Constraints on Common Envelope Ejection from Double Helium White Dwarfs

TL;DR

This study models the formation of double helium white dwarfs, revealing how common envelope ejection efficiency depends on progenitor masses and aligning with observed properties.

Contribution

It introduces a semi-analytic and stellar evolution simulation approach to constrain common envelope ejection physics in double helium white dwarfs.

Findings

Most double He WDs have mass ratios slightly greater than 1.

Orbital period and mass ratio relations are consistent with observations.

CE ejection efficiency increases with WD progenitor mass.

Abstract

Double helium white dwarfs (He WDs) are one type of gravitational wave source and are greatly important in the studies of binary interaction, particularly in the common envelope (CE) ejection physics. Most double He WDs with mass ratios of q~1 are formed through a particular channel. In this channel, one He WD is initially produced from a red giant (RG) with a degenerate core via stable Roche lobe overflow, and another He WD is formed from an RG with a degenerate core via CE ejection. They may have significant implications for the binary evolution processes yet have not received specific studies, especially for the CE phase. This paper adopts a semi-analytic method and a detailed stellar evolution simulation to model the formation of double He WDs. We find that most double He WDs show mass ratios being slightly greater than 1, and their orbital periods and mass ratios relation are broadly consistent with observations. There is also a relation between the mass ratios and the progenitors' masses of the He WDs produced via CE ejection for double He WDs with determined WD masses. Based on this relation, the mass of the He WD progenitor can be inferred from the mass ratio. Then, the CE ejection efficiency can be constrained with the orbital period. In addition, we constrain the CE ejection efficiency for two double He WDs, J1005-2249 and WD0957-666. The results show that the CE ejection efficiencies increase with the WD progenitor masses.