Evolution of the First Stellar Generations

¹ Physics and Astronomy, University of Basel, Klingelberstr. 82, 4056 Basel, Switzerland
² Geneva Observatory, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
³ Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italia

Corresponding author: raphael.hirschi@unibas.ch

Abstract

Although the theoretical study of very low metallicity (Z) and metal–free stars is not new, their importance has recently greatly increased since two related fields have been developing rapidly. The first is cosmological simulations of the formation of the first stars and of the reionisation period. The second is the observations of extremely metal poor stars. In this paper, we present pre–supernova evolution models of massive rotating stars at very low Z (Z = 10^-8) and at Z = 0. Rotation has a strong impact on mass loss and nucleosynthesis. Models reaching break–up velocities lose up to ten percents of their initial mass. In very low Z models, rotational and convective mixing enhances significantly the surface content in carbon, nitrogen and oxygen (CNO) when the star becomes a red supergiant. This induces a strong mass loss for stars more massive than about 60 M_๏. Our models predict type Ib,c supernovae and gamma–ray bursts at very low Z. Rotational mixing also induces a large production of CNO elements, in particular of primary nitrogen. The stellar wind chemical composition is compatible with the most metal–poor star know to date, HE 1327–2326, for CNO elements. Our models reproduce the early evolution of nitrogen in the Milky Way.