The Properties of Hypernovae : SNe Ic 1998 bw , 1997 ef

We discuss the properties of the hyper-energetic Type Ic supernovae (SNe Ic) 1998bw and 1997ef and Type IIn supernova (SN IIn) 1997cy. SNe Ic 1998bw and 1997ef are characterized by their large luminosity and the very broad spectral features. Their observed properties can be explained if they are very energetic SN explosions with the kinetic energy of EK ∼ > 1 × 10 52 erg, originating probably from the core collapse of the bare C+O cores of massive stars (∼ 30 − 40M⊙). At late times, both the light curves and the spectra suggest that the explosions may have been asymmetric; this may help us understand the claimed connection with GRB's. The Type IIn SN 1997cy is even more luminous than SN 1998bw and the light curve declines more slowly than 56 Co decay. We model such a light curve with circumstellar interaction, which requires the explosion energy of ∼ 3 × 10 52 erg. Because these kinetic energies of explosion are much larger than in normal core-collapse SNe, we call objects like these SNe " hypernovae ". The mass of 56 Ni in SN 1998bw is estimated to be as large as 0.5-0.7 M⊙ from both the maximum brightness and late time emission spectra, which suggests that the asymmetry may not be extreme.