SN 1991T - Further evidence of the heterogeneous nature of type IA supernovae

Optical spectra of SN 1991T obtained approximately one week before B maximum showed no evidence of the Si, Ca, and S absorption lines which normally dominate the spectra of type Ia supernovae at early phases. Nevertheless, within two weeks after maximum, the spectrum of SN 1991T had evolved to resemble closely that of other type Ia events. These observations suggest that the abundances of Si, Ca, and S in the outer ejecta of SN 1991T were unusually low. The B and V light curves of SN 1991T resembled those of other type Ia supernovae, but the slope of the initial decline following maximum was less steep than normal in both colors. In spite of these spectroscopic and photometric peculiarities the absolute magnitude of SN 1991T at B maximum was apparently similar to that of a typical type Ia event. Thus, although the data suggest that SN 1991T was a type Ia supernova with peculiar surface abundances, in most other ways it seems to have been a completely normal type Ia explosion.