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.