Difficulties of Two Exploding White Dwarfs to Account for Type Ia Supernovae with Bimodal Nebular Emission Profiles

We use a simple dynamical scheme to simulate the ejecta of type Ia supernova (SN Ia) scenarios with two exploding white dwarfs (WDs) and find that the velocity distribution of the ejecta has difficulties accounting for bimodal emission line profiles with a large separation between the two emission peaks. The essence of the dynamical code is in including the fact that the ejecta does not leave the system instantaneously. We find that the final separation velocity between the centers of masses of the two WDs’ ejecta is ≃80% of the pre-explosion WDs’ orbital velocity, i.e., we find separation velocities of 4200-5400 km s⁻¹ for two WDs of masses M₁ = M₂ = 0.94 M_⊙. The lower separation velocities we find challenge scenarios with two exploding WDs to explain bimodal emission line profiles with observed velocity separations of up to ≃7000 km s⁻¹. Only the mass in the ejecta of one WD with an explosion velocity lower than the separation velocity contributes to one peak of the bimodal profile; this is the inner ejecta. We find the inner ejecta to be only ≲15% of the ejecta mass in energetic explosions. Less energetic explosions yield higher inner mass but lower separation velocities. We encourage searching for alternative explanations of bimodal line profiles.