A dense $\mathbf{0.1 M_{\rm \odot}}$ star in a 51-minute orbital period eclipsing binary

TL;DR

This paper reports the discovery of ZTF J1813+4251, a 51-minute orbital period eclipsing binary with a helium-rich star, providing key insights into the evolutionary pathway leading to helium cataclysmic variables.

Contribution

It presents the first detailed characterization of a short-period, helium-rich binary system that is a direct progenitor of helium CVs, filling a crucial gap in evolutionary models.

Findings

ZTF J1813+4251 has a 51-minute orbital period and a helium-rich donor star.

The system's properties suggest it will evolve into a helium CV with an orbital period under 20 minutes.

Evolutionary modeling confirms its role as a missing link in CV evolution.

Abstract

In over a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes. One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution prior to interacting with the white dwarf, and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs. However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute orbital period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater due to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modeling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs.