The origin of primary nitrogen in galaxies

Abstract

We investigate the role of stellar axial rotation on the nitrogen nucleosynthesis at low metallicities Z. For this purpose, we have calculated models with initial masses between 2 and 60 M$_\odot$ at Z=0.00001 from the zero age sequence to the phase of thermal pulses for models below or equal to 7 M$_\odot$, and up to the end of central C--burning for the more massive stars. The models include all the main physical effects of rotation. We show that intermediate mass stars with rotation naturally reproduce the occurrence and amount of primary nitrogen in the early star generations in the Universe. We identify two reasons why rotating models at low Z produce primary $^{14}$N: 1) Since the stars lose less angular momentum, they rotate faster. Simultaneously, they are more compact, thus differential rotation and shear mixing are stronger. 2) The H--burning shell has a much higher temperature and is thus closer to the core, which favours mixing between the two.