Explosive nucleosynthesis from gamma-ray burst and hypernova progenitors: Direct collapse versus fallback

The collapsar engine behind long-duration gamma-ray bursts extracts the energy released from the rapid accretion of a collapsing star onto a stellar mass black hole. In a collapsing star, this black hole can form in two ways: the direct collapse of the stellar core into a black hole and the delayed collapse of a black hole caused by fallback in a weak supernova explosion. In the case of a delayed-collapse black hole, the strong collapsar-driven explosion overtakes the weak supernova explosion before shock breakout, and it is very difficult to distinguish this black hole formation scenario from the direct-collapse scenario. However, the delayed-collapse mechanism, with its double explosion, produces explosive nucleosynthetic yields that are very different from those in the direct-collapse scenario. We present one-dimensional studies of the nucleosynthetic yields from both black hole formation scenarios, deriving differences and trends in their nucleosynthetic yields.