Supernova Neutrino Effects on r-Process Nucleosynthesis in Black Hole Formation

Very massive stars with mass ≥8 M☉ culminate their evolution by supernova explosions, which are presumed to be the most viable candidates for the astrophysical sites of r-process nucleosynthesis. If the models for the supernova r-process are correct, then the results of nucleosynthesis could also put a significant constraint on the remnants of supernova explosions, i.e., a neutron star or black hole. In the case of very massive core collapse for a progenitor mass 20-40 M☉, a remnant stellar black hole is thought to be formed. Intense neutrino flux from the neutronized core and the neutrinosphere might suddenly cease during the Kelvin-Helmholtz cooling phase because of the black hole formation. It is important and interesting to explore the observable consequences of such a neutrino flux truncation. It has recently been argued in the literature that even the neutrino mass can be determined from the time delay of the deformed neutrino energy spectrum after the cessation of neutrino ejection (neutrino cutoff effect). Here we study the expected theoretical response of the r-process nucleosynthesis to the neutrino cutoff effect in order to look for another independent signature of this phenomenon. We found a sensitive response of the r-process yield if the neutrino cutoff occurs after the critical time when the expanding materials in the neutrino-driven wind drop out of nuclear statistical equilibrium (NSE). The r-process nucleosynthesis yields drastically change if the cutoff occurs during the r-process, having maximal effect on the change in abundance of 232Th and 235,238U. There is a large probability of finding this effect in elemental abundances of r-process-enhanced metal-deficient halo stars whose chemical composition is presumed to be affected by Population III supernovae in the early Galaxy. Using this result, connected with future detection of the time variation of the SN neutrino spectrum, we are able to identify when the black hole formation occurs in the course of SN collapse.