PopII 1/2 stars: very high ¹⁴N and low ¹⁶O yields

Abstract

Nine 20$\,M_\odot$ models were computed with metallicities ranging from solar, through $Z=10^{-5}$ ([Fe/H]∼−3.1) down to $Z=10^{-8}$ ([Fe/H]∼−6.1) and with initial rotational velocities between 0 and 600 km s⁻¹ to study the impact of initial metallicity and rotational velocity (Hirschi (2005)). The very large amounts of ¹⁴N observed (∼0.03 M_{[odot ]}) are only produced at $Z=10^{-8}$ (PopII 1/2). The strong dependence of the ¹⁴N yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of ¹⁴N abundance. The metallicity trends are best reproduced by the models with $\upsilon_{ini}/\upsilon_c \sim 0.75$, which is slightly above the mean observed value for OB solar metallicity stars. Indeed, in the model with $\upsilon_{ini}$ = 600 km s⁻¹ at $Z=10^{-8}$, the ¹⁶O yield is reduced due to strong mixing. This allows in particular to reproduce the upturn for C/O and a slightly decreasing [C/Fe], which are observed below [Fe/H]∼−3.