The (54Fe+58Ni)/56Ni ratio as a second parameter for Type Ia supernova properties

A variation of the relative content of (Fe-54+Ni-58) versus Ni-56 may be responsible for the observed scatter of Type Ia supernovae (SNe Ia) about a mean relation between their intrinsic brightness and the shape of their light curve. Synthetic light curves are computed from parametrized Chandrasekhar-mass explosion models of constant kinetic energy, where the ejecta are divided into an inner nuclear statistical equilibrium (NSE) zone, composed of (Fe-54 + Ni-58) inside and Ni-56 outside, an outer zone with intermediate-mass elements, and a CO zone. Both the size of the NSE zone and the fraction of (Fe-54+Ni-58) versus Ni-56 are varied systematically. Models with the same original NSE content but different (Fe-54+Ni-58)/Ni-56 ratios reach different peak brightness but have similar light curve shapes. Synthetic spectra indicate that the V-band decline rate is not affected by the (Fe-54+Ni-58)/Ni-56 ratio. While the Ni-56 mass and the total NSE mass are the dominant parameters that determine the peak luminosity and the shape of the light curve, respectively, a variation in the (Fe-54+Ni-58)/Ni-56 ratio, which may depend on the metallicity of the progenitor is likely to account for a significant part of the observed scatter of local SNe Ia about the mean brightness - decline rate relation.