2026ApJ...997...96M

SN 2020eyj is the first Type Ia supernova (SN Ia) showing the signature of compact helium-rich circumstellar material (CSM). Such a large amount of CSM is difficult to explain in a single-degenerate scenario where the donor star is a helium star. Here we show that, under certain conditions, it is possible that the transfer of helium leads to a common envelope (CE) engulfing the system, similar to the CE wind model proposed by X. Meng & P. Podsiadlowski. If in such a helium CE wind model the initial white dwarf mass is larger than 1.1 M_⊙ and the helium star is more massive than 1.8 M_⊙, the mass of a helium CE can be larger than 0.3 M_⊙ prior to supernova explosion. The CE mass heavily depends on the initial parameters of the binary system. A dynamical CE ejection event could occur shortly before the supernova, and then our model may naturally explain the properties of SN 2020eyj, specifically the massive He-rich CSM, and its dim peak brightness, low ejecta velocity, and low birth rate.