Origins of Slowly Fading Super-Luminous Supernovae

November 12, 2013

Super-luminous supernovae that radiate more than 1044 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1–4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140–260 times that of the Sun that now have carbon–oxygen cores of 65–130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron–positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56Ni are synthesized; this isotope decays to 56Fe via 56Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. In a Letter in Nature, researchers from several institutions, including the Harvard-Smithsonian Center for Astrophysics, report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Their early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling their observations with 10–16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10−6 times that of the core-collapse rate.

Prof. Christopher Stubbs is one of the authors of the Letter.

See M. Nicholl, S. J. Smartt, A. Jerkstrand, et al., "Slowly fading super-luminous supernovae that are not pair-instability explosions," Nature 502: 346–349 | doi:10.1038/nature12569