Post-merger evolution of double helium white dwarfs and distribution of helium-rich hot subdwarfs

TL;DR

This study uses binary population synthesis to explore how double helium white dwarf mergers produce helium-rich hot subdwarfs, explaining their observed subgroups and estimating their galactic population and proportions.

Contribution

It demonstrates that the merger channel accounts for the two observed subgroups of helium-rich hot subdwarfs and provides quantitative predictions of their birth rates and distributions.

Findings

Merger channel explains the two subgroups in the $T_{\rm{eff}}-\log g$ plane.

Estimated galactic birth rate of helium-rich hot subdwarfs is $\sim 4.82 \times 10^{-3}$ yr$^{-1}$.

Proportion of carbon-rich to carbon-normal hot subdwarfs is 32% to 68%.

Abstract

The mergers of double helium white dwarfs are believed to form isolated helium-rich hot subdwarfs. Observation shows that the helium-rich hot subdwarfs can be divided into two subgroups based on whether the surface is carbon-rich or carbon-normal. But it is not clear whether this distribution directly comes from binary evolution. We adopt the binary population synthesis (BPS) to obtain the population of single helium-rich hot subdwarfs according to the channel of double helium white dwarfs merger. We find that the merger channel can represent the two subgroups in the $T_{\rm{eff}}-\log g$ plane related to different masses of progenitor helium white dwarfs. For $Z$ = 0.02, the birth rates and local density of helium-rich hot subdwarf stars by the mergers of two helium white dwarfs is $\sim 4.82 \times 10^{-3}$ $\rm yr^{-1}$ and $\sim$ 290.0 $\rm kpc^{-3}$ at 13.7 Gyr in our Galaxy, respectively. The proportion of carbon-rich and carbon-normal helium-rich hot subdwarfs are 32$\%$ and 68$\%$, respectively.