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1. New ²⁶ P(p, γ) ²⁷ S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process

   https://doi.org/10.3847/1538-4357/accf9c  

   Hou, S. Q.; Liu, J. B.; Trueman, T. C. L.; Li, J. G.; Pignatari, M.; Bertulani, C. A.; Xu, X. X. (June 2023, The Astrophysical Journal)

   Abstract Accurate nuclear reaction rates for²⁶P(p,γ)²⁷S are pivotal for a comprehensive understanding of therp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of²⁶P(p,γ)²⁷S because of the lack of nuclear mass and energy level structure information for²⁷S. We reevaluate this reaction rate using the experimentally constrained²⁷S mass, together with the shell model predicted level structure. It is found that the²⁶P(p,γ)²⁷S reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances atE= 1.104, 1.597, and 1.777 MeV above the proton threshold in²⁷S. The new rate is overall smaller than the other previous rates from the Hauser–Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new²⁷S mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in therp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of²⁷S/²⁶P obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path²⁶P(p,γ)²⁷S(β⁺,ν)²⁷P is not as important as previously thought for producing²⁷P. The adoption of the new reaction rates for²⁶P(p,γ)²⁷S only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements. 

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2. Thermonuclear ²⁸ P(p, γ ) ²⁹ S reaction rate and astrophysical implication in ONe nova explosion

   https://doi.org/10.1051/0004-6361/202449536  

   Liu, J B; José, J; Hou, S Q; Pignatari, M; Trueman, T_C L; Longland, R; Li, J G; Bertulani, C A; Xu, X X (July 2024, Astronomy & Astrophysics)

   Context.An accurate²⁸P(p,γ)²⁹S reaction rate is crucial to defining the nucleosynthesis products of explosive hydrogen burning in ONe novae. Using the recently released nuclear mass of²⁹S, together with a shell model and a direct capture calculation, we reanalyzed the²⁸P(p,γ)²⁹S thermonuclear reaction rate and its astrophysical implication. Aims.We focus on improving the astrophysical rate for²⁸P(p,γ)²⁹S based on the newest nuclear mass data. Our goal is to explore the impact of the new rate and associated uncertainties on the nova nucleosynthesis. Methods.We evaluated this reaction rate via the sum of the isolated resonance contribution instead of the previously used Hauser-Feshbach statistical model. The corresponding rate uncertainty at different energies was derived using a Monte Carlo method. Nova nucleosynthesis is computed with the 1D hydrodynamic code SHIVA. Results.The contribution from the capture on the first excited state at 105.64 keV in²⁸P is taken into account for the first time. We find that the capture rate on the first excited state in²⁸P is up to more than 12 times larger than the ground-state capture rate in the temperature region of 2.5 × 10⁷K to 4 × 10⁸K, resulting in the total²⁸P(p,γ)²⁹S reaction rate being enhanced by a factor of up to 1.4 at ~1 × 10⁹K. In addition, the rate uncertainty has been quantified for the first time. It is found that the new rate is smaller than the previous statistical model rates, but it still agrees with them within uncertainties for nova temperatures. The statistical model appears to be roughly valid for the rate estimation of this reaction in the nova nucleosynthesis scenario. Using the 1D hydrodynamic code SHIVA, we performed the nucleosynthesis calculations in a nova explosion to investigate the impact of the new rates of²⁸P(p,γ)²⁹S. Our calculations show that the nova abundance pattern is only marginally affected if we use our new rates with respect to the same simulations but statistical model rates. Finally, the isotopes whose abundance is most influenced by the present²⁸P(p,γ)²⁹S uncertainty are²⁸Si,^33,34S,^35,37Cl, and³⁶Ar, with relative abundance changes at the level of only 3% to 4%. 

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3. ⁵⁸ Ni( ³ He,t) ⁵⁸ Cu*(γ) measurements with GODDESS to constrain the astrophysical rate of ⁵⁷ Ni(p,γ) ⁵⁸ Cu

   https://doi.org/10.1088/1742-6596/2586/1/012109  

   Carmichael, S R; Pain, S D; Siciliano, M; Allen, J; Bardayan, D W; Boomershine, C; Campbell, C M; Carpenter, M P; Chipps, K A; Cizewski, J A; et al (September 2023, Journal of Physics: Conference Series)

   Abstract The observation ofγrays from the decay of⁴⁴Ti in the remnants of core-collapse supernovae (CCSNe) provides crucial information regarding the nucleosynthesis occurring in these events, as⁴⁴Ti production is sensitive to CCSNe conditions. The final abundance of⁴⁴Ti is also sensitive to specific nuclear input parameters, one of which is the⁵⁷Ni(p,γ)⁵⁸Cu reaction rate. A precise rate for⁵⁷Ni(p,γ)⁵⁸Cu is thus critical if⁴⁴Ti production is to be an effective probe into CCSNe. To experimentally constrain the⁵⁷Ni(p,γ)⁵⁸Cu rate, the structure properties of⁵⁸Cu were measured via the⁵⁸Ni(³He,t)⁵⁸Cu*(γ) reaction using GODDESS (GRETINA ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Laboratory’s ATLAS facility. Details of the experiment, ongoing analysis, and plans are presented. 

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4. The impact of (n,γ) reaction rate uncertainties on the predicted abundances of i-process elements with 32 ≤ Z ≤ 48 in the metal-poor star HD94028

   https://doi.org/10.1093/mnras/stz3322  

   McKay, John E.; Denissenkov, Pavel A.; Herwig, Falk; Perdikakis, Georgios; Schatz, Hendrik (December 2019, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Several anomalous elemental abundance ratios have been observed in the metal-poor star HD94028. We assume that its high [As/Ge] ratio is a product of a weak intermediate (i) neutron-capture process. Given that observational errors are usually smaller than predicted nuclear physics uncertainties, we have first set-up a benchmark one-zone i-process nucleosynthesis simulation results of which provide the best fit to the observed abundances. We have then performed Monte Carlo simulations in which 113 relevant (n,γ) reaction rates of unstable species were randomly varied within Hauser–Feshbach model uncertainty ranges for each reaction to estimate the impact on the predicted stellar abundances. One of the interesting results of these simulations is a double-peaked distribution of the As abundance, which is caused by the variation of the 75Ga (n,γ) cross-section. This variation strongly anticorrelates with the predicted As abundance, confirming the necessity for improved theoretical or experimental bounds on this cross-section. The 66Ni (n,γ) reaction is found to behave as a major bottleneck for the i-process nucleosynthesis. Our analysis finds the Pearson product–moment correlation coefficient rP > 0.2 for all of the i-process elements with 32 ≤ Z ≤ 42, with significant changes in their predicted abundances showing up when the rate of this reaction is reduced to its theoretically constrained lower bound. Our results are applicable to any other stellar nucleosynthesis site with the similar i-process conditions, such as Sakurai’s object (V4334 Sagittarii) or rapidly accreting white dwarfs. 

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5. The impact of (n,γ) reaction rate uncertainties of unstable isotopes on the i -process nucleosynthesis of the elements from Ba to W

   https://doi.org/10.1093/mnras/stab772  

   Denissenkov, Pavel A; Herwig, Falk; Perdikakis, Georgios; Schatz, Hendrik (April 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The abundances of neutron (n)-capture elements in the carbon-enhanced metal-poor (CEMP)-r/s stars agree with predictions of intermediate n-density nucleosynthesis, at Nn ∼ 1013–1015 cm−3, in rapidly accreting white dwarfs (RAWDs). We have performed Monte Carlo simulations of this intermediate-process (i-process) nucleosynthesis to determine the impact of (n,γ) reaction rate uncertainties of 164 unstable isotopes, from 131I to 189Hf, on the predicted abundances of 18 elements from Ba to W. The impact study is based on two representative one-zone models with constant values of Nn = 3.16 × 1014 and 3.16 × 1013 cm−3 and on a multizone model based on a realistic stellar evolution simulation of He-shell convection entraining H in a RAWD model with [Fe/H] = −2.6. For each of the selected elements, we have identified up to two (n,γ) reactions having the strongest correlations between their rate variations constrained by Hauser–Feshbach computations and the predicted abundances, with the Pearson product–moment correlation coefficients |rP| > 0.15. We find that the discrepancies between the predicted and observed abundances of Ba and Pr in the CEMP-i star CS 31062−050 are significantly diminished if the rate of 137Cs(n,γ)138Cs is reduced and the rates of 141Ba(n,γ)142Ba or 141La(n,γ)142La increased. The uncertainties of temperature-dependent β-decay rates of the same unstable isotopes have a negligible effect on the predicted abundances. One-zone Monte Carlo simulations can be used instead of computationally time-consuming multizone Monte Carlo simulations in reaction rate uncertainty studies if they use comparable values of Nn. We discuss the key challenges that RAWD simulations of i process for CEMP-i stars meet by contrasting them with recently published low-Z asymptotic giant branch (AGB) i process. 

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