Off-centre ignition of sub-Chandrasekhar white dwarfs does not resolve the tension with the observed t0–MNi56 relation of type Ia supernovae

Abstract

Type Ia supernovae (SNe Ia) are likely the thermonuclear explosions of carbon-oxygen (CO) white-dwarf (WD) stars, but the exact nature of their progenitor systems remains uncertain. Recent studies have suggested that a propagating detonation within a thin helium shell surrounding a sub-Chandrasekhar mass CO core can subsequently trigger a detonation within the core (the double-detonation model, DDM). The resulting explosion resembles a central ignition of a sub-Chandrasekhar mass CO WD (SCD), which is known to be in tension with the observed t0 −MNi56 relation, where t0 (the γ-rays’ escape time from the ejecta) is positively correlated with MNi56 (the synthesized 56Ni mass). SCD predicts an anti-correlation between t0 and MNi56, with t0 ≈ 30 day for luminous (MNi56 ≳ 0.5 M⊙) SNe Ia, while the observed t0 is in the range of 35 − 45 day. In this study, we apply our recently developed numerical scheme to calculate in 2D the impact of off-center ignition in sub-Chandrasekhar mass CO WD, aiming to better emulate the behavior expected in the DDM scenario. Our calculations of the t0 − MNi56 relation, which do not require radiation transfer calculations, achieve convergence to within a few percent with a numerical resolution of ${\sim }1\, \rm {km}$. We find that the results only slightly depend on the ignition location, mirroring the SCD model, and consequently, the discrepancy with the observed t0-MNi56 relation remains unresolved.