Explosive nucleosynthesis in aspherical hypernova explosions and late-time spectra of SN 1998bw

Aspherical explosion models for the hypernova (hyperenergetic supernova) SN 1998bw are presented. Nucleosynthesis in aspherical explosions is examined with a two-dimensional hydrodynamical code and a detailed nuclear reaction network. Aspherical explosions lead to a strong alpha-rich freezeout, thus enhancing the abundance ratios [Ca-44, Ti-48, Zn-64/Fe] in the ejecta. The nebular line profiles of the Fe-dominated blend near 5200 Angstrom and of [O I] 6300, 6363 Angstrom are calculated and compared with the observed late-time spectra of SN 1998bw. Compared with the spherical model, the unusual features of the observed nebular spectra can be better explained if SN 1998bw is a strongly aspherical explosion with a kinetic energy of similar to10(5 2) ergs viewed from near the jet direction.