The r-process nucleosynthesis: connecting FRIB with the cosmos
(Dated: January 4, 2016)
BACKGROUND
Heavy element abundance patterns, roughly matching the solar r-process residuals, have now been found in many metal poor halo stars. There are two main contenders as astrophysical site of the r-process: core collapse supernovae and neutron star mergers. The potential astrophysical site that perhaps most comfortably fits galactic chemical evolution models based on the observation in metal poor halo stars is the core-collapse supernova. The supernova site that has received the most attention is the neutrino-driven wind of the newly formed protoneutron star. However, conditions required for the nucleosynthesis to reach uranium and thorium in this wind are not achieved in most modern simulations although they are not terribly far off, sometimes reaching the second peak of the r-process. An alternate, more robustly neutron-rich r-process site is within a neutron star-neutron star or neutron star-black hole merger. Modern simulations of the cold or mildly-heated merger tidal tail ejecta show a vigorous r-process with fission recycling. Fission recycling can produce an r-process abundance pattern between the second and third r-process peaks that is relatively insensitive to variations in the initial conditions. This appears consistent with observations of r- process-enhanced halo stars for which we have relatively complete 56 < Z < 82 patterns - most are strikingly similar and a good match to the solar r-process residuals within this element range. Thus the data favors different aspects of each astrophysical site, and in the first week of the workshop, we hope to obtain a list of constraints from the observation of metal poor halo stars that can be applied to not only the two main contenders, but other possibilities as well.
The astrophysical sites under consideration for the r-process are all characterized by distinct conditions of tempera- ture and density as a function of time, initial composition and neutron-richness that should produce unique abundance pattern signatures. Even within a given astrophysical event, different conditions characterize different components of the ejecta. In neutron star mergers, outflows from the remnant accretion disk have higher entropy and electron fractions than the colder tidal tails. Recent general relativistic simulations also find an important contribution from polar outflows heated at the shocked interface of the merging neutron stars. In principle, the unique abundance pattern should lead us directly to the astrophysical site of the main r-process, since this pattern is known to excellent precision and appears to be nearly universal. However, the nuclear network calculations currently used to generate r-process predictions are still too uncertain for such detailed comparisons to be reliable. The uncertainties arise from the difficulties in modeling astrophysical environments as well as the unknown nuclear properties of the thousands of neutron-rich nuclear species that participate in the r-process. r-process nucleosynthesis calculations require nuclear properties and reaction rates for thousands of nuclei from stability to the neutron drip line. Arguably the most im- portant nuclear data sets for the r-process are nuclear masses, beta decay properties, fission properties, and neutron capture rates. Measurements that tell us about all of these properties are possible at FRIB. Theoretical models of these quantities are on relatively sure footing close to stability, where experimental information is currently available. Moving toward more neutron rich nuclei, where data is not available, different theoretical approaches make markedly different predictions so future experimental measurements are crucial.
R-PROCESS EXPERIMENTS
With FRIB coming online, a large number of neutron rich nuclei will become accessible for experiments. In addition, a broad set of experimental approaches exists, or is under development, to address various aspects of nuclear structure and reactions that are relevant for the r-process. These include beta decay studies with beta-gamma, beta-neutron, and beta-multi-neutron coincidences, mass measurements using different techniques, level density measurements, for example with the beta-Oslo method, investigations of gamma strength functions with various techniques, and studies of fission barriers and fission fragment distributions. In addition reaction theory and experimental developments continue to open new avenues to constrain neutron capture rates through indirect techniques.
Given the vast experimental and theoretical possibilities, and the large number of nuclear data entering the various r-process model calculations it is important to develop a strategy for experiments, nuclear theory, astrophysics theory and observations that maximizes the impact of the new opportunities. We will explore the most critical data needs to make progress on key open questions related to the r-process, including the origin of the light r-process elements, the origin of the rare earth peak, the role of fission in the r-process, the observed enhancement of uranium and thorium in some metal poor stars (the so called actinide boost) and radioactive dating of the r-process. Likely each of these problems requires a different set of key experiments. In addition, nuclear theory developments are essential to predict nuclear properties that are out of reach of experiments. We will also determine the most critical data and mass regions for future theoretical investigations, and we will identify the experiments that will maximize the guidance provided to those most important theoretical developments. All this will be done by taking into account the expected experimental and theoretical capabilities, as well as expected developments in astronomy, for example on abundance measurements for specific elements.
(Dated: January 4, 2016)
BACKGROUND
Heavy element abundance patterns, roughly matching the solar r-process residuals, have now been found in many metal poor halo stars. There are two main contenders as astrophysical site of the r-process: core collapse supernovae and neutron star mergers. The potential astrophysical site that perhaps most comfortably fits galactic chemical evolution models based on the observation in metal poor halo stars is the core-collapse supernova. The supernova site that has received the most attention is the neutrino-driven wind of the newly formed protoneutron star. However, conditions required for the nucleosynthesis to reach uranium and thorium in this wind are not achieved in most modern simulations although they are not terribly far off, sometimes reaching the second peak of the r-process. An alternate, more robustly neutron-rich r-process site is within a neutron star-neutron star or neutron star-black hole merger. Modern simulations of the cold or mildly-heated merger tidal tail ejecta show a vigorous r-process with fission recycling. Fission recycling can produce an r-process abundance pattern between the second and third r-process peaks that is relatively insensitive to variations in the initial conditions. This appears consistent with observations of r- process-enhanced halo stars for which we have relatively complete 56 < Z < 82 patterns - most are strikingly similar and a good match to the solar r-process residuals within this element range. Thus the data favors different aspects of each astrophysical site, and in the first week of the workshop, we hope to obtain a list of constraints from the observation of metal poor halo stars that can be applied to not only the two main contenders, but other possibilities as well.
The astrophysical sites under consideration for the r-process are all characterized by distinct conditions of tempera- ture and density as a function of time, initial composition and neutron-richness that should produce unique abundance pattern signatures. Even within a given astrophysical event, different conditions characterize different components of the ejecta. In neutron star mergers, outflows from the remnant accretion disk have higher entropy and electron fractions than the colder tidal tails. Recent general relativistic simulations also find an important contribution from polar outflows heated at the shocked interface of the merging neutron stars. In principle, the unique abundance pattern should lead us directly to the astrophysical site of the main r-process, since this pattern is known to excellent precision and appears to be nearly universal. However, the nuclear network calculations currently used to generate r-process predictions are still too uncertain for such detailed comparisons to be reliable. The uncertainties arise from the difficulties in modeling astrophysical environments as well as the unknown nuclear properties of the thousands of neutron-rich nuclear species that participate in the r-process. r-process nucleosynthesis calculations require nuclear properties and reaction rates for thousands of nuclei from stability to the neutron drip line. Arguably the most im- portant nuclear data sets for the r-process are nuclear masses, beta decay properties, fission properties, and neutron capture rates. Measurements that tell us about all of these properties are possible at FRIB. Theoretical models of these quantities are on relatively sure footing close to stability, where experimental information is currently available. Moving toward more neutron rich nuclei, where data is not available, different theoretical approaches make markedly different predictions so future experimental measurements are crucial.
R-PROCESS EXPERIMENTS
With FRIB coming online, a large number of neutron rich nuclei will become accessible for experiments. In addition, a broad set of experimental approaches exists, or is under development, to address various aspects of nuclear structure and reactions that are relevant for the r-process. These include beta decay studies with beta-gamma, beta-neutron, and beta-multi-neutron coincidences, mass measurements using different techniques, level density measurements, for example with the beta-Oslo method, investigations of gamma strength functions with various techniques, and studies of fission barriers and fission fragment distributions. In addition reaction theory and experimental developments continue to open new avenues to constrain neutron capture rates through indirect techniques.
Given the vast experimental and theoretical possibilities, and the large number of nuclear data entering the various r-process model calculations it is important to develop a strategy for experiments, nuclear theory, astrophysics theory and observations that maximizes the impact of the new opportunities. We will explore the most critical data needs to make progress on key open questions related to the r-process, including the origin of the light r-process elements, the origin of the rare earth peak, the role of fission in the r-process, the observed enhancement of uranium and thorium in some metal poor stars (the so called actinide boost) and radioactive dating of the r-process. Likely each of these problems requires a different set of key experiments. In addition, nuclear theory developments are essential to predict nuclear properties that are out of reach of experiments. We will also determine the most critical data and mass regions for future theoretical investigations, and we will identify the experiments that will maximize the guidance provided to those most important theoretical developments. All this will be done by taking into account the expected experimental and theoretical capabilities, as well as expected developments in astronomy, for example on abundance measurements for specific elements.