WD mass and orbital period relation of sdB + He WD binaries

TL;DR

This study investigates the relationship between white dwarf mass and orbital period in sdB + He WD binaries, using models to improve understanding of their formation and evolution, and to aid in parameter estimation.

Contribution

It establishes a WD mass and orbital period relation through semi-analytic and detailed stellar evolution models, aligning with observations and aiding parameter inference.

Findings

The relation is consistent with observed data.

It allows inference of orbital period or WD mass from the other.

Supports constraining CE ejection efficiency and mass transfer stability.

Abstract

Most subdwarf B (sdB) + Helium white dwarf (He WD) binaries are believed to be formed from a particular channel. In this channel, the He WDs are produced first from red giants (RGs) with degenerate cores via stable mass transfer and sdB stars are produced from RGs with degenerate cores via common envelope (CE) ejection. They are important for the studies of CE evolution, binary evolution, and binary population synthesis. However, the relation between WD mass and orbital period of sdB + He WD binaries has not been specifically studied. In this paper, we first use a semi-analytic method to follow their formation and find a WD mass and orbital period relation. Then we use a detailed stellar evolution code to model their formation from main-sequence binaries. We find a similar relation between the WD mass and orbital period, which is in broad agreement with observations. For most sdB + He WD systems, if the WD mass (orbital period) can be determined, the orbital period (WD mass) can be inferred with this relation and then the inclination angle can be constrained with the binary mass function. In addition, we can also use this relation to constrain the CE ejection efficiency and find that a relative large CE ejection efficiency is favoured. If both the WD and sdB star masses can be determined, the critical mass ratios of dynamically unstable mass transfer for RG binaries can also be constrained.