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An Unusually Fast-Evolving Supernova

We Are Stardust Supernovae form as the result of stellar explosions and are classified according to the properties of their spectra. Poznanski et al. (p. 58, published online 5 November) present a peculiar supernova that is characterized by extremely fast temporal evolution and unusual spectroscopic features, such that it defies classification. SN2002bj appears to be a member of a new class of supernovae, possibly formed by a helium detonation on a white dwarf ejecting a small envelope of material. The distinctive properties of this supernova suggest that it is of a kind predicted by theory but not previously observed. Analyses of supernovae (SNe) have revealed two main types of progenitors: exploding white dwarfs and collapsing massive stars. Here we describe SN 2002bj, which stands out as different from any SN reported to date. Its light curve rose and declined very rapidly, yet reached a peak intrinsic brightness greater than −18 magnitude. A spectrum obtained 7 days after discovery shows the presence of helium and intermediate-mass elements, yet no clear hydrogen or iron-peak elements. The spectrum only barely resembles that of a type Ia SN, with added carbon and helium. Its properties suggest that SN 2002bj may be representative of a class of progenitors that previously has been only hypothesized: a helium detonation on a white dwarf, ejecting a small envelope of material. New surveys should find many such objects, despite their scarcity.