A New Route to Massive Hot Subdwarfs: Common Envelope Ejection from Asymptotic Giant Branch Stars

Abstract

The hot subdwarf O/B stars (sdO/Bs) are known as extreme horizontal branch stars, which is of great importance in stellar evolution theory. The sdO/Bs are generally thought to have a helium-burning core and a thin hydrogen envelope ( M _env < 0.02 M _⊙ ). In the canonical binary evolution scenario, sdO/Bs are considered to be the stripped cores of red giants. However, such a scenario cannot explain the recently discovered sdO/B binary SMSS J1920, where the strong Ca H and K lines in the spectrum are found. It suggests that this binary likely originated from the recent ejection of the common envelope (CE). In this work, we propose a new formation channel of massive sdO/Bs, namely, sdO/Bs produced from a CE ejection process with an asymptotic giant branch (AGB) star (hereafter the AGB CE channel). We constructed the evolutionary model of sdO/Bs and successfully explained most of the important observed parameters of the sdO/B star in SMSS J1920, including the evolutionary age, sdO/B mass, effective temperature, surface gravity, and surface helium abundance. The minimum sdO/B mass produced from the AGB CE channel is about 0.48 M _⊙ . The evolutionary tracks in the $\mathrm{log}{T}_{\mathrm{eff}}\mbox{--}\mathrm{log}g$ plane may explain a fraction of the observational samples with high $\mathrm{log}{T}_{\mathrm{eff}}$ and low $\mathrm{log}g$ . Considering the wind mass loss of sdO/Bs, the model could produce helium-rich hot subdwarfs with $\mathrm{log}({n}_{\mathrm{He}}/{n}_{{\rm{H}}})\gtrsim -1$ .

Keywords

• Subdwarf stars
• Common envelope evolution
• Asymptotic giant branch
• Binary stars