Motivated by the discovery of extremely bright supernovae SNe1999as and 2006gy , we have investigated how much ^ { 56 } Ni mass can be synthesized in core-collapse massive supernovae ( SNe ) .
We calculate the evolution of several very massive stars with initial masses M \leq 100 M _ { \odot } 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 only consider metal-poor stars with initial metallicity Z = Z _ { \odot } / 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 _ { \odot } model is M _ { CO } = 42.6 M _ { \odot } and this is the heaviest model in the literature for which Fe-core collapse SN is explored .
The synthesized ^ { 56 } Ni mass increases with the increasing explosion energy and progenitor mass .
For the explosion energy of E _ { 51 } \equiv E / 10 ^ { 51 } erg =30 , for example , the ^ { 56 } Ni masses of M ( ^ { 56 } Ni ) = 2.2 , 2.3 , 5.0 , and 6.6 M _ { \odot } can be produced for the progenitors with initial masses of 30 , 50 , 80 and 100 M _ { \odot } ( or C+O core masses M _ { CO } = 11.4 , 19.3 , 34.0 and 42.6 M _ { \odot } ) , respectively .
We find that producing M ( ^ { 56 } Ni ) \sim 4 M _ { \odot } as seen in SN1999as is possible for M _ { CO } \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { % $ > $ } } 34 M _ { \odot } and E _ { 51 } \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ > $ } % } 20 .
Producing M ( ^ { 56 } Ni ) \sim 13 M _ { \odot } as suggested for SN2006gy requires a too large explosion energy for M _ { CO } \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { % $ < $ } } 43 M _ { \odot } , but it may be possible with a reasonable explosion energy if M _ { CO } \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { % $ > $ } } 60 M _ { \odot } .