Modeling the iron-dominated spectra of the type IA supernova SN 1991T at premaximum

SN 199IT in NGC 4527, a galaxy of the Virgo Cluster, has been an unusual Type Ia supernova, being at least one magnitude brighter at maximum than other SN Ia's in Virgo. Unlike SN 1990N, another recent Type Ia SN observed also at premaximum, this supernova has not shown in this phase lines of Si II or Ca II. To study the composition of the ejecta an approximate NLTE treatment of excitation and ionization based on the Sobolev escape probability concept has been developed and incorporated in the Monte Carlo transfer code of Lucy. This spectral synthesis method has been applied to analyze the premaximum observations taken at ESO-La Silla, which are also presented and discussed in this work. The analysis of the optical spectra indicates that the outermost layers of SN 199IT have undergone complete burning, giving a composition rich in Fe-peak elements (Fe is clearly detected and Ni seems to be present as well), without any evidence of intermediate-mass elements such as Si, Ca or O. This can be interpreted in terms of a fast deflagration that induced a detonation near the surface of the star that burned the outermost layers into nuclear statistical equilibrium (NSE). Comparison of the observations with delayed-detonation models show significant differences in chemical composition from that predicted.