Recombination energy in double white dwarf formation

TL;DR

This study demonstrates that including recombination energy in modeling common envelope events enables formation of complete, circular double white dwarf binaries, providing new insights into their evolution and progenitor systems.

Contribution

It introduces a novel modeling approach that incorporates recombination energy, allowing for realistic simulation of complete common envelope events leading to double white dwarf formation.

Findings

Recombination energy helps avoid circumbinary envelope formation.

Complete common envelope events can be modeled successfully.

The progenitor system of WD 1101+364 involved a 1.5 solar mass red giant.

Abstract

In this Letter we investigate the role of recombination energy during a common envelope event. We confirm that taking this energy into account helps to avoid the formation of the circumbinary envelope commonly found in previous studies. For the first time, we can model a complete common envelope event, with a clean compact double white dwarf binary system formed at the end. The resulting binary orbit is almost perfectly circular. In addition to considering recombination energy, we also show that between 1/4 and 1/2 of the released orbital energy is taken away by the ejected material. We apply this new method to the case of the double-white dwarf system WD 1101+364, and we find that the progenitor system at the start of the common envelope event consisted of a $\sim1.5M_\odot$ red giant star in a $\sim 30$ day orbit with a white dwarf companion.