Recently, Monthly Notices of the Royal Astronomical Society (MNRAS) published a research by the Binary Population Synthesis Group at Yunnan Observatories (YNAO), Chinese Academy of Sciences. The study revealed the ubiquitous presence of circumbinary matter ejected during the common envelope (CE) phase around hot subdwarf (sdB) stars, providing crucial observational evidence for understanding CE evolution in binary systems.
Binary stars are common in the universe, comprising nearly half of all stars. The CE phase is a critical stage in their evolution, determining their final fate. When one star expands and engulfs its companion, a common envelope forms around both stars. This envelope may eventually be ejected, leaving behind a short-period binary, or the stars may merge. First proposed by Paczyński in 1976, this process has remained elusive due to its transient and complex nature, with direct observational evidence lacking for decades.
This study analyzed low-resolution spectroscopic data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), focusing on 727 sdB candidates. Notably, around 20% exhibited Ca II K absorption lines exceeding interstellar medium (ISM) expectations, indicating the presence of significant circumstellar material. This material is likely remnants of common envelope ejecta. It persists throughout the sdB stars' lifetime, forming a long-lived circumbinary environment. By estimating its mass and radius, the team inferred that the circumbinary material extends between 10000 and 30000 AU, with an anisotropic distribution concentrated near the equatorial plane of the binary system.
This work was conducted by LI Jiangdan, HAN Zhanwen, CHEN Xuefei Chen from YNAO and Christian Wolf from ANU, along with their collaborators. This work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, the International Centre of Supernovae, Yunnan Key Laboratory, and the "Yunnan Revitalization Talent Support Program" Science & Technology Champion Project.
Figure 1, correlation between the equivalent width of the Ca II K line and the reddening extinction coefficient. The typical linear relationship between the Ca II K absorption in the interstellar medium and the reddening coefficient allows differentiation between absorption features originating from the interstellar medium and those from circumstellar material. Image by LI.
Figure 2, relationship between circumstellar material and the evolutionary stages of hot subdwarf stars. The effective temperature–surface gravity diagram shows no significant correlation between the presence of circumbinary material and the evolutionary stage of hot subdwarfs. This suggests that the material ejected during the common envelope phase may persist throughout the hot subdwarf’s lifetime. Image by LI.
Contact:
LI Jiangdan
Yunnan Observatories , CAS
Email: lijiangdan@ynao.ac.cn
