Supernovae interacting with Si and S-rich circumstellar matter from double white dwarf mergers

We present that supernovae interacting with a dense Si and S-rich circumstellar matter like SN 2021yfj can originate from mergers of two white dwarfs. A C+O white dwarf accreting He from its non-degenerate He companion star can initiate a C burning frame at its surface propagating inward under certain conditions. Such a burning frame synthesizes intermediate mass elements such as Si and S, forming a hybrid WD with an outer Si+S-rich layer. After the He star companion becomes a white dwarf, the two white dwarfs can eventually merge. During the merger, the outer layers of the hybrid white dwarf can be tidally stripped, forming a dense Si and S-rich circumstellar matter. If a thermonuclear explosion is triggered in the merging white dwarfs, an explosion within a dense Si and S-rich circumstellar matter can be realized, resulting in SN 2021yfj-like events. We argue that the properties of SN 2021yfj can be reproduced by a dense Si and S-rich circumstellar matter having ~ 0.3 Msun within which an explosion having kinetic energy of ~ 4e50 erg and ejecta mass of ~ 0.3 Msun occurred. These properties are consistent with the double white dwarf merger scenario. This scenario can naturally explain the existence of He observed in SN 2021yfj. Because white dwarf mergers can also lead to the formation of He and C+O dense circumstellar matter, some Type Ibn and Icn supernovae may also originate from a similar evolutionary path.