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

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 $0.35\pm 0.03M_{\odot }$ BHB star and a likely $1.26\pm 0.17M_{\odot }$ white dwarf (WD). The BHB star has an effective temperature of $15,524\pm 310$ K and a luminosity of $39.7\pm 4.1L_{\odot }$. 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.