Sub-Chandrasekhar-mass detonations are in tension with the observed t₀-M_Ni56 relation of type Ia supernovae

Type Ia supernovae (SNe Ia) are likely the thermonuclear explosions of carbon–oxygen (CO) white-dwarf (WD) stars, but their progenitor systems remain elusive. 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 outcome of this explosion is similar to a central ignition of a sub-Chandrasekhar mass CO WD (SCD). While SCD is consistent with some observational properties of SNe Ia, several computational challenges prohibit a robust comparison to the observations. We focus on the observed t0−MNi56 relation, where t0 (the γ-rays’ escape time from the ejecta) is positively correlated with MNi56 (the synthesized 56Ni mass). We apply our recently developed numerical scheme to calculate SCD and show that the calculated t0−MNi56 relation, which does not require radiation transfer calculations, converges to an accuracy of a few per cent. We find a clear tension between our calculations and the observed t0−MNi56 relation. SCD predicts an anticorrelation between t0 and MNi56, with t0≈30d for luminous (⁠MNi56≳0.5M⊙⁠) SNe Ia, while the observed t0 is in the range of 35−45d⁠. We show that this tension is larger than the uncertainty of the results, and that it exists in all previous studies of the problem. Our results hint that more complicated models are required, but we argue that DDM is unlikely to resolve the tension with the observations.