CNO Cycle Burning in Ultra-low Metallicity Solar Mass Stars

We use the 1D stellar evolution code MESA to run select models at one solar mass at near zero metallicity to examine the effect of a lack of catalysts for CNO burning on the evolution of these models. We find that for metallicities below 10^-10 ${Z}_{\odot }$ a loop occurs on the HR diagram as the star evolves off the main sequence, corresponding to the onset of significant CNO burning in the core of the star. This feature is visible in the Kippenhahn diagram as a period where the core becomes convective while the burning takes place. During this loop, the degeneracy of the core is lifted as CNO burning produces around 3.7% of the total nuclear energy.