A massive star origin for an unusual helium-rich supernova in an elliptical galaxy

The novel properties of the faint supernova SN 2005E mean that it does not fit readily into the established supernova categories. Types Ib, Ic and II, core-collapse supernovae, are thought to form when a massive star explodes at the end of its life, and type Ia as a result of the thermonuclear explosion of an accreting white dwarf. From spectroscopic data, Perets et al. conclude that SN 2005E is helium rich, like a type Ib, and lacks the hydrogen, silicon and sulphur spectral lines typical of type Ia. But based on its presence in an 'old' stellar environment, and with a low derived ejected mass, they argue against a core-collapse origin and for an origin from a low-mass, old progenitor, probably a helium-accreting white dwarf in a binary system. Kawabata et al. see it differently. SN 2005E resembles SN 2005cz, they say, a type Ib supernova that is unusual in being found in an elliptical galaxy. Both SN 2005E and SN 2005cz, they suggest, are best explained as products of the core collapse of massive stars at the low (6–12 solar mass) end of massiveness. In the accompanying News & Views, David Branch discusses these two models in the context of the latest thinking on how stars explode.