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arXiv · 2608.11557

A Helium-shell Burning Blue Horizontal Branch Star Produced from Common Envelope Evolution

Jiao Li·Changqing Luo·Hai-Liang Chen·Zhicun Liu·Bo Zhang·Shi Jia·Hongwei Ge·Tao Wu·Yuhan Yao·Pei Wang·Marat Gilfanov·You Wu·Zhenwei Li·Zhengwei Liu·Xiangcun Meng·Xue-Fei Chen·Philipp Podsiadlowski·Chao Liu·Zhan-Wen Han
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Abstract

Observationally, blue horizontal branch (BHB) stars are defined as hot stars occupying a characteristic region between the extreme blue horizontal branch and RR Lyrae variables in the Hertzsprung-Russell diagram. Most of them are interpreted as stripped core-helium-burning stars, but the role of binary interaction in their formation remains unclear. Here, we report the discovery of a metal-rich BHB star in a 0.82628-day binary system (Feige 64) comprising a $0.35\pm0.03\,M_{\odot}$ BHB star and a likely $1.26\pm0.17\,M_{\odot}$ white dwarf (WD). The BHB star has an effective temperature of $15{,}524\pm310$ K and a luminosity of $39.7\pm4.1\,L_{\odot}$. Stellar evolution modelling indicates that it is a helium-shell-burning star produced through the common-envelope channel, retaining a hydrogen-rich envelope that is more massive than previously thought for low-mass stars. This finding provides direct evidence for binary interaction in the formation of BHB stars, offering a fresh perspective on interpreting this emerging population.

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Jiao Li, Changqing Luo, Hai-Liang Chen, Zhicun Liu, Bo Zhang, Shi Jia, Hongwei Ge, Tao Wu, Yuhan Yao, Pei Wang, Marat Gilfanov, You Wu, Zhenwei Li, Zhengwei Liu, Xiangcun Meng, Xue-Fei Chen, Philipp Podsiadlowski, Chao Liu, Zhan-Wen Han. 2026-08-12. A Helium-shell Burning Blue Horizontal Branch Star Produced from Common Envelope Evolution. https://arxiv.org/abs/2608.11557

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