Nitrogen production in population III stars

Abstract

The first stars in the Universe have inherited their composition from primordial nucleosynthesis, so they have no metal. These stars, which are also named population III (pop III) stars, began the process of reionization in the Universe and contributed to the metal enrichment with heavy elements. Previous studies showed that they should have been rotating fast due to small or no angular momentum loss, reaching easily the critical velocity since they are massive and have very low stellar winds, thus their mass loss is very low or zero. Our aim is to study how the production of primary nitrogen is affected due to high rotation in the pop III stars. So, we compared grids of pop III stars with zero, average, and high rotation. All these models have been computed using Geneva code (GENEC) in the mass range of 9M_⊙ ≤ M_ini ≤ 120M_⊙. Due to the rotational mixing, the carbon produced in the He-burning core is diffused towards the H-burning shell, triggering the CNO cycle and producing primary nitrogen. In some models the transition of the shell from a pp-chain H-burning to a CNO H-burning induces a strong energy release and a complete change of the stellar structure and the nucleosynthesis. The production of nitrogen is boosted for the high rotation models.