Evidence for sub-Chandrasekhar-mass progenitors of Type Ia supernovae at the faint end of the width–luminosity relation

The faster light-curve evolution of low-luminosity Type Ia supernovae (SNe Ia) suggests that they could result from the explosion of white dwarf (WD) progenitors below the Chandrasekhar mass (M-Ch). Here we present 1D non-local thermodynamic equilibrium time-dependent radiative transfer simulations of pure central detonations of carbon-oxygen WDs with a mass (M-tot) between 0.88 and 1.15 M-circle dot and a Ni-56 yield between 0.08 and 0.84 M-circle dot. Their lower ejecta density compared to M-Ch models results in a more rapid increase of the luminosity at early times and an enhanced gamma-ray escape fraction past maximum light. Consequently, their bolometric light curves display shorter rise times and larger post-maximum decline rates. Moreover, the higher M(Ni-56)/M-tot ratio at a given Ni-56 mass enhances the temperature and ionization level in the spectrum-formation region for the less luminous models, giving rise to bluer colours at maximum light and a faster post-maximum evolution of the B - V colour. For sub-M-Ch models fainter than M-B approximate to -18.5 mag at peak, the greater bolometric decline and faster colour evolution lead to a larger B-band post-maximum decline rate, Delta M-15(B). In particular, all of our previously published M-Ch models (standard and pulsational delayed detonations) are confined to Delta M-15(B) < 1.4 mag, while the sub-M-Ch models with M-tot less than or similar to 1 M-circle dot extend beyond this limit to Delta M-15(B) approximate to 1.65 mag for a peak M-B approximate to -17 mag, in better agreement with the observed width-luminosity relation (WLR). Regardless of the precise ignition mechanism, these simulations suggest that fast-declining SNe Ia at the faint end of the WLR could result from the explosion of WDs whose mass is significantly below the Chandrasekhar limit.