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

0585616 - ASÚ 2025 RIV US eng J - Journal Article
Lin, J. - Wu, Ch. - Xiong, H. - Németh, Péter … Total 31 authors
A seven-Earth-radius helium-burning star inside a 20.5-min detached binary.
Nature Astronomy. Roč. 8, April (2024), s. 491-503. ISSN 2397-3366. E-ISSN 2397-3366
R&D Projects: GA ČR(CZ) GA22-34467S
Institutional support: RVO:67985815
Keywords : solar and stellar astrophysics * Earth and planetary astrophysics * high energy astrophysical phenomena
OECD category: Astronomy (including astrophysics,space science)
Impact factor: 14.1, year: 2022
Method of publishing: Limited access
https://doi.org/10.1038/s41550-023-02188-2

Binary evolution theory predicts that the second common envelope ejection can produce low-mass (0.32-0.36 M⊙) 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 min. However, only four sdB binaries with orbital periods below an hour have been reported so far, and none of them has an orbital period approaching the above theoretical limit. Here we report the discovery of a 20.5-min-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 companion. The visible component is inferred to be an sdB star with a mass ∼0.33 M⊙ approaching the helium-ignition limit, although a He-core white dwarf cannot be completely ruled out. In particular, the radius of this low-mass sdB star is only 0.066 R⊙, about seven Earth radii. Such a system provides a key clue in mapping the binary evolution scheme from the second common envelope ejection to the formation of AM CVn stars having a helium-star donor. It may also serve as a crucial verification binary of space-borne GW observatories such as LISA and TianQin in the future.
Permanent Link: https://hdl.handle.net/11104/0353294