O 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 pp chains, thereby requiring the star to contract to reach temperatures high enough to eventually trigger 3α-reactions. Even when small amounts of the ¹²C(α,γ)¹⁶O reactions begin to feed the CNO mass range and enable the CNO cycle to generate energy, this occurs at higher temperatures compared to later stellar generations. It is currently controversial if the observed enhanced abundances of Ca in the most metal-poorstars 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 ¹⁹F(p,γ)²⁰Ne reaction. In this work, the rates of both the ¹⁹F(p,γ)²⁰Ne and competing ¹⁹F(p,α)¹⁶O reactions are re-evaluated using phenomenological R-matrix, simultaneously considering several ¹⁹F(p,γ)²⁰Ne, ¹⁹F(p,α)¹⁶O, and ¹⁹F(p,p)¹⁹F data sets, in order to better characterize the rate uncertainties. It is found that the rate uncertainty for ¹⁹F(p,γ)²⁰Ne 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 higher lying resonances. Additional experimental measurements are therefore needed to determine if ¹⁹F(p,γ)²⁰Ne CNO breakout is responsible for Ca enrichment in metal poor stars.