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1. Regulated NiCu Cycles with the New ⁵⁷ Cu(p, γ ) ⁵⁸ Zn Reaction Rate and the Influence on Type-I X-Ray Bursts: GS 1826–24 Clocked Burster

   https://doi.org/10.1051/epjconf/202226011023  

   Lam, Yi Hua; Lu, Ning; Heger, Alexander; Jacobs, Adam Michael; Smirnova, Nadezda A.; Nieto, Teresa Kurtukian; Johnston, Zac; Kubono, Shigeru (January 2022, EPJ Web of Conferences)

   Liu, W.; Wang, Y.; Guo, B.; Tang, X.; Zeng, S. (Ed.)

   In Type-I X-ray bursts (XRBs), the rapid-proton capture (rp-) process passes through the NiCu and ZnGa cycles before reaching the region above Ge and Se isotopes that hydrogen burning actively powers the XRBs. The sensitivity study performed by Cyburt et al . [1] shows that the 57 Cu(p, γ ) 58 Zn reaction in the NiCu cycles is the fifth most important rp-reaction influencing the burst light curves. Langer et al . [2] precisely measured some low-lying energy levels of 58 Zn to deduce the 57 Cu(p, γ ) 58 Zn reaction rate. Nevertheless, the order of the 1 + 1 and 2 + 3 resonance states that dominate at 0:2 ≲ T (GK) ≲ 0:8 is not confirmed. The 1 + 2 resonance state, which dominates at the XRB sensitive temperature regime 0:8 ≲ T (GK) ≲ 2 was not detected. Using isobaric-multipletmass equation (IMME), we estimate the order of the 1 + 1 and 2 + 3 resonance states and estimate the lower limit of the 1 + 2 resonance energy. We then determine the 57 Cu(p, γ ) 58 Zn reaction rate using the full pf -model space shell model calculations. The new rate is up to a factor of four lower than the Forstner et al . [3] rate recommended by JINA REACLIBv2.2. Using the present 57 Cu(p, γ ) 58 Zn, the latest 56 Ni(p, γ ) 57 Cu and 55 Ni(p, γ ) 56 Cu reaction rates, and 1D implicit hydrodynamic K epler code, we model the thermonuclear XRBs of the clocked burster GS 1826–24. We find that the new rates regulate the reaction flow in the NiCu cycles and strongly influence the burst-ash composition. The 59 Cu(p, γ ) 56 Ni and 59 Cu(p, α ) 60 Zn reactions suppress the influence of the 57 Cu(p, γ ) 58 Zn reaction. They strongly diminish the impact of the nuclear reaction flow that bypasses the 56 Ni waiting point induced by the 55 Ni(p, γ ) 56 Cu reaction on burst light curve. 

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2. The Regulated NiCu Cycles with the New ⁵⁷ Cu(p,γ) ⁵⁸ Zn Reaction Rate and Its Influence on Type I X-Ray Bursts: the GS 1826–24 Clocked Burster

   https://doi.org/10.3847/1538-4357/ac4d89  

   Lam, Yi Hua; Lu, Ning; Heger, Alexander; Jacobs, Adam Michael; Smirnova, Nadezda A.; Nieto, Teresa Kurtukian; Johnston, Zac; Kubono, Shigeru (April 2022, The Astrophysical Journal)

   Abstract During the X-ray bursts of GS 1826−24, a “clocked burster”, the nuclear reaction flow that surges through the rapid-proton capture process path has to pass through the NiCu cycles before reaching the ZnGa cycles that moderate further hydrogen burning in the region above the germanium and selenium isotopes. The 57 Cu(p, γ ) 58 Zn reaction that occurs in the NiCu cycles plays an important role in influencing the burst light curves found by Cyburt et al. We deduce the 57 Cu(p, γ ) 58 Zn reaction rate based on the experimentally determined important nuclear structure information, isobaric-multiplet-mass equation, and large-scale shell-model calculations. Based on the isobaric-multiplet-mass equation, we propose a possible order of 1 1 + - and 2 3 + -dominant resonance states and constrain the resonance energy of the 1 2 + state. The latter reduces the contribution of the 1 2 + -dominant resonance state. The new reaction rate is up to a factor of 4 lower than the Forstner et al. rate recommended by JINA REACLIB v2.2 at the temperature regime sensitive to clocked bursts of GS 1826−24. Using the simulation from the one-dimensional implicit hydrodynamic code K epler to model the thermonuclear X-ray bursts of the GS 1826−24 clocked burster, we find that the new 57 Cu(p, γ ) 58 Zn reaction rate, coupled with the latest 56 Ni(p, γ ) 57 Cu and 55 Ni(p, γ ) 56 Cu reaction rates, redistributes the reaction flow in the NiCu cycles and strongly influences the burst ash composition, whereas the 59 Cu(p, α ) 56 Ni and 59 Cu(p, γ ) 60 Zn reactions suppress the influence of the 57 Cu(p, γ ) 58 Zn reaction and diminish the impact of nuclear reaction flow that bypasses the important 56 Ni waiting point induced by the 55 Ni(p, γ ) 56 Cu reaction on the burst light curve. 

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3. Skeletal Kinetics Reduction for Astrophysical Reaction Networks

   https://doi.org/10.3847/1538-4365/ad454a  

   Nouri, A G; Liu, Y; Givi, P; Babaee, H; Livescu, D (May 2024, The Astrophysical Journal Supplement Series)

   AAS (Ed.)

   Abstract A novel methodology is developed to extract accurate skeletal reaction models for nuclear combustion. Local sensitivities of isotope mass fractions with respect to reaction rates are modeled based on the forced optimally time-dependent (f-OTD) scheme. These sensitivities are then analyzed temporally to generate skeletal models. The methodology is demonstrated by conducting skeletal reduction of constant density and temperature burning of carbon and oxygen relevant to Type Ia supernovae (SNe Ia). The 495-isotopes Torch model is chosen as the detailed reaction network. A map of maximum production of⁵⁶Ni in SNe Ia is produced for different temperatures, densities, and proton-to-neutron ratios. The f-OTD simulations and the sensitivity analyses are then performed with initial conditions from this map. A series of skeletal models are derived and their performances are assessed by comparison against currently existing skeletal models. Previous models have been constructed intuitively by assuming the dominance ofα-chain reactions. The comparison of the newly generated skeletal models against previous models is based on the predicted energy release and⁴⁴Ti and⁵⁶Ni abundances by each model. The consequences ofy_e≠ 0.5 in the initial composition are also explored wherey_eis the electron fraction. The simulated results show that⁵⁶Ni production decreases by decreasingy_eas expected, and that the⁴³Sc is a key isotope in proton and neutron channels toward⁵⁶Ni production. It is shown that an f-OTD skeletal model with 150 isotopes can accurately predict the⁵⁶Ni abundance in SNe Ia fory_e≲ 0.5 initial conditions. 

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4. The Enge Split-Pole Spectrograph at the University of Notre Dame

   https://doi.org/10.1051/epjconf/202430402002  

   Carmichael, Scott; O’Malley, Patrick; Bardayan, Daniel; Bailey, Thomas; Boomershine, Chevelle; Brodeur, Maxime; Coil, Sydney; Dembski, Cade; Gore, Tom; Jones, Chloe; et al (January 2024, EPJ Web of Conferences)

   Pakou, A; Souliotis, G; Moustakidis, C (Ed.)

   Nuclear reactions play a crucial role in determining the nucleosynthesis that occurs in astrophysical events. The rates of many reactions that significantly impact certain nucleosynthesis processes can not be currently measured via direct means. These reactions must be constrained in another manner, such as determining the level energies and other structure properties of the compound nuclei. In order to measure level energies of nuclei relevant to nuclear astrophysics, the Enge split-pole spectrograph has been installed and commissioned at the University of Notre Dame’s Nuclear Science Laboratory. The first scientific measurement has also been performed. Structure properties of⁵⁸Cu were measured via the reaction⁵⁸Ni(³He,t)⁵⁸Cu to provide the first experimental constraint of the⁵⁷Ni(p,γ)⁵⁸Cu reaction rate, which impacts the production of of⁴⁴Ti,⁵⁷Fe, and⁵⁹Ni in core-collapse supernovae. Preliminary analysis of this measurement confirms the level energies of states in⁵⁸Cu that could lead to significant resonances in the⁵⁷Ni(p,γ)⁵⁸Cu reaction rate, while suggesting the presence of additional states that have not been previously observed but could also lead to significant resonances. 

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5. Impact of the New ⁶⁵ As(p,γ) ⁶⁶ Se Reaction Rate on the Two-proton Sequential Capture of ⁶⁴ Ge, Weak GeAs Cycles, and Type I X-Ray Bursts Such as the Clocked Burster GS 1826−24

   https://doi.org/10.3847/1538-4357/ac4d8b  

   Lam, Yi Hua; Liu, Zi Xin; Heger, Alexander; Lu, Ning; Jacobs, Adam Michael; Johnston, Zac (April 2022, The Astrophysical Journal)

   Abstract We reassess the 65 As(p, γ ) 66 Se reaction rates based on a set of proton thresholds of 66 Se, S p ( 66 Se), estimated from the experimental mirror nuclear masses, theoretical mirror displacement energies, and full p f -model space shell-model calculation. The self-consistent relativistic Hartree–Bogoliubov theory is employed to obtain the mirror displacement energies with much reduced uncertainty, and thus reducing the proton-threshold uncertainty up to 161 keV compared to the AME2020 evaluation. Using the simulation instantiated by the one-dimensional multi-zone hydrodynamic code, K epler , which closely reproduces the observed GS 1826−24 clocked bursts, the present forward and reverse 65 As(p, γ ) 66 Se reaction rates based on a selected S p ( 66 Se) = 2.469 ± 0.054 MeV, and the latest 22 Mg( α ,p) 25 Al, 56 Ni(p, γ ) 57 Cu, 57 Cu(p, γ ) 58 Zn, 55 Ni(p, γ ) 56 Cu, and 64 Ge(p, γ ) 65 As reaction rates, we find that though the GeAs cycles are weakly established in the rapid-proton capture process path, the 65 As(p, γ ) 66 Se reaction still strongly characterizes the burst tail end due to the two-proton sequential capture on 64 Ge, not found by the Cyburt et al. sensitivity study. The 65 As(p, γ ) 66 Se reaction influences the abundances of nuclei A = 64, 68, 72, 76, and 80 up to a factor of 1.4. The new S p ( 66 Se) and the inclusion of the updated 22 Mg( α ,p) 25 Al reaction rate increases the production of 12 C up to a factor of 4.5, which is not observable and could be the main fuel for a superburst. The enhancement of the 12 C mass fraction alleviates the discrepancy in explaining the origin of the superburst. The waiting point status of and two-proton sequential capture on 64 Ge, the weak-cycle feature of GeAs at a region heavier than 64 Ge, and the impact of other possible S p ( 66 Se) are also discussed. 

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