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Session 6 - Novae and Supernovae.
Display session, Monday, June 08
Atlas Ballroom,
SN 1991T was a spectroscopically well observed and conspicuously peculiar Type Ia event. We present results of an analysis of its photospheric--phase spectra using the parameterized supernova spectrum--synthesis code, SYNOW. We confirm the existence of freshly synthesized ^56Ni and its decay products in the outer layers of the ejecta as suggested by previous studies. However, we also present evidence in the pre--maximum--light spectra for C II \lambda6580, blueshifted by 14,500 - 17,000 km s^-1 (rather than Si II \lambda6355 with a much smaller blueshift). It is not clear whether the carbon and the freshly synthesized ^56Ni overlap in velocity space. Analysis of post--maximum spectra shows that silicon and sulfur were confined to an unusually narrow velocity interval, and that the maximum velocity of the inner iron--peak core was unusually high.
NGC 4527, the parent galaxy of SN 1991T, is assumed to be at the same distance as NGC 4536 and NGC 4496, the parent galaxies of the normal SNe Ia 1981B and 1960F, both of which have Cepheid--based distances. (This assumption is to be tested by the HST Supernova Consortium of Saha, Sandage, et al.) If so, the high peak luminosity of SN 1991T (M_bol \sim -20.3) probably requires that the mass ejection of SN 1991T was super-Chandrasekhar, presumably as the result of the merger of two white dwarfs. More powerful versions of the super--Chandrasekhar DETENV hydrodynamical models of Khokhlov, Müller, and Höflich (1993, Aamp;A, 270, 223) should be explored, to raise the minimum velocity of the carbon--rich zone in the models.
