WD. The interaction of the circumbinary disk with the binary system will reduce the orbital separation much more than expected from the dynamical phase (where the envelope is ejected) of the CE alone. The smaller orbital separation favors a merger at the end of the CE phase or a short time after, while the core is still hot. This is another channel to form a massive WD with super-Chandrasekhar mass that might explode as a type Ia supernova. We term this the core-degenerate (CD) scenario.