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 HanAbstract
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 ± 0.03 M⊙ BHB star and a likely 1.26 ± 0.17 M⊙ white dwarf (WD). The BHB star has an effective temperature of 15, 524 ± 310 K and a luminosity of 39.7 ± 4.1 L⊙. 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.