The peculiar Type IA SN 1991T - Detonation of a white dwarf?

SN 1991T was a peculiar object whose premaximum optical spectrum did not resemble that of any known supernova; it appears to have been dominated by lines of iron-group elements. Near maximum brightness, however, lines of intermediate-mass elements slowly appeared, and the spectrum began to resemble that of Type Ia supernovae (SNs Ia). With time, the spectral similarity to classical SNs Ia grew progressively stronger. Two months after the explosion, the spectrum was once again dominated by iron-group elements and appeared almost identical to that of typical SNs Ia. At visual wavelengths, SN 1991T was probably > 0.6 mag more luminous than classical SNs Ia, but the shape of its light curve was reasonably normal. We suggest that SN 1991T was a double detonation of a white dwarf initiated at the boundary layer between the carbon-oxygen core and the helium envelope. Alternatively, the explosion may have been the result of a delayed detonation in a carbon-oxygen white dwarf.