Neutrino-induced neutron spallation and supernovar-process nucleosynthesis

Abstract

It is quite likely that the site of the r process is the hot, neutron-rich ``bubble'' that expands off a protoneutron star during a core-collapse supernova. The $r$ process would then occur in an intense flux of neutrinos. In order to explore the consequences of the neutrino irradiation, we calculate the rates of charged-current and neutral-current neutrino reactions on neutron-rich heavy nuclei, and estimate the average number of neutrons emitted in the resulting spallation. Our results suggest, for a dynamic $r$ process occurring in an expanding bubble, that charged-current ${\ensuremath{\nu}}_{e}$ captures might help shorten the time scale for the $r$ process, bringing it into better accord with our expectations about the conditions in the hot bubble: neutrino reactions can be important in breaking through the waiting-point nuclei at $N=50$ and 82, while still allowing the formation of abundance peaks. Furthermore, after the $r$ process freezes out, there appear to be distinctive neutral-current and charged-current postprocessing effects. These include a spreading of the abundance peaks and damping of the most pronounced features (e.g., peaks and valleys) in the unpostprocessed abundance distribution. Most importantly, a subtraction of the neutrino postprocessing effects from the observed solar $r$-process abundance distribution shows that two mass regions, $A=124$--126 and 183--187, are inordinately sensitive to neutrino postprocessing effects. This imposes very stringent bounds on the freeze-out radii and dynamic time scales governing the $r$ process. Moreover, we find that the abundance patterns within these mass windows are entirely consistent with synthesis by neutrino interactions. This strongly argues that the $r$ process must occur in the intense neutrino flux provided by a core-collapse supernova. It also greatly restricts dynamic models for the supernova $r$-process nucleosynthesis.