21 - On variations in the peak luminosities of Type Ia supernovae

Summary

Abstract

We explore whether the observed variations in the peak luminosities of Type Ia supernovae originate in part from a scatter in metallicity of the main-sequence stars that become white dwarfs. Previous, numerical, studies have not self-consistently explored metallicities greater than solar. One-dimensional, Chandrasekhar mass models of SNe Ia produce most of their ⁵⁶Ni in a burn to nuclear statistical equilibrium between the mass shells 0.2 M_⊙ and 0.8 M_⊙, for which the electron to nucleon ratio Y_e is constant during the burn. We show analytically that, under these conditions, charge and mass conservation constrain the mass of ⁵⁶Ni produced to depend linearly on the original metallicity of the white dwarf progenitor. This effect is most evident at metallicities greater than solar. Detailed post-processing of W7-like models confirms this linear dependence, and our calculations are in agreement with previous self-consistent calculations over the metallicity range common to both calculations. The observed scatter in the metallicity (1/3 Z_⊙-3 Z_⊙) of the solar neighborhood is enough to induce a 25% variation in the mass of ⁵⁶Ni ejected by Type Ia supernova and is sufficient to vary the peak V-band brightness by |ΔM_V| ≈ 0.2. This scatter in metallicity is present out to the limiting redshifts of current observations (z ≲ 1). Sedimentation of ²²Ne can possibly amplify the variation in ⁵⁶Ni mass to ≲50%. Further numerical studies can determine if other metallicity-induced effects, such as a change in the mass of the ⁵⁶Ni-producing region, offset or enhance the variation we identify.