EVR-CB-001: An evolving, progenitor, white dwarf compact binary discovered with the Evryscope

TL;DR

This paper reports the discovery of EVR-CB-001, a rare compact binary system with a pre-helium white dwarf and a helium white dwarf companion, providing insights into the evolution of low-mass white dwarfs and hot subdwarfs.

Contribution

It presents the first detailed characterization of EVR-CB-001, a unique progenitor system bridging pre-He WD and hot subdwarf stages, expanding understanding of binary evolution.

Findings

The primary is a pre-He WD with specific mass, temperature, and gravity.

The system is expected to evolve into a double degenerate, possibly forming a hot subdwarf.

EVR-CB-001 is a rare example of a progenitor system for single hot subdwarfs.

Abstract

We present EVR-CB-001, the discovery of a compact binary with an extremely low mass ($.21 \pm 0.05 M_{\odot}$) helium core white dwarf progenitor (pre-He WD) and an unseen low mass ($.32 \pm 0.06 M_{\odot}$) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low mass He WDs are exotic objects (only about .2$\%$ of WDs are thought to be less than .3 $M_{\odot}$), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius ($\approx .2 R_{\odot}$) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature ($T_{\rm eff}=18,500 \pm 500 K$), and surface gravity ($\log(g)=4.96 \pm 0.04$) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-RGB, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color-magnitude and $T_{\rm eff}$-$\log(g)$ diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.