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 velocities1,2. They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae3,4,5,6, but their composition (dominated by iron-group and intermediate-mass elements1,7) 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 rate1. The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon–oxygen white dwarfs that do not completely unbind the star8,9,10, 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 Puppis11,12,13 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 star14,15.
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We dedicate this paper to the memory of our friend and colleague Weidong Li, whose work on the Lick Observatory Supernova Search, SN 2002cx-like supernovae, and Hubble Space Telescope observations of supernova progenitors continues to inspire us. We thank the SH0ES team for assistance with data from HST programme GO-12880, E. Bertin for the development of the STIFF software to produce colour images, and A. Dolphin for software and guidance in photometry. This research at Rutgers University was supported through NASA/HST grant GO-12913.01 and National Science Foundation (NSF) CAREER award AST-0847157 to S.W.J.; NASA/HST grant GO-12999.01 to R.J.F. supported this work at the University of Illinois. At UC Santa Barbara, this work was supported by NSF grants PHY 11-25915 and AST 11-09174 to L.B. The Danish Agency for Science, Technology, and Innovation supported M.D.S. through a Sapere Aude Level 2 grant. Support for HST programmes GO-12913 and GO-12999 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
The authors declare no competing financial interests.
Extended data figures and tables
SN 2012Z1 is similar to SN 2005hk4 and SN 2008A6, all classified as type Iax supernovae. The SN 2012Z spectrum was taken on 2012 February 16 ut with the Whipple Observatory 1.5 m telescope (+FAST spectrograph) with a total exposure time of 1,800 s. Each flux density (Fλ) spectrum is labelled by its rest-frame phase past maximum light in B band. Prominent features due to intermediate-mass and iron-group elements are indicated; these features are also observed in luminous, slowly declining type Ia supernova spectra at maximum light, though with higher expansion velocities.
a, Chance alignment probability between a random position and a detected source within a square box of the given width (top axis, in arcsec; bottom axis, in pixels) centred at the position of SN 2012Z. This calculation is based on a 1σ position coincidence (0.0103″ = 0.2575 WFC3/UVIS pixels); a 3σ position coincidence gives probabilities approximately 9 times higher. The offset between SN 2012Z and S1 is 0.8σ. The shaded regions show a naive Poisson-like uncertainty estimate with a fractional error given by . b, Number of detected sources (including S1) as a function of the box size. The lines show results for all stellar sources (>3σ detection in any band; blue) and just those as bright as S1 (red).
The SN 2012Z progenitor system S1 (blue) is shown along with nearby stars (all with 1σ photometric uncertainties), with three isochrones24 with ages of 10.5 (dotted), 17.0 (dashed), and 41.7 Myr (solid). a, The F435W − F555W colour (roughly B − V), b, the F555W − F814W colour (roughly V − I), both plotted against the F555W (roughly V) absolute magnitude. The large, filled circles correspond to stars within 10 WFC3/UVIS pixels (0.4″) of the supernova location, small filled circles are within 20 pixels (0.8″), and the small open circles are within 30 pixels (1.2″). Objects in the grey shaded regions would not be detected given the depth of the combined images.
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McCully, C., Jha, S., Foley, R. et al. A luminous, blue progenitor system for the type Iax supernova 2012Z. Nature 512, 54–56 (2014). https://doi.org/10.1038/nature13615
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