Session EB: Mini-Symposium: The Properties of rp-Process Nuclei I

Comparing the Impacts of Nuclear Physics and Astrophysics Inputs on X-ray Burst Model Calculations

Type-I X-ray burst light curves encode information about the underlying neutron star, such as its compactness and distance. This information can be gleaned by performing detailed comparisons between astronomical observations and astrophysics model calculations. However, model calculation results are sensitive to astrophysics and nuclear physics input. A natural question is to what degree the nuclear physics uncertainties matter when compared to modifications to the astrophysical conditions assumed for a calculation. I will present results from model calculations performed with the code MESA that show nuclear physics uncertainties can have as large of an impact as relatively substantial modifications to some astrophysical conditions on modeled x-ray burst light curves and ash abundances. I will briefly discuss prospects for removing some of the most critical nuclear physics uncertainties.   

Nuclear reaction rates and x-ray burst abundances from the ab initio symmetry-adapted no-core shell model

I outline a new formalism using the symmetry-adapted (SA) basis that begins with ab initio symmetry-adapted no-core shell-model (SA-NCSM) wave functions, and leads to a description of alpha-capture reaction rates and impacts on abundance patterns from x-ray burst nucleosynthesis. For narrow resonances, the physics of the system is described by the overlap of a wave function for the complete A-particle system and a cluster wave function. In this study, we consider a single partitioning (a single reaction channel) and no excitations of the clusters. We show results for O, Ne, and Mg isotopes, and abundance patterns determined from XRB nucleosynthesis simulations.   

A case study of the transition from isolated resonances to the continuum: the ${}^{34}{\rm Ar}(\alpha,p){}^{37}{\rm K}$ reaction

Many nuclear reactions of astrophysical importance are modeled by Hauser-Feshbach (HF) calculations, the well-established approach for computing average cross sections when many resonant levels are involved. This approach assumes that the density of levels is sufficient that only average properties, such as optical potentials and level densities, are sufficient to model the reaction. However, for intermediate masses or near the drip lines, these assumptions may break down. One example where these considerations are likely important is ${}^{34}{\rm Ar}(\alpha,p){}^{37}{\rm K}$, a reaction which may provide a pathway beyond the waiting point nucleus ${}^{34}{\rm Ar}$ in $x$-ray bursts. This reaction is also the focus of a number of recent and ongoing experiments. We have modeled this reaction using both a HF calculation and an $R$-matrix calculation using resonance parameters sampled from distributions which are consistent with the level density and optical potentials involved. The $R$-matrix calculation includes interference effects between levels with the same spin and parity. These results provide a measure of the statistical error in the cross section or reaction rate prediction arising from low density of states.