The Peculiar Type Ic Supernova 1997ef: Another Hypernova

SN 1997ef has been recognized as a peculiar supernova from its light curveand spectral properties. The object was classified as a Type Ic supernova (SNIc) because its spectra are dominated by broad absorption lines of oxygen andiron, lacking any clear signs of hydrogen or helium line features. The lightcurve is very different from that of previously known SNe Ic, showing a verybroad peak and a slow tail. The strikingly broad line features in the spectraof SN 1997ef, which were also seen in the hypernova SN 1998bw, suggest theinteresting possibility that SN 1997ef may also be a hypernova. The light curveand spectra of SN 1997ef were modeled first with a standard SN~Ic modelassuming an ordinary kinetic energy of explosion $E_{\rm K} = 10^{51}$ erg. Theexplosion of a CO star of mass $M_{\rm CO} \approx 6 M_\odot$ gives areasonably good fit to the light curve but clearly fails to reproduce the broadspectral features. Then, models with larger masses and energies were explored.Both the light curve and the spectra of SN 1997ef are much better reproduced bya C+O star model with $E_{\rm K} =$ 8 \e{51} erg and $M_{\rm CO} = 10 M_\odot$.Therefore, we conclude that SN 1997ef is very likely a hypernova on the basisof its kinetic energy of explosion. Finally, implications for the deviationfrom spherical symmetry are discussed in an effort to improve the light curveand spectral fits.Comment: "To appear in the Astrophysical Journal, Vol.534 (2000)