Stars with initial masses such that 10 ≤ Minitial ≤ 100, where is the solar mass, fuse progressively heavier elements in their centres, until the core is inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion—an iron-core-collapse supernova1,2. By contrast, extremely massive stars with Minitial ≥ 140 (if such exist) develop oxygen cores with masses, Mcore, that exceed 50, where high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron–positron pairs occurs before oxygen ignition and leads to a violent contraction which triggers a nuclear explosion3,4,5 that unbinds the star in a pair-instability supernova. Transitional objects with 100 < Minitial < 140 may end up as iron-core-collapse supernovae following violent mass ejections, perhaps as a result of brief episodes of pair instability, and may already have been identified6,7,8. Here we report observations of supernova SN 2007bi, a luminous, slowly evolving object located within a dwarf galaxy. We estimate the exploding core mass to be Mcore ≈ 100, in which case theory unambiguously predicts a pair-instability supernova. We show that >3 of radioactive 56Ni was synthesized during the explosion and that our observations are well fitted by models of pair-instability supernovae9,10. This indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic stellar mass limit11, which perhaps result from processes similar to those that created the first stars in the Universe.
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We gratefully acknowledge advice and help from E. Pian and discussions with Z. Barkat, E. Livne, E. Nakar, N. Langer and P. Podsiadlowski. This work benefited from useful interaction during the Fireworks meetings held at the Weizmann Institute (2008) and at the University of Bonn (2009). Work related to the CSS data reported here was supported by the US National Aeronautics and Space Administration (NASA) under a grant issued through the Science Mission Directorate Near-Earth Object Observations program. The joint work of A.G.-Y. and P.M. is supported by a Weizmann-Minerva grant. A.G.-Y. acknowledges support from the Israeli Science Foundation, a European Union Seventh Framework Programme Marie Curie IRG fellowship, the Benoziyo Center for Astrophysics, a research grant from the Peter and Patricia Gruber Awards, and the William Z. and Eda Bess Novick New Scientists Fund at the Weizmann Institute. P.E.N. is supported by the US Department of Energy’s Scientific Discovery through Advanced Computing programme. The A.V.F. group at the University of California, Berkeley is grateful for financial support from the US National Science Foundation, the US Department of Energy, the TABASGO Foundation, Gary and Cynthia Bengier, and the Richard and Rhoda Goldman Fund. J.D. is supported by the National Natural Science Foundation of China and by the Chinese 973 Program. This work is based in part on data from the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA; it was made possible by the generous financial support of the W. M. Keck Foundation. This work made use of the NASA/IPAC Extragalactic Database, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. R.C.T. is a Luis W. Alvarez Fellow at the Lawrence Berkeley National Laboratory. R.J.F is a Clay Fellow at the Harvard-Smithsonian Center for Astrophysics.
Author Contributions A.G.-Y. initiated, coordinated and managed the project, carried out photometric and spectroscopic analysis, and wrote the manuscript. P.M. was responsible for obtaining the Very Large Telescope late-time observations, carried out spectroscopic modelling and led the theoretical interpretation. E.O.O. led the Palomar photometry, obtained P200 and Keck observations, and performed the photometric calibration analysis. P.E.N. discovered SN 2007bi, identified its peculiarity and similarity to SN 1999as, initiated some of the early spectroscopic analysis and led the recovery of pre-discovery data from DeepSky and the Catalina Real-Time Transient Survey. S.R.K., M.M.K. and R.M.Q. obtained key late-time Keck spectra and helped with the P60 observations. A.V.F., S.B.C. and R.C. analysed early Keck data and contributed to manuscript preparation and editing, including final proofreading (A.V.F.). R.W. and D.K. carried out custom PISN modelling for comparison with the observations. M.S. undertook custom reduction of the key late-time Keck spectrum. E.C.B. is the principal investigator for the CSS, and his team acquired the CSS data and provided preliminary calibration of the results. A.J.D. helped recover CSS data and advised about their calibration. R.C.T. analysed early spectra using his automated SYNOW code. J.S.B., D.P. and A.A.M. obtained early spectroscopic observations of SN 2007bi as well as infrared observations using the Peters Automated Infrared Imaging Telescope, and contributed to analysis and manuscript editing. R.J.F. and J.M.S. contributed to spectral observations and reductions, and advised during manuscript preparation. I.A. helped with P60 photometry and calibration, and with manuscript editing. R.S.E. obtained Keck observations of SN 2007bi. J.D. contributed to the Very Large Telescope programme that resulted in the observations of SN 2007bi, and proofread the manuscript.
This file contains Supplementary Notes and Data, Supplementary Figures 4 - 6 with Legends, Supplementary Table 1 and Supplementary References.
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Nebular-phase Spectra of Superluminous Supernovae: Physical Insights from Observational and Statistical Properties
The Astrophysical Journal (2019)