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

• Published in: The Astrophysical journal Vol. 964; no. 1; pp. 22 - 28
• Main Authors: Li, Zhenwei, Zhang, Yangyang, Chen, Hailiang, Ge, Hongwei, Jiang, Dengkai, Li, Jiangdan, Chen, Xuefei, Han, Zhanwen
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.03.2024; IOP Publishing
• Subjects: Asymptotic giant branch; Asymptotic giant branch stars; B stars; Binary stars; Common envelope evolution; Ejection; Helium; Horizontal branch stars; K lines; Red giant stars; Stars; Stellar evolution; Subdwarf stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad2206

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 log T eff – log g plane may explain a fraction of the observational samples with high log T eff and low log g . Considering the wind mass loss of sdO/Bs, the model could produce helium-rich hot subdwarfs with log ( n He / n H ) ≳ − 1 .