Double-detonation Models for Type Ia Supernovae: Trigger of Detonation in Companion White Dwarfs and Signatures of Companions’ Stripped-off Materials.

We have studied double-detonation explosions in double-degenerate (DD) systems with different companion white dwarfs (WDs) for modeling Type Ia supernovae (SNe Ia) by means of high-resolution smoothed particle hydrodynamics (SPH) simulations. We have found that only the primary WDs explode in some of the DD systems, while the explosions of the primary WDs induce the explosions of the companion WDs in the other DD systems. The former case is a so-called dynamically-driven double-degenerate double-detonation (D⁶) explosion, or helium-ignited violent merger explosion. The SN ejecta of the primary WDs strip materials from the companion WDs, whose mass is ∼10⁻³M_⊙. The stripped materials contain carbon and oxygen when the companion WDs are carbon–oxygen (CO) WDs with He shells ≲0.04 M_⊙. Since they contribute to low-velocity ejecta components as observationally inferred for iPTF14atg, D⁶ explosions can be counterparts of subluminous SNe Ia. The stripped materials may contribute to low-velocity C seen in several SNe Ia. In the latter case, the companion WDs explode through He detonation if they are He WDs and through the double-detonation mechanism if they are CO WDs with He shells. We name these explosions “triple” and “quadruple” detonation (TD/QD) explosions after the number of detonations. The QD explosion may be counterparts of luminous SNe Ia, such as SN 1991T and SN 1999aa, since they yield a large amount of ⁵⁶Ni, and their He-detonation products contribute to the early emissions accompanying such luminous SNe Ia. On the other hand, the TD explosion may not yield a sufficient amount of ⁵⁶Ni to explain luminous SNe Ia. [ABSTRACT FROM AUTHOR]