The peculiar type Ic supernova 1997ef: Another hypernova

SN 1997ef has been recognized as a peculiar supernova from its light curve and spectral properties. The object was classified as a Type Ic supernova ( SN Ic) because its spectra were dominated by broad absorption lines of oxyg en and iron, lacking any clear signs of hydrogen or helium line features. T he light curve is very different from that of previously known SNe Ic, show ing a very broad peak and a slow tail. The strikingly broad line features i n the spectra of SN 1997ef, which were also seen in the hypernova SN 1998bw , suggest the interesting possibility that SN 1997ef may also be a hypernov a. The light curve and spectra of SN 1997ef were modeled first with a stand ard SN Ic model assuming an ordinary kinetic energy of explosion E-K = 10(5 1) ergs. The explosion of a CO star of mass M-CO approximate to 6 M-circle dot gives a reasonably good fit to the light curve but clearly fails to rep roduce the broad spectral features. Then, models with larger masses and ene rgies were explored. Both the light curve and the spectra of SN 1997ef are much better reproduced by a C+O star model with E-K = 8 x 10(51) ergs and M -CO = 10 M-circle dot. Therefore, we conclude that SN 1997ef is very likely a hypernova on the basis of its kinetic energy of explosion. Finally, impl ications for the deviation from spherical symmetry are discussed in an effo rt to improve the fits to the observations.