How much 56Ni can be produced in core-collapse supernovae?: Evolution and explosions of 30-100 M⊙ stars

Motivated by the discovery of extremely bright supernovae SNe 1999as and 2006gy, we have investigated how much Ni-56 mass can be synthesized in core-collapse massive supernovae ( SNe). We calculate the evolution of several very massive stars with initial masses M <= 100 M-circle dot from the main sequence to the beginning of the Fe core collapse and simulate their explosions and nucleosynthesis. In order to avoid complications associated with strong mass loss, we consider only metal-poor stars with initial metallicity Z=Z(circle dot)/200. However, our results are applicable to higher metallicity models with similar C+O core masses. The C+O core mass for the 100 M-circle dot model isMCO 42:6 M-circle dot, and this is the heaviest model in the literature for which Fe-core-collapse SN is explored. The synthesized Ni-56 mass increases with the increasing explosion energy and progenitor mass. For the explosion energy of E-51 E/10(51) ergs = 30, for example, the Ni-56 masses of M(Ni-56) 2.2, 2.3, 5.0, and 6.6 M-circle dot can be produced for the progenitors with initial masses of 30, 50, 80, and 100 M-circle dot (or C+O core masses M-CO 11.4, 19.3, 34.0, and 42.6 M-circle dot), respectively. We find that producing M(Ni-56) similar to 4 M-circle dot as seen in SN 1999as is possible for M-CO greater than or similar to 34 M-circle dot and E-51 greater than or similar to 20. Producing M(Ni-56) similar to 13 M-circle dot, as suggested for SN 2006gy, requires a too large explosion energy for M-CO less than or similar to 43 M-circle dot, but it may be possible with a reasonable explosion energy if M-CO less than or similar to 60 M-circle dot.