The detonation of a sub-Chandrasekhar-mass white dwarf at the origin of the low-luminosity type Ia supernova 1999 by

Abstract

While Chandrasekhar-mass (MCh) models with a low 56Ni yield can match the peak lumi- nosities of fast-declining, 91bg-like Type Ia supernovae (SNe Ia), they systematically fail to reproduce their faster light-curve evolution. Here we illustrate the impact of a low ejecta mass on the radiative display of low-luminosity SNe Ia, by comparing a sub-MCh model resulting from the pure central detonation of a C-O White Dwarf (WD) to a MCh delayed-detonation model with the same 56Ni yield of 0.12 M⊙. Our sub-MCh model from a 0.90 M⊙ WD progen- itor has a ∼ 5 d shorter rise time in the integrated UV-optical-IR (uvoir) luminosity, as well as in the B-band, and a ∼ 20 per cent higher peak uvoir luminosity (∼ 1 mag brighter peak MB ). This sub-MCh model also displays bluer maximum-light colours due to the larger specific heating rate, and larger post-maximum uvoir and B-band decline rates. The luminosity decline at nebular times is also more pronounced, reflecting the enhanced escape of gamma rays result- ing from the lower density of the progenitor WD. The deficit of stable nickel in the innermost ejecta leads to a notable absence of forbidden lines of [Ni ii] in the nebular spectra. In contrast, the MCh model displays a strong line due to [Ni ii] 1.939 μm, which could in principle serve to distinguish between different progenitor scenarios. Our sub-MCh model offers an unprece- dented agreement with optical and near-infrared observations of the 91bg-like SN 1999by, making a strong case for a WD progenitor significantly below the Chandrasekhar-mass limit for this event and other low-luminosity SNe Ia.