SPINSTARS at low metallicities

Abstract

The main effect of axial rotation on the evolution of massive PopIII stars is to trigger internal mixing processes which allow stars to produce significant amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars produce much more primary nitrogen than PopIII stars for a given initial mass and rotation velocity. The very metal poor stars undergo strong mass loss induced by rotation. One can distinguish two types of rotationnaly enhanced stellar winds: 1) Rotationally mechanical winds occurs when the surface velocity reaches the critical velocity at the equator, {\it i.e.} the velocity at which the centrifugal acceleration is equal to the gravity; 2) Rotationally radiatively line driven winds are a consequence of strong internal mixing which brings large amounts of CNO elements at the surface. This enhances the opacity and may trigger strong line driven winds. These effects are important for an initial value of $\upsilon/\upsilon_{\rm crit}$ of 0.54 for a 60 M$_\odot$ at $Z=10^{-8}$, {\it i.e.} for initial values of $\upsilon/\upsilon_{\rm crit}$ higher than the one ($\sim$0.4) corresponding to observations at solar $Z$. These two effects, strong internal mixing leading to the synthesis of large amounts of primary nitrogen and important mass losses induced by rotation, occur for $Z$ between about 10$^{-8}$ and 0.001. For metallicities above 0.001 and for reasonable choice of the rotation velocities, internal mixing is no longer efficient enough to trigger these effects.