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

A variation of the relative content of (Fe+Ni) versus Ni 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 of parametrised Chandrasekhar-mass explosion models of constant kinetic energy, where the ejecta are divided into an inner NSE zone, composed of (Fe+Ni) inside and Ni outside, an outer zone with Intermediate Mass Elements and a CO zone. Both the size of the NSE zone and the fraction of (Fe+Ni) v. Ni are varied systematically. Models with the same original NSE content but different (Fe+Ni)/Ni 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+Ni)/Ni ratio. While the Ni mass and the total NSE mass are the dominant parameters determining the peak luminosity and the shape of the light curve, respectively, a variation in the (Fe+Ni)/Ni ratio, which may depend on the metallicity of the progenitor (Timmes, Brown & Truran 2003) is likely to account for a significant part of the observed scatter of local SNe Ia about the mean brightness–decline rate relation.