A seven-Earth-radius helium-burning star inside a 20.5-min detached binary

TL;DR

This paper reports the discovery of an ultra-compact binary system with a 20.5-minute orbital period, featuring a helium-burning star nearly as small as Earth, providing insights into binary evolution and gravitational wave sources.

Contribution

The discovery of a 20.5-minute orbital period binary with a helium-burning star and a white dwarf companion, approaching the theoretical limit for such systems, advancing understanding of binary evolution.

Findings

The binary system has a 20.5-minute orbital period.

The visible star is a nearly Earth-sized helium-burning star.

The system offers a key test for binary evolution models.

Abstract

Binary evolution theory predicts that the second common envelope (CE) ejection can produce low-mass (0.32-0.36 Msun) subdwarf B (sdB) stars inside ultrashort-orbital-period binary systems, as their helium cores are ignited under nondegenerate conditions. With the orbital decay driven by gravitational-wave (GW) radiation, the minimum orbital periods of detached sdB binaries could be as short as ~20 minutes. However, only four sdB binaries with orbital periods below an hour have been reported so far, while none of them has an orbital period approaching the above theoretical limit. Here we report the discovery of a 20.5-minute-orbital-period ellipsoidal binary, TMTS J052610.43+593445.1, in which the visible star is being tidally deformed by an invisible carbon-oxygen white dwarf (WD) companion. The visible component is inferred to be an sdB star with a mass of ~0.33 Msun, approaching that of helium-ignition limit, although a He-core WD cannot be completely ruled out. In particular, the radius of this low-mass sdB star is only 0.066 Rsun, about seven Earth radii, possibly representing the most compact nondegenerate star ever known. Such a system provides a key clue to map the binary evolution scheme from the second CE ejection to the formation of AM CVn stars having a helium-star donor, and it will also serve as a crucial verification binary of space-borne GW detectors in the future.