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

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 t(0)-M-Ni56 relation, where t(0) (the gamma-rays' escape time from the ejecta) is positively correlated with M-Ni56 (the synthesized Ni-56 mass). SCD predicts an anticorrelation between t(0) and M-Ni56, with t(0)approximate to 30day for luminous (M-Ni56 greater than or similar to 0.5M(circle dot)) SNe Ia, while the observed t(0) is in the range of 35-45day. In this study, we apply our recently developed numerical scheme to calculate in 2D the impact of off-centre ignition in sub-Chandrasekhar mass CO WD, aiming to better emulate the behaviour expected in the DDM scenario. Our calculations of the t(0)-M-Ni56 relation, which do not require radiation transfer calculations, achieve convergence to within a few per cent with a numerical resolution of similar to 1km. We find that the results only slightly depend on the ignition location, mirroring the SCD model, and consequently, the discrepancy with the observed t(0)-M-Ni56 relation remains unresolved.