Supernovae come in two flavors: The type Ia are thermonuclear explosions of white dwarfs, whereas the types Ib, Ic, and II are produced by core collapse in massive stars. The different types of supernovae are distinguished by their spectra and brightness variations over time.
Benetti et al. noticed that one of the brightest supernovae ever observed, SN 1991D, has spectral characteristics of type Ia and types Ib or Ic. The progenitor may have been a binary system consisting of a white dwarf and a low-mass helium star, and it is possible that the white dwarf exploded within the helium envelope of its companion or while the white dwarf was still spiraling into the companion. This rare combination of events would explain the mixed spectral signals and could also have produced an extremely high luminosity as the explosion interacted with the helium envelope. In addition to aiding our understanding of the progenitors and mechanisms of stellar explosions, which are relevant to star formation and nucleosynthesis, the brightest type Ia supernovae are used to estimate the age, geometry, and expansion rate of the universe. Bright peculiar supernovae, such as SN 1991D, could complicate these estimates because they do not fit the standard model of type Ia explosions. -- LR
Mon. Not. R. Astron. Soc. 336, 91 (2002).
