We report the first (in)elastic scattering measurement of 25 Al + p with the capability to select and measure in a broad energy range the proton resonances in 26 Si contributing to the 22 Mg(α, p) reaction at type I x-ray burst energies. We measured spin-parities of four resonances above the α threshold of 26 Si that are found to strongly impact the 22 Mg(α, p) rate. The new rate advances a state-ofthe-art model to remarkably reproduce lightcurves of the GS 1826-24 clocked burster with mean deviation <9% and permits us to discover a strong correlation between the He abundance in the accreting envelope of photospheric radius expansion burster and the dominance of 22 Mg(α, p) branch.
Background: Globular clusters are known to exhibit anomalous abundance trends such as the sodium-oxygen anticorrelation. This trend is thought to arise via pollution of the cluster interstellar medium from a previous generation of stars. Intermediate-mass asymptotic giant branch stars undergoing hot bottom burning (HBB) are a prime candidate for producing sodium-rich oxygen-poor material, and then expelling this material via strong stellar winds. The amount of 23 Na produced in this environment has been shown to be sensitive to uncertainties in the 22 Ne(p, γ ) 23 Na reaction rate. The 22 Ne(p, γ ) 23 Na reaction is also activated in classical nova nucleosynthesis, strongly influencing predicted isotopic abundance ratios in the Na-Al region. Therefore, improved nuclear physics uncertainties for this reaction rate are of critical importance for the identification and classification of pre-solar grains produced by classical novae. Purpose: At temperatures relevant for both HBB in AGB stars and classical nova nucleosynthesis, the 22 Ne(p, γ ) 23 Na reaction rate is dominated by narrow resonances, with additional contribution from direct capture. This study presents new strength values for seven resonances, as well as a study of direct capture. Method: The experiment was performed in inverse kinematics by impinging an intense isotopically pure beam of 22 Ne onto a windowless H 2 gas target. The 23 Na recoils and prompt γ rays were detected in coincidence using a recoil mass separator coupled to a 4π bismuth-germanate scintillator array surrounding the target. Results: For the low-energy resonances, located at center of mass energies of 149, 181, and 248 keV, we recover stength values of ωγ 149 = 0.17 +0.05 −0.04 , ωγ 181 = 2.2 ± 0.4, and ωγ 248 = 8.2 ± 0.7 μeV, respectively. These results are in broad agreement with recent studies performed by the LUNA and TUNL groups. However, for the important reference resonance at 458 keV we obtain a strength value of ωγ 458 = 0.44 ± 0.02 eV, which is significantly lower than recently reported values. This is the first time that this resonance has been studied completely independently from other resonance strengths. For the 632-keV resonance we recover a strength value of ωγ 632 = 0.48 ± 0.02 eV, which is an order of magnitude higher than a recent study. For reference resonances at 610-and 1222-keV, our strength values are in agreement with the literature. In the case of direct capture, we recover an S factor of 60 keV b, consistent with prior forward kinematics experiments. Conclusions: In summary, we have performed the first direct measurement of 22 Ne(p, γ ) 23 Na in inverse kinematics. Our results are in broad agreement with the literature, with the notable exception of the 458-keV
A promising astrophysical site to produce the lighter heavy elements of the first r-process peak (Z = 38 − 47) is the moderately neutron-rich (0.4 < Y e < 0.5) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak r-process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from (α, xn) reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved (α, xn) rates based on the Atomki-V2 α optical model potential. We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] < −1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of (α, xn) reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims to motivate more nuclear physics experiments on (α, xn) reactions using the current and new generation of radioactive beam facilities and also more observational studies of metal-poor stars.
We investigate the post-explosion phase in core-collapse supernovae with 2D hydrodynamical simulations and a simple neutrino treatment. The latter allows us to perform 46 simulations and follow the evolution of the 32 explosion models during several seconds. We present a broad study based on three progenitors (11.2, 15, and 27 M ⊙), different neutrino heating efficiencies, and various rotation rates. We show that the first seconds after shock revival determine the final explosion energy, remnant mass, and properties of ejected matter. Our results suggest that a continued mass accretion increases the explosion energy even at late times. We link the late-time mass accretion to initial conditions such as rotation strength and shock deformation at explosion time. Only some of our simulations develop a neutrino-driven wind (NDW) that survives for several seconds. This indicates that NDWs are not a standard feature expected after every successful explosion. Even if our neutrino treatment is simple, we estimate the nucleosynthesis of the exploding models for the 15 M ⊙ progenitor after correcting the neutrino energies and luminosities to get a more realistic electron fraction.
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