X-ray bursts are thermonuclear flashes on the surface of accreting neutron stars and reliable burst models are needed to interpret observations in terms of properties of the neutron star and the binary system. We investigate the dependence of X-ray burst models on uncertainties in (p,γ), (α,γ), and (α,p) nuclear reaction rates using fully self-consistent burst models that account for the feedbacks between changes in nuclear energy generation and changes in astrophysical conditions. A two-step approach first identified sensitive nuclear reaction rates in a single-zone model with ignition conditions chosen to match calculations with a state-of-the-art 1D multi-zone model based on the Kepler stellar evolution code. All relevant reaction rates on neutron deficient isotopes up to mass 106 were individually varied by a factor of 100 up and down. Calculations of the 84 highest impact reaction rate changes were then repeated in the 1D multi-zone model. We find a number of uncertain reaction rates that affect predictions of light curves and burst ashes significantly. The results provide insights into the nuclear processes that shape X-ray burst observables and guidance for future nuclear physics work to reduce nuclear uncertainties in X-ray burst models.

Background: Clustering phenomena in N = Z nuclei provide an opportunity to understand the interplay between cluster and nucleon degrees of freedom. Purpose:To study resonances in the 18 O spectrum, populated in 14 C+α elastic scattering.Method: The Thick Target Inverse Kinematics (TTIK) technique was used to measure the excitation function for the 14 C+α elastic scattering. A 42 MeV 14 C beam was used to populate states of excitation energy up to 14.9 MeV in 18 O. The analysis was performed using a multi-level, multi-channel R-Matrix approach.Results: Detailed spectroscopic information, including spin-parities, partial α-and neutron-decay widths and dimensionless reduced widths, was obtained for excited states in 18 O between 8 and 14.9 MeV in excitation energy. Cluster-Nucleon Configuration Interaction Model calculations of the same quantities are performed and compared to the experimental results.Conclusions: Strong fragmentation of large α-cluster strengths is observed in the spectrum of 18 O making the α-cluster structure of 18 O quite different from the pattern of known quasi-rotational bands of alternating parity that are characteristic of N = Z, even-even nuclei like 16 O and 20 Ne.

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The reaction 13C(alpha,n) is considered to be the main source of neutrons for the s process in asymptotic giant branch stars. At low energies, the cross section is dominated by the 1/2+ 6.356 MeV subthreshold resonance in (17)O whose contribution at stellar temperatures is uncertain by a factor of 10. In this work, we performed the most precise determination of the low-energy astrophysical S factor using the indirect asymptotic normalization (ANC) technique. The alpha-particle ANC for the subthreshold state has been measured using the sub-Coulomb alpha-transfer reaction ((6)Li,d). Using the determined ANC, we calculated S(0), which turns out to be an order of magnitude smaller than in the nuclear astrophysics compilation of reaction rates.

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