2008ApJ...673.1014U

Motivated by the discovery of extremely bright supernovae SNe 1999as and 2006gy, we have investigated how much ⁵⁶Ni 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_☉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_☉/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_ ☉_ model is M_CO=42.6 M_☉, and this is the heaviest model in the literature for which Fe-core-collapse SN is explored. The synthesized ⁵⁶Ni mass increases with the increasing explosion energy and progenitor mass. For the explosion energy of E₅₁≡E/10⁵¹ ergs=30, for example, the ⁵⁶Ni masses of M(⁵⁶Ni)=2.2, 2.3, 5.0, and 6.6 M_☉can be produced for the progenitors with initial masses of 30, 50, 80, and 100 M_☉(or C+O core masses M_CO=11.4, 19.3, 34.0, and 42.6 M_☉), respectively. We find that producing M(⁵⁶Ni)∼4 M_☉ as seen in SN 1999as is possible for M_CO≳34 M_☉ and E₅₁≳20. Producing M(⁵⁶Ni)∼13 M_☉, as suggested for SN 2006gy, requires a too large explosion energy for M_CO≲43 M_☉, but it may be possible with a reasonable explosion energy if M_CO≳60 M_☉.