19F(p, γ)20Ne and 19F(p, αγ)16O reaction rates and their effect on calcium production in Population III stars from hot CNO breakout

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 3 alpha reactions. Even small amounts of the C-12(alpha, gamma)O-16 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 F-19(p, gamma)Ne-20 reaction. In this work, the rates of both the F-19(p, gamma)Ne-20 and competing F-19(p, alpha)O-16 reactions are re-evaluated using the phenomenological R-matrix approach, simultaneously considering several F-19(p, gamma)Ne-20, F-19(p, alpha)O-16 and F-19(p, p)F-19 data sets, to better characterize the rate uncertainties. It is found that the rate uncertainty for F-19(p, gamma)Ne-20 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 F-19(p, gamma)Ne-20 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.

Automated summary

First generation stars evolve differently due to their initial primordial abundance composition, lacking carbon, oxygen, and nitrogen. The enhanced abundance of calcium in the most metal-poor stars could potentially be a result of high temperature H-burning in the first massive stars, but the rate uncertainty for this enrichment is considerable. Additional experimental measurements are necessary to determine the cause of calcium enrichment in metal-poor stars.