Multidimensional Nebular-Phase Calculations of Dynamically-Driven Double-Degenerate Double-Detonation Models for Type Ia Supernovae

The dynamically-driven double-degenerate double-detonation model has emerged as a promising progenitor candidate for Type Ia supernovae. In this scenario, the primary white dwarf ignites due to dynamical interaction with a companion white dwarf, which may also undergo a detonation. Consequently, two scenarios exist: one in which the secondary survives and another in which both white dwarfs detonate. In either case, substantial departures from spherical symmetry are imprinted on the ejecta. Here, we compute full non-local thermodynamic equilibrium nebular-phase spectra in 1D and 3D to probe the innermost asymmetries. Our simulations reveal that the multidimensional structures significantly alter the overall ionisation balance, width and velocity of features, especially when the secondary detonates. In this scenario, some element distributions may produce orientation-dependent line profiles that can be centrally peaked from some viewing-angles and somewhat flat-topped from others. Comparison to observations reveals that both scenarios produce most observed features from the optical to mid-infrared. However, the current model realisations do not consistently reproduce all line shapes or relative strengths, and yield prominent optical Ar III emission which is inconsistent with the data. When the secondary detonates, including 3D effects improves the average agreement with observations, however when compared to observations, particularly weak optical Co III emission and the presence of optical O I and near-infrared S I challenge its viability for normal Type Ia supernovae. Thus, overall, our comparisons with normal Type Ia's tentatively favour detonation of only the primary white dwarf, but we stress that more model realisations and mid-infrared observations are needed.