ANALYSIS OF THE PHOTOSPHERIC EPOCH SPECTRA OF TYPE-IA SUPERNOVAE SN-1990N AND SN-1991T

An analysis of the photospheric epoch optical and UV spectra (obtained with the IUE satellite) of Type Ia supernovae SN 1990N and SN 1991T is presented. To perform the analysis, synthetic spectra calculated using an LTE radiative transfer procedure and white dwarf explosion models of Type la supernovae were fitted to the observations. For both supernovae, the observed spectra are consistent with a white dwarf explosion model that possesses a core of newly synthesized iron peak elements, a core boundary moving at approximately 9000 km s-1, and an envelope with a composition of intermediate-mass elements. In the well known white dwarf explosion model W7, the inner envelope (i.e., matter with velocities in the range approximately 9000-15,000 km s-1) consists of newly synthesized elements produced as the burning front of the explosion died out. For SN 1990N the observed spectra are consistent with a composition of the inner envelope that is like that of model W7. For SN 1991T the observed spectra suggest that the inner envelope of this supernova has silicon underabundant by a factor of order 3 and calcium underabundant by a factor of order 10 relative to model W7. The near-UV spectra (2500-3500 angstrom) for both supernovae are best fitted by assuming that there are iron peak elements in the ejecta moving at velocities greater than 15,000 km s-1 (i.e., in the outer envelope) and that these iron peak elements are dominated by newly synthesized Ni-Co. This result is inconsistent with the model W7 prediction of an outer envelope consisting of unburned matter dominated by carbon and oxygen. The absolute abundance of intermediate-mass elements in the outer ejecta is quite uncertain for both supernovae, but spectrum fitting shows that abundances higher than solar are probable and suggests that silicon, sulfur, and calcium are underabundant in SN 1991T relative to SN 1990N by factors of order 3, 3, and 10, respectively. In the case of SN 1990N, there are ejecta moving at a velocity of approximately 40,000 km s-1 and newly synthesized material moving perhaps as fast as 25,000 km s-1. SN 1991T has matter moving at least as fast as approximately 20,000 km s-1. Model W7 has no ejecta moving faster than 22,027 km s-1. Some variation on model W7 or the delayed/late detonation models for Type Ia supernovae may be able to explain the high velocities required for SN 1990N and the newly synthesized elements that are probably present in the outer ejecta of both supernovae. The synthetic spectra calculated for the analysis and the expanding photosphere method are used for absolute B maximum and distance determinations for SN 1990N and SN 1991T.