2023MNRAS.521.4561S

We argue that the core-degenerate (CD) scenario of type Ia supernovae (SNe Ia) can explain the compact helium-rich circumstellar material (CSM) of SN 2020eyj. In the new channel of the CD scenario, we propose that there are two major common envelope evolution (CEE) phases. After the white dwarf (WD) companion removes the hydrogen-rich envelope of the asymptotic giant branch star, its spiralling-in halts at few solar radii from the core rather than continuing to the carbon oxygen (CO) core as in the hydrogen-rich SNe Ia-CSM CD scenario. Only hundreds to tens of thousands of years later, after the helium-rich core expands, does the WD enters a CEE with the helium-rich layer. By that time, the hydrogen-rich envelope is at a large distance from the centre. The WD merges with the CO core during the second CEE phase, and only after a merger to explosion delay (MED) time of weeks to tens of years, the merger remnant explodes. The SN Ia ejecta collides with a helium-rich CSM at tens to hundreds of AU. We follow the evolution of two stellar models with initial masses of 5M_⊙ and 7M_⊙ to their asymptotic giant branch phase when they are supposed to engulf the WD companion. We find that there is a sufficiently massive CO core to merge with the WD in the frame of the CD scenario as well as a massive helium-rich layer, ≃ 0.3-1M_⊙, to account for the helium-rich CSM of SN 2020eyj.