LATE DETONATION MODELS FOR THE TYPE-IA SUPERNOVAE SN-1991T AND SN-1990N

We present hydrodynamical models of exploding white dwarfs to account for the newly observed premaximum features of Type la 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.