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-circle dot of Ni-56 and was less luminous than SN 2011iv, which produced 0.31 M-circle dot of Ni-56. SN 2007on had a lower central density (rho(c)) and higher explosion energy (E-kin similar to 1.3 +/- 0.3x10(51)erg) than SN 2011iv, and it produced less nuclear statistical equilibrium (NSE) elements (0.06 M-circle dot). Whereas, SN 2011iv had a larger rho(c), which increased the electron capture rate in the lowest velocity regions, and produced 0.35 M-circle dot of stable NSE elements. SN 2011iv had an explosion energy of similar to E-kin similar to 0.9 +/- 0.2x10(51)erg. 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 (rho(tr)) but similar rho(c). Whereas, SN 1986G and SN 2007on had a similar rho(tr) but different rho(c). Finally, we examine the colour-stretch parameter sBV vs. L-max 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 rho(tr). We also find rho(c) to be driving the second order scatter observed at the faint end of the luminosity-width relationship.