A series of early-time optical spectra of the peculiar SNIa 1991T, obtained from 2 weeks before to 4 weeks after maximum, have been computed with our Monte Carlo code.

The earlier spectra can be successfully modelled if 56Ni and its decay products, 56Co and 56Fe, dominate the composition of the outer part of the ejecta. This atypical distribution confirms that the explosion mechanism in SN 1991T was different from a simple deflagration wave, the model usually adopted for SNe Ia.

As the photosphere moves further into the ejecta the Ni Co Fe fraction drops, while intermediate mass elements become more abundant. The spectra obtained 3–4 weeks after maximum look very much like those of the standard SN Ia 1990N. A mixed W7 composition produces good fits to these spectra, although Ca and Si are underabundant. Thus, in the inner parts of the progenitor white dwarf the explosion mechanism must have been similar to the standard deflagration model.

The fits were obtained adopting a reddening E(B – V) = 0.13. A Tully-Fisher distance modulus μ = 30.65 to NGC 4527 implies that SN 1991T was about 0.5 mag brighter than SN 1990N. At comparable epochs, the photosphere of SN 1991T was thus hotter than that of SN 1990N. The high temperature, together with the anomalous composition stratification, explains the unusual aspect of the earliest spectra of SN 1991T.

The model results allow us to follow the abundances as a function of mass. In particular, spectroscopic evidence is found that about 0.6M⊙ of 56Ni must have been synthesized in the outermost 1M⊙ of the exploding white dwarf. This implies that almost twice as much 56Ni was produced in SN 1991T than in normal SNe Ia, and explains the unusual brightness of this SN.