Bulletin of the American Physical Society
5th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 63, Number 12
Tuesday–Saturday, October 23–27, 2018; Waikoloa, Hawaii
Session EP: The r-process and Neutron Star Mergers |
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Chair: Hye Young Lee, Los Alamos National Laboratory Room: Hilton Kona 1 |
Thursday, October 25, 2018 7:00PM - 7:15PM |
EP.00001: A New Survey to Constrain the Astrophysical r-Process Timothy C Beers There are presently some 35 highly r-process-element-enhanced metal-poor (r-II) stars known in the Galactic halo, roughly twenty-five years after their first recognition. These stars exhibit enhancements of their r-process-element to iron ratios, relative to Solar ratios, by a factor of 10 to 100+ ([r-element/Fe] > +1.0). Despite their very low metallicities ([Fe/H < –2.0), these stars exhibit an apparently universal [r-element/Fe] pattern that is very well-matched to the Solar r-process pattern. As such, they have long been thought to provide fundamental information on the likely astrophysical site of the r-process. A new large-scale effort to dramatically increase the numbers of recognized r-II stars (from ~35 to ~100-150) is now underway; current results will be reported on, including the identification of numerous new bright r-II and r-I stars, several of which have detectable U and Th, and new examples of stars that exhibit the actinide-boost phenomenon – and likely to be a key to understanding the nature of the r-process.
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Thursday, October 25, 2018 7:15PM - 7:30PM |
EP.00002: Constraints on the Nuclear Equation of State and r-Process Nucleosynthesis from the Multi-Messenger Detection of Binary Neutron-Star Merger GW170817 Grant J Mathews, InSaeng Suh, Nguyen Q Lan The first detection of gravitational waves a binary neutron star merger GW170817 by the LIGO-Virgo Collaboration has provided fundamental new insights into the astrophysical site for the r process nucleosynthesis and on the nature of dense neutron-star matter. The detected gravitational wave signal depends upon the tidal distortion of the neutron stars as they approach merger. We present new numerical relativistic hydrodynamic simulations of how the detected “chirp” depends the adopted equation of state. The detected evidence of heavy-element nucleosynthesis also provides insight into the nature of the r-process and the fission properties of the heaviest nuclei. Parametrically, one can divide models for the r-process into three scenarios roughly characterized by the number of neutron captures per seed nucleus (n/s). In addition to neutron-star mergers, these include magneto-hydrodynamic jets from supernovae and the neutrino heated wind above the proto neutron star in core-collapse supernovae. Insight from GW170817 allows one to better quantify the relative contributions of each astrophysical site and to clarify the strength of fission barriers of heavy nuclei. |
Thursday, October 25, 2018 7:30PM - 7:45PM |
EP.00003: The formation of the r-process rare-earth peak in neutron star mergers Nicole Vassh, Rebecca A Surman, Matthew R Mumpower, Gail C McLaughlin
The electromagnetic counterpart of the GW170817 neutron star merger event suggested lanthanides were synthesized. However lanthanide production in heavy element nucleosynthesis is subject to large uncertainties. For instance the rare-earth abundance peak, a feature of enhanced lanthanide production at A~164 in the solar r-process residuals, is not robustly produced in r-process calculations. The proposed dynamical mechanism of peak formation requires the r-process path to encounter a nuclear physics feature which may be within reach of experiments performed at, for example, the upcoming FRIB. We employ Markov Chain Monte Carlo studies to "reverse engineer" the nuclear masses capable of producing a peak compatible with the observed solar r-process abundances. Here I will compare neutron-rich mass measurements from the CPT at CARIBU with the results for the masses found in both "cold" and "hot" accretion disk wind and dynamical ejecta conditions. Such direct comparisons between theory and experiment could soon be in a position to make definitive statements regarding the astrophysical site of rare-earth peak formation. |
Thursday, October 25, 2018 7:45PM - 8:00PM |
EP.00004: Constraining the astrophysical r-process by mass measurements on very neutron-rich isotopes at CARIBU and the N=126 factory Guy Savard, Jason A Clark, Filip G Kondev, Rodney Orford, Dwaipayan Ray, Kumar Sharma The Californium Rare Isotope Breeder Upgrade (CARIBU) facility at ANL can provide access to beams of mass separated very neutron-rich isotopes which can then have their mass determined to high-precision with the Canadian Penning Trap (CPT) mass spectrometer or their decay properties studied with the X-array spectrometer. Recent mass measurement results (R. Orford et al, PRL 120, 262702 (2018)) have reached the region of nuclear masses relevant to the formation of the rare-earth abundance peak at A ∼165 by the rapid neutron-capture process. These results are now being completed in his region and will be extended to the region responsible for the formation of the heaviest abundance peak in the r-process at the N=126 factory. The technique and recent results will be presented, together with the on-going work to help constrain the r-process astrophysical conditions. The additional information coming from the decay studies which significantly expand on the data obtained at fragmentation facilities, will also be presented. |
Thursday, October 25, 2018 8:00PM - 8:15PM |
EP.00005: The tidal polarizability of neutron stars, the neutron skin thickness of heavy nuclei, and the nature of dense matter Charles J Horowitz, Farrukh Fattoyev, Jorge Piekarewicz The historic first detection of a binary neutron star merger is providing fundamental insights into the nature of dense matter. A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Limits on the tidal polarizability translate into constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Indeed, we deduce an upper limit on the radius of a 1.4M_sun neutron star of R_1.4 < 13.76 km. Given the sensitivity of the neutron-skin thickness of 208Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of about R208 ≲ 0.25 fm. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities—likely indicative of a phase transition in the interior of neutron stars. |
Thursday, October 25, 2018 8:15PM - 8:30PM |
EP.00006: Neutron star tidal deformabilities constrained by nuclear theory and experiment Jeremy W Holt, Yeunhwan Lim
We confront observational data from gravitational wave event GW170817 with microscopic modeling of the cold neutron star equation of state. We develop and employ a Bayesian statistical framework that enables us to implement constraints on the equation of state from laboratory measurements of nuclei and state-of-the-art chiral effective field theory methods. We find that the 95% credibility range of predicted neutron star tidal deformabilities is between 136 and 519 for a 1.4 solar-mass neutron star, which is already consistent with the upper bound deduced from observations of the GW170817 event. However, we find that lower bounds on the neutron star tidal deformability will very strongly constrain microscopic models of the dense matter equation of state. We also demonstrate a strong correlation between the neutron star tidal deformability and the pressure of beta-equilibrated matter at twice saturation density. |
Thursday, October 25, 2018 8:30PM - 8:45PM |
EP.00007: Half-Lives of the Neutron-Rich N=82 Isotopes 130Cd and 131In Ryan A Dunlop Half-lives of N=82 nuclei below doubly-magic 132Sn are key input parameters for calculations of any astrophysical r-process scenario and play an important role in the formation and shape of the second r-process abundance peak. In the past, shell-model calculations of neutron-rich nuclei near the N=82 neutron shell closure that are not yet experimentally accessible have been performed by adjusting the quenching of the Gamow-Teller (GT) operator to reproduce the half-life of 130Cd [1]. The calculated half-lives of other nuclei in the region are known to be systematically too long. Recently, a shorter half-life for 130Cd was reported [2,3]. A re-scaling of the GT quenching to reproduce this 130Cd half-life resolved the discrepancy [2,3]. However, this created a new disagreement in the calculated half-life of 131In. This region is complicated due to the presence of isomers and β-n decays, making photopeak gating an ideal method to measure nuclear half-lives. In this talk, the half-lives of 130Cd and 131In, as well as the spectroscopy of 131Sn measured with the GRIFFIN γ-ray spectrometer at TRIUMF will be presented. |
Thursday, October 25, 2018 8:45PM - 9:00PM |
EP.00008: Progress toward direct measurements of neutron-induced reactions in a ring Shea Mosby, Aaron J Couture, Nathan Moody, Michelle Mosby, Rene Reifarth Neutron-induced reactions play a significant role in both nuclear astrophysics and nuclear technology, and many of the desired reaction rates are on nuclei too short-lived to measure with existing techniques. Indirect methods continue to progress, but systematic uncertainties associated with the techniques remain a persistent issue. It has been shown that an ion storage ring coupled to a spallation neutron source and fed by a radioactive ion beam facility could enable direct measurements of neutron-induced reactions for short-lived nuclei. We are evaluating the feasibility of using the LANSCE accelerator complex as the basis for such a facility. Results from our ongoing feasibility analysis will be presented. |
Thursday, October 25, 2018 9:00PM - 9:15PM |
EP.00009: Measuring Multi-Neutron Emission in Neutron-Rich Nuclei; First Results from the BRIKEN Collaboration Bertis C Rasco Exploring the beta decay of extremely neutron rich nuclei gives insight into nuclear structure far from stability and gives improved inputs into r process calculations that inform the production of the elements in the universe. The BRIKEN Collaboration has finished a commissioning run and two experimental runs at RIKEN using the BigRIPS separator. The BRIKEN detector setup is a collection of 140+ 3He tubes along with ion-implant detectors such as the silicon stack arrays of AIDA and WAS3ABI and a YSO scintillator implant detector. Over 250 neutron-rich isotopes were measured and around 45 new half lives and 165 new single neutron emission probabilities are expected to be measured. In addition at least 10 nonzero two neutron emission probabilities in the Nickel 78 region were measured. We will present an overview of the BRIKEN detector, the setup at RIKEN, and a brief overview of the analysis technique used to extract multi-neutron emission probabilities. First results on some two neutron emission probabilities will be presented. |
Thursday, October 25, 2018 9:15PM - 9:30PM |
EP.00010: Study of hot nuclear matter within rellativistic mean field models Abdul Quddus, Shakeb Ahmad, Suresh Kumar Patra Core-collapse supernovae are one of the most important events in the universe. This is a site for the production of heavier nuclei through r-processes. At the moment of explosion, the temperature of nuclear matter (Neutron star) is high and the density at bounce of the collapsing core goes up to 1.5-2.0 times the nuclear saturation density. It is impossible to study nuclear matter at extreme temperature and density in a laboratory due to its incompressible nature. Hence, an equation of state (EOS), the relation between energy and pressure, is a tool to study nuclear matter at extreme conditions. Relativistic mean field (RMF) model is one of the most successful and widely used models to study nuclear matter ranging from finite nuclei to infinite nuclear matter. We have studied symmetric and asymmetric infinite nuclear matter at finite temperature within RMF models by considering NL3, G2, FSUGarnet and newly predicted IOPB-I and G3 force parameters. We have put different values of temperature and asymmetric coefficient and find the critical parameters; the parameters where both liquid and gas phases of matter exist. |
Thursday, October 25, 2018 9:30PM - 9:45PM |
EP.00011: Actinide Production in Neutron Star Mergers Erika M Holmbeck, Trevor M Sprouse, Rebecca A Surman, Matthew R Mumpower, Nicole Vassh, Timothy Carter Beers, Toshihiko Kawano The rapid-neutron-capture ("r") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r-process-enhanced stars. We investigate actinide production in the dynamical ejecta of a neutron star merger as an explanation for the actinide boost. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. Our study suggests that while the dynamical ejecta of a neutron star merger is a likely production site for the formation of actinides, a significant contribution from another site or sites (e.g., the neutron star merger accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars. |
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