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 $(M_{\rm env }<0.02M_\odot)$. 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$\&$K lines in the spectrum are found. It suggests that this binary is likely originated from the recent ejection of common envelope (CE). In this {work}, we proposed 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 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 $0.48M_\odot$. The evolutionary tracks in $\log T_{\rm eff}-\log g$ plane {may explain a fraction of the observational samples} with high-$\log T_{\rm eff}$ and low-$\log g$. Considering wind mass-loss of sdO/Bs, the model could produce helium-rich hot subdwarfs with $\log (n_{\rm He}/n_{\rm H})\gtrsim-1$.