Letter to the Editor

The origin of primary nitrogen in galaxies

Geneva Observatory, 1290 Sauverny, Switzerland

Corresponding author: G. Meynet, Georges.Meynet@obs.unige.ch

Received: 24 October 2001
Accepted: 8 November 2001

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  at from the zero age sequence to the phase of thermal pulses for models below or equal to 7 , 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 ¹⁴N: 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.