Testing common envelopes on double white dwarf binaries

TL;DR

This paper investigates the role of recombination energy in the ejection of common envelopes during the formation of double white dwarf binaries, using hydrodynamic simulations to match observed systems.

Contribution

It demonstrates that recombination energy is crucial for fully ejecting the common envelope in binary evolution models.

Findings

Recombination energy aids in complete envelope ejection.

Without recombination energy, a significant envelope remains bound.

The progenitor for WD 1101+364 is a 1.5 solar mass red giant.

Abstract

The formation of a double white dwarf binary likely involves a common envelope (CE) event between a red giant and a white dwarf (WD) during the most recent episode of Roche lobe overflow mass transfer. We study the role of recombination energy with hydrodynamic simulations of such stellar interactions. We find that the recombination energy helps to expel the common envelope entirely, while if recombination energy is not taken into account, a significant fraction of the common envelope remains bound. We apply our numerical methods to constrain the progenitor system for WD 1101+364 -- a double WD binary that has well-measured mass ratio of $q=0.87\pm0.03$ and an orbital period of 0.145 days. Our best-fit progenitor for the pre-common envelope donor is a 1.5 $M_\odot$ red giant.