The Peculiar Type Ic Supernova 1997ef: Another Hypernova

Abstract

SN 1997ef has been recognized as a peculiar supernova from its light curve\nand spectral properties. The object was classified as a Type Ic supernova (SN\nIc) because its spectra are dominated by broad absorption lines of oxygen and\niron, lacking any clear signs of hydrogen or helium line features. The light\ncurve is very different from that of previously known SNe Ic, showing a very\nbroad peak and a slow tail. The strikingly broad line features in the spectra\nof SN 1997ef, which were also seen in the hypernova SN 1998bw, suggest the\ninteresting possibility that SN 1997ef may also be a hypernova. The light curve\nand spectra of SN 1997ef were modeled first with a standard SN~Ic model\nassuming an ordinary kinetic energy of explosion $E_{\\rm K} = 10^{51}$ erg. The\nexplosion of a CO star of mass $M_{\\rm CO} \\approx 6 M_\\odot$ gives a\nreasonably good fit to the light curve but clearly fails to reproduce the broad\nspectral features. Then, models with larger masses and energies were explored.\nBoth the light curve and the spectra of SN 1997ef are much better reproduced by\na C+O star model with $E_{\\rm K} =$ 8 \\e{51} erg and $M_{\\rm CO} = 10 M_\\odot$.\nTherefore, we conclude that SN 1997ef is very likely a hypernova on the basis\nof its kinetic energy of explosion. Finally, implications for the deviation\nfrom spherical symmetry are discussed in an effort to improve the light curve\nand spectral fits.\n