The electron-capture origin of supernova 2018zd
Van Dyk, Schuyler D.; Filippenko, Alexei V.; Kelly, Patrick L.; Smith, Nathan; Sand, David J.; Tominaga, Nozomu; Nomoto, Ken'ichi; Anand, Gagandeep S.; Andrews, Jennifer E.; Bostroem, K. Azalee; Smith, Paul S.; Hosseinzadeh, Griffin; Burke, Jamison; Hiramatsu, Daichi; Howell, D. Andrew; McCully, Curtis; Folatelli, Gastón; Maeda, Keiichi; Bersten, Melina C.; Brown, Peter J.; Arcavi, Iair; Moriya, Takashi J.; Valenti, Stefano; Goldberg, Jared A.; Dong, Yize; Xu, Chengyuan; Bilinski, Christopher; Williams, G. Grant; Noguchi, Toshihide; Itagaki, Koichi
United States, Japan, Australia, Israel, Canada, Argentina
Abstract
In the transitional mass range (~8-10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O + Ne + Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse1-4. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova with strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition5-7, explosion energy, light curve and nucleosynthesis8-12. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained by the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis and remnant populations in the transitional mass range.