On the Origin of the Lightest Molybdenum Isotopes

We discuss implications of recent precision measurements for the Rh proton separation energy for the production of the lightest molybdenum isotopes in proton-rich type II supernova ejecta. It has recently been shown that a novel neutrino-induced process makes these ejecta a promising site for the production of the light molybdenum isotopes and other “p-nuclei” with atomic mass near 100. The origin of these isotopes has long been uncertain. A distinguishing feature of nucleosynthesis in neutrino-irradiated outflows is that the relative production of Mo and Mo is set by a competition governed by the proton separation energy of Rh. We use detailed nuclear network calculations and the recent experimental results for this proton separation energy to place constraints on the outflow characteristics that produce the lightest molybdenum isotopes in their solar proportions. It is found that for the conditions calculated in recent two-dimensional supernova simulations, and also for a large range of outflow characteristics around these conditions, the solar ratio of Mo to Mo cannot be achieved. This suggests that either proton-rich winds from type II supernova do not exclusively produce both isotopes, or that these winds are qualitatively different than calculated in today’s supernova models. Subject headings: supernovae, nuclear reactions, nucleosynthesis Burbidge et al. (1957) described the synthesis of elements heavier than the iron group in terms of three main processes: slow (s-process) neutron capture, rapid (r-process) neutron capture, and proton capture (the p-process). It was believed that proton capture could account for the nuclei blocked from synthesis in neutron processes by stable isotopes that prevent their production through β-decay. Subsequent studies of nucleosynthesis in stellar environments found the densities and temperatures needed for the p-process difficult to obtain. Today the origin of the p-nuclei between A = 92–126, and in particular Mo and Mo, is one of the great outstanding mysteries in nuclear astrophysics.