Late detonation models for the type IA supernovae SN 1991T and SN 1990N

We present hydrodynamical models of exploding white dwarf's to account for the newly observed premaximum features of Type Ia supernovae 1990N and 1991T in a unified manner. In these models, a carbon deflagration, producing a central Fe/Co/Ni core and an intermediate Si/S/Ca layer, is later transformed into a detonation in the outermost layer. Depending on the transition density, the nucleosynthesis in such late detonations results in the formation of Fe-dominated surface layers or Si/Fe-rich surface layers, which are qualitatively consistent with the inferred composition of SN 1991T and 1990N, respectively.