A luminous, blue progenitor system for the type Iax supernova 2012Z

The detection of the luminous, blue progenitor system of the type Iax supernova 2012Z suggests that this supernova was the explosion of a white dwarf accreting material from a helium-star companion. A type Iax supernova progenitor SN 2012Z, discovered in the Lick Observatory Supernova Search on 29 January 2012, is a type Iax supernova. Sometimes referred to as 'mini supernovae', these are initially spectroscopically similar to some type-Ia supernovae but diverge with time and are much less energetic and fainter. It is not clear what triggers a type Iax explosion. This paper reports the detection of a progenitor in deep observations of NGC 1309, the host galaxy of SN 2012Z, obtained with the Hubble Space Telescope and including the location of the supernova before its explosion. Its optical properties and similarity to the progenitor of the helium nova V445 Puppis suggest that SN 2012Z was probably an explosion of a white dwarf accreting from a helium-star companion. Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities 1 , 2 . They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae 3 , 4 , 5 , 6 , but their composition (dominated by iron-group and intermediate-mass elements 1 , 7 ) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate 1 . The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon–oxygen white dwarfs that do not completely unbind the star 8 , 9 , 10 , implying that they are ‘less successful’ versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis 11 , 12 , 13 suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star 14 , 15 .