On the type Ia supernovae 2007on and 2011iv: evidence for Chandrasekhar-mass explosions at the faint end of the luminosity–width relationship

Radiative transfer models of two transitional type Ia supernova (SNe Ia) have been produced using the abundance stratification technique. These two objects --designated SN 2007on and SN 2011iv-- both exploded in the same galaxy, NGC 1404, which allows for a direct comparison. SN 2007on synthesised 0.25 M_ of ⁵⁶Ni and was less luminous than SN 2011iv, which produced 0.31 M_ of ⁵⁶Ni. SN 2007on had a lower central density (ρc) and higher explosion energy (Eₖᵢₙ ~ 1.3±0.3×10⁵¹erg) than SN 2011iv, and it produced less nuclear statistical equilibrium (NSE) elements (0.06 M_). Whereas, SN 2011iv had a larger ρc, which increased the electron capture rate in the lowest velocity regions, and produced 0.35 M_ of stable NSE elements. SN 2011iv had an explosion energy of Eₖᵢₙ ~ 0.9 ±0.2×10⁵¹erg. Both objects had an ejecta mass consistent with the Chandrasekhar mass (Ch-mass), and their observational properties are well described by predictions from delayed-detonation explosion models. Within this framework, comparison to the sub-luminous SN 1986G indicates SN 2011iv and SN 1986G have different transition densities (ρₜᵣ) but similar ρc. Whereas, SN 1986G and SN 2007on had a similar ρₜᵣ but different ρc. Finally, we examine the colour-stretch parameter sBV vs. Lₘₐₓ relation and determine that the bulk of SNe Ia (including the sub-luminous ones) are consistent with Ch-mass delayed-detonation explosions, where the main parameter driving the diversity is ρₜᵣ. We also find ρc to be driving the second order scatter observed at the faint end of the luminosity-width relationship.