Where, oh where has the r-process gone?

Abstract

We present a review of the possible sources for r-process nuclei ( r-nuclei). It is known that there is as yet no self-consistent mechanism to provide abundant neutrons for a robust r-process in the neutrino-driven winds from nascent neutron stars. We consider that the heavy r-nuclei with mass numbers A > 130 (Ba and above) cannot be produced in the neutrino-driven winds. Nonetheless, the r-process and the neutrino-driven winds may be directly or indirectly related by some unknown additional mechanism, which, for example, could provide ejecta with very short dynamic timescales of ≲ 0.004 s . This undetermined mechanism must supply a neutron source within the same general stellar sites that undergo core collapse to produce the neutron star. Observational data on low-metallicity stars in the Galactic halo show that sites producing the heavy r-nuclei do not produce Fe or any other elements between N and Ge. Insofar as a forming neutron star is key to producing the heavy r-nuclei, then the only possible sources are supernovae resulting from collapse of O–Ne–Mg cores or accretion-induced collapse of white dwarfs, neither of which produce the elements of the Fe group or those of intermediate mass (above C and N). Observational evidence on s and r-nuclei in low-metallicity stars with high C and N abundances shows that the r-process is also active in binary systems. The nuclei with A ∼ 90 –110 produced by charged-particle reactions (CPR) in the neutrino-driven winds are in general present in metal-poor stars with high or low abundances of heavy r-nuclei. The CPR nuclei and the heavy r-nuclei are not strongly coupled. Some metal-poor stars show extremely high enrichments of heavy r-nuclei and have established that the abundance patterns of these nuclei are universally close to the solar abundance pattern of heavy r-nuclei. Using a template star with high enrichments of heavy r-nuclei and another with low enrichments we develop a two-component model based on the abundances of Eu (from sources for heavy r-nuclei) and Fe (from Fe core-collapse supernovae). This model gives very good quantitative predictions for the abundances of all the other elements in those metal-poor stars with [ Fe / H ] ≲ - 1.5 for which the Eu and Fe abundances are known. We attribute the CPR elements such as Sr, Y, and Zr to reactions in the neutrino-driven winds from a nascent neutron star and the heavy r-nuclei to the hypothecated true “ r-process”. The CPR nuclei should be produced whenever a neutron star is formed regardless of whether heavy r-nuclei are produced or not. Using the two-component model we estimate the yield of the CPR element Sr to be ∼ 10 - 6 M ⊙ for a single neutron star formation event. Self-consistent astrophysical models are needed to establish that the CPR nuclei are common to the neutron stars produced in both sources for the heavy r-nuclei and those for Fe. We show that the observational data appear fully consistent with the two-component model. The specific mechanism and site for the production of heavy r-nuclei remains to be found.