Parametrizations of thermal bomb explosions for core-collapse supernovae and 56Ni production

Thermal bombs are a widely used method to artificially trigger explosions of core-collapse supernovae (CCSNe) to determine their nucleosynthesis or ejecta and remnant properties. Recently, their use in spherically symmetric (1D) hydrodynamic simulations led to the result that Ni-56,Ni-57 and Ti-44 are massively underproduced compared to observational estimates for Supernova 1987A, if the explosions are slow, i.e. if the explosion mechanism of CCSNe releases the explosion energy on long time-scales. It was concluded that rapid explosions are required to match observed abundances, i.e. the explosion mechanism must provide the CCSN energy nearly instantaneously on time-scales of some ten to order 100 ms. This result, if valid, would disfavour the neutrino-heating mechanism, which releases the CCSN energy on time-scales of seconds. Here, we demonstrate by 1D hydrodynamic simulations and nucleosynthetic post-processing that these conclusions are a consequence of disregarding the initial collapse of the stellar core in the thermal-bomb modelling before the bomb releases the explosion energy. We demonstrate that the anticorrelation of Ni-56 yield and energy-injection time-scale vanishes when the initial collapse is included and that it can even be reversed, i.e. more Ni-56 is made by slower explosions, when the collapse proceeds to small radii similar to those where neutrino heating takes place in CCSNe. We also show that the Ni-56 production in thermal-bomb explosions is sensitive to the chosen mass cut and that a fixed mass layer or fixed volume for the energy deposition cause only secondary differences. Moreover, we propose a most appropriate setup for thermal bombs.