Role of (a,n) reactions in the nucleosynthesis of light r-elements in neutrino-driven winds

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1 Role of (a,n) reactions in the nucleosynthesis of light r-elements in neutrino-driven winds Jorge Pereira, Fernando Montes National Superconducting Cyclotron Laboratory, MSU, USA Joint Institute for Nuclear Astrophysics, USA Fernando Montes, Jorge Pereira, Almudena Arcones, Almudena Arcones, Julia Bliss and Zach Meisel (on behalf of Jorge Pereira) Institut für Kernphysik, TU-Darmstadt, Germany GSI Helmholtzzentrum für Schwerionenforschung, Germany J. Pereira and F. Montes, PRC 93, (2016)

2 Nucleosynthesis of heavy elements Ultra metal-poor stars with high (H) and low (L) enrichment of heavy r-process nuclei (e.g Eu) suggest at least TWO components or sites (Qian and Wasserburg) [Cowan and Sneden, Nature, 2006] 38<Z<47 Z>47 H component: Core-collapse supernovae, NS mergers r-process L component: neutrino-driven winds (e.g. Core-collapse SNe) Ye < 0.5: a-process (Woosley and Hoffman, ApJ 1992)

3 Nucleosynthesis in neutrino-driven winds: The a process Wind parameters S~ k B /nuc, Y e <0.5, t exp ~10 ms [1] a process (+ weak r process) [1] Arcones and Thielemann, JPG 2013 Initial wind composition (T 10 GK): neutrons, protons, a (NSE) NSE breakdown (a rich freeze-out): a particles recombine (e.g. 3a reaction) seed nuclei Fast expansion other CPR reactions (a process): (n,g), (a,n), (p,n) (equilibrium with inverse, T 4.5 GK) (n,g) (g,n) in equilibrium Isotopic composition (a,n) (T< 4.5 GK) faster than (n,a) Increase Z (a,1n) main engine driving matter to heavier Z Janka et al., Phys Rep. 442 (2007) Isotopic composition via (a,xn) [only if (n,g) (g,n) break equilibrium] At T 1.5 GK CPR freeze-out: b decay and (n,g) (g,n) weak r-process

4 Impact of (a,n) reaction rates in the synthesis of light r-elements Bliss et al. (to be submitted to JPG) Abundance network calculations are sensitive to (a,n) rates Experimentally unknown at a-process temperatures ( GK) Need to use GLOBAL codes based on Hauser-Feshbach model (e.g TALYS, NON-SMOKER) Calculated vs. measured rates (energies above a-process T range) : differences ~5-10 What is the theoretical uncertainty of (a,n) at a-process temperatures ( GK)?

5 Theoretical uncertainty of (a,n) reaction rates Nuclear-physics inputs: e.g. optical potentials, level densities, etc Reaction mechanism: mostly compound nucleus (Hauser-Feshbach). Others: direct, preequilibrium Technical aspects: internal algorithms used Our approach: study representative (a,n) case for a process, e.g. 86 Se(a,n) 89 Kr Analyze sensitivity to nuclear-inputs and reaction mechanisms using TALYS

6 Theoretical uncertainty of (a,n) reaction rates: Nuclear Inputs Nuclear Input TALYS 1 packet Alpha OP Masses (and deformations) Level densities Proton/neutron OP Level structure Global Skyrme-HFB Constant temperature Global RIPL-3 + theoretical extension (up to 100 levels) Radiative transmission coefficients Kopecky-Uhl (E1,E2,M1,M2) Width fluctuation correction Preequilibrium Moldauer Exciton

7 Theoretical uncertainty of (a,n) reaction rates: Nuclear Inputs Rate Ratio = Reaction Rate (AOP, TALYS1) Reaction Rate (TALYS 1) Alpha optical potentials

8 Theoretical uncertainty of (a,n) reaction rates: Nuclear Inputs Alpha optical potentials Masses Level densities Proton/Neutron OP Level structure Preequilibrium

9 Theoretical uncertainty of (a,n) reaction rates: Technical Aspects Nuclear-physics inputs: e.g. optical potentials, level densities, masses Reaction mechanism: mostly compound nucleus (Hauser-Feshbach). Others: direct to discrete, preequilibrium Technical aspects: internal algorithms used (e.g. binning of excitation energy) Sensitivity to technical aspects: Compare TALYS and NON-SMOKER rates for 86 Se(a,n) 89 Kr when both use same nuclear inputs and reaction mechanisms

10 Theoretical uncertainty of (a,n) reaction rates: Technical Aspects Differences between TALYS vs. NON-SMOKER due to technical aspects: Very important at high T (~10 GK), moderate for a process (T<4.5 GK) Binning of excitation energy: differences ~2 at T 4.5 GK

11 Theoretical uncertainty of (a,n) reaction rates: Summary At temperatures relevant for the a-process (T<4.5 GK): Strongest effects (~10-100): Alpha optical potentials Medium effects (~2-10): Contributions from other exclusive channels: (a,2n) Binning of excitation energy Minor effects (~10%-50%): Preequilibrium, masses, level densities Proton/neutron OP, level structure No effects: Radiative transmission coefficients Width fluctuation correction

12 Sensitivity of light r-process nuclei to (a,n) rates WINNET [1] reaction network: 4412 n-, p-rich nuclei between Z=1-77 Wind trajectories by Arcones et al. [2] (e.g. S=86 k B /nuc, t exp =11 ms, Ye=0.47) Rates [(n,g); (p,n); (a,n) ]: experimental (if available) and calculated Neutrino interactions [4] Abundance sensitivity to (a,n) reaction rates (~200 reactions) Network calculations using (a,n) baseline rates scaled by factor p [1] Winterer, Käppeli et al., ApJ L750 (2012) [2] Arcones, Janka et al., A&A 467 (2007) [3] Cyburt et al., ApJ L750 (2012) [4] Frölich et al., ApJ 637 (2006)

13 Sensitivity of light r-process nuclei to (a,n) rates Preliminary studies: scale rates of groups of elements by same p (0.1, 10) [J. Bliss et al., in preparation] Z=30-40 Z=10-30 Z=30-40 Z=10-30

14 Sensitivity of light r-process nuclei to (a,n) rates Refined studies: Monte Carlo sampling of p for each (a,n) reaction (~200 reactions) 1) Randomly select a different p for each reaction (assuming log-normal distribution) 2) Run network calculation 3) Repeat times! [J. Bliss et al., in preparation] 96 Zr(a,n) 99 Mo

15 Experiment studies of (a,n) reactions (a,n) cross sections at T~1-5 GK Detect exclusive channels 1n, 2n Energy and angle of neutrons: separate compound-nucleus, preequilibrium, direct Zn, Ga, Ge, Se, Br, Kr, Sr, Zr (a,a) elastic scattering: alpha OP Experiments in e.g. ReA3 facility in FRIB using different devices He gas cells + neutron long counters, e.g. HABANERO ReA3 experiment, July 2016: 75 Ga(a,n) (Montes, et al.) He gas cells + neutron walls, e.g. LENDA, VANDLE Use of He active targets, e.g. AT-TPC (Y. Ayyad, private communication)

16 Conclusions Alpha process operating in neutrino-driven winds contributes to the synthesis of light r-process elements (Sr-Ag) All (a,n) reaction rates involved are experimentally unknown. Network calculations rely on reaction codes like TALYS and NON-SMOKER Study of theoretical uncertainty of (a,n) reactions: Most important source: Alpha optical potentials Others: contribution from exclusive channels (a,2n), (a,3n) Sensitivity studies show strong impact of (a,n) uncertainties on networkcalculated abundances. Same effects for different astrophysical conditions Important experimental studies (isotopes Zn-Zr elements in a-process path): (a,n) cross sections at T~1-5 GK Measurement of exclusive channels (a,1n), (a,2n) Energy and angle of emitted neutrons (separate reaction mechanisms) Elastic (a,a) scattering AOP