F19(p,γ)20Ne and F19(p,α)O16 reaction rates and their effect on calcium production in Population III stars from hot CNO breakout

Abstract

First generation, or Population III, stars have a different evolution than those of later generations owing to their initial primordial abundance composition. Most notably, the lack of carbon, oxygen, and nitrogen means that primordial massive stars must rely on the less efficient p−p chains, thereby requiring the star to contract to reach temperatures high enough to eventually trigger 3α reactions. Even small amounts of the C12(α,γ)O16 reactions begin feeding the CNO mass range and enable the CNO cycle to generate energy, but this occurs at higher temperature compared to later stellar generations. It is currently controversial if the observed enhanced abundances of Ca in the most metal-poor stars could be a result of the high temperature H-burning conditions in the first massive stars. The level of this enrichment depends on the hot breakout path from the CNO cycles via the F19(p,γ)Ne20 reaction. In this work, the rates of both the F19(p,γ)Ne20 and competing F19(p,α)O16 reactions are re-evaluated using the phenomenological R-matrix approach, simultaneously considering several F19(p,γ)Ne20, F19(p,α)O16, and F19(p,p)F19 data sets, to better characterize the rate uncertainties. It is found that the rate uncertainty for F19(p,γ)Ne20 reaction is considerably larger than previously reported. This is the result of undetermined interferences between observed resonances, a possible threshold state, possible subthreshold states, direct capture, and background levels. Additional experimental measurements are therefore needed to determine if F19(p,γ)Ne20 CNO breakout is responsible for Ca enrichment in metal-poor stars. Astrophysically, the breakout reaction revision makes it less likely that Ca observed in the most Fe-poor stars can originate in hot CNO breakout H-burning nucleosynthtesis, thereby casting doubt on the prevailing faint supernova scenario to explain the abundances observed in these stars.17 MoreReceived 5 January 2021Accepted 9 April 2021Corrected 1 June 2021DOI:https://doi.org/10.1103/PhysRevC.103.055815©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levels & level densitiesFormation & evolution of stars & galaxiesH & He induced nuclear reactionsHydrostatic stellar nucleosynthesisLifetimes & widthsNuclear astrophysicsNuclear spin & parityNuclear structure & decaysRadiative captureResonance reactionsProperties20 ≤ A ≤ 38TechniquesNuclear data analysis & compilationNuclear PhysicsGravitation, Cosmology & Astrophysics