Clustering and alpha-capture reaction rate from ab initio symmetry-adapted descriptions of Ne-20

We introduce a new framework for studying clustering and for calculating alpha partial widths using ab initio wave functions. We demonstrate the formalism for Ne-20, by calculating the overlap between the O-16 + alpha cluster configuration and states in Ne-20 computed in the ab initio symmetry-adapted no-core shell model. We present spectroscopic amplitudes and spectroscopic factors, and compare those to no-core symplectic shell-model results in larger model spaces, to gain insight into the underlying physics that drives alpha clustering. Specifically, we report on the alpha partial width of the lowest 1- resonance in Ne-20, which is found to be in good agreement with experiment. We also present first no-core shell-model estimates for asymptotic normalization coefficients for the ground state, as well as for the first excited 4(+) state in Ne-20 that lies in a close proximity to the alpha + O-16 threshold. This outcome highlights the importance of correlations for developing cluster structures and for describing alpha widths. The widths can then be used to calculate alpha-capture reaction rates for narrow resonances of interest to astrophysics. We explore the reaction rate for the alpha-capture reaction O-16(alpha, gamma)Ne-20 at astrophysically relevant temperatures and determine its impact on simulated x-ray burst abundances.