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

Abstract

Radiative transfer models of two transitional type Ia supernovae (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 synthesized 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 (ρ_c) and higher explosion energy (E_kin ∼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 ρ_c, 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 ∼E_kin ∼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 (ρ_tr) but similar ρ_c. Whereas SN 1986G and SN 2007on had a similar ρ_tr but different ρ_c. Finally, we examine the colour-stretch parameter s_BV versus 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 ρ_tr. We also find ρ_c to be driving the second-order scatter observed at the faint end of the luminosity-width relationship.