Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

This article first reviews the basic physics of rotating stars and their evolution. The changes of the mechanical and thermal equilibrium of rotating stars are examined. An important, predicted and observed, effect is that rotating stars are hotter at the poles and cooler at the equator. The mass loss by stellar winds, which are influenced by the anisotropic temperature distribution, is discussed. These anisotropies in the interior are also driving circulation currents, which transports the chemical elements and the angular momentum in stars. Internal differential rotation, if present, creates instabilities and mixing, in particular, the shear mixing, the horizontal turbulence and their interactions. A major check of the model predictions concerns the changes of the surface abundances, which are modified by mass loss in the very massive stars and by rotational mixing in O-type and B-type stars. The observations are shown to confirm the existence of rotational mixing, with much larger effects at lower metallicities. The predictions of stellar models concerning the evolution of the surface velocities, the evolutionary tracks in the Hertzsprung-Russell diagram and lifetimes, the populations of blue, red supergiants and Wolf-Rayet stars, and the progenitors of type Ibc supernovae are discussed. In many aspects, rotating models are shown to provide a much better fit than nonrotating ones. Using the same physical ingredients as those which fit the best observations of stars at near solar metallicities, the consequences of rotating models for the status of Be stars, the progenitors of gamma ray bursts, the evolution of Pop III stars and of very metal-poor stars, the early chemical evolution of galaxies, the origin of the C-enhanced metal poor stars and of the chemical anomalies in globular clusters are explored. Rotation together with mass loss are two key physical ingredients shaping the evolution of massive stars during the whole cosmic history.

The goal of this paper is to analyze the impact of a primary neutron source on the s-process nucleosynthesis in massive stars at halo metallicity. Recent stellar models including rotation at very low metallicity predict a strong production of primary N14. Part of the nitrogen produced in the H-burning shell diffuses by rotational mixing into the He core where it is converted to Ne22 providing additional neutrons for the s process. We present nucleosynthesis calculations for a 25 Msun star at [Fe/H] = -3, -4, where in the convective core He-burning about 0.8 % in mass is made of primary Ne22. The usual weak s-process shape is changed by the additional neutron source with a peak between Sr and Ba, where the s-process yields increase by orders of magnitude with respect to the yields obtained without rotation. Iron seeds are fully consumed and the maximum production of Sr, Y and Zr is reached. On the other hand, the s-process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the amount of Ne22 and by nuclear uncertainties, first of all by the Ne22(alpha,n)Mg25 reaction. Finally, assuming that Ne22 is primary in the considered metallicity range, the s-process efficiency decreases with metallicity due to the effect of the major neutron poisons Mg25 and Ne22. This work represents a first step towards the study of primary neutron source effect in fast rotating massive stars, and its implications are discussed in the light of spectroscopic observations of heavy elements at halo metallicity.Comment: Accepted for publication in ApJ Letters, 11 pages, 2 figures, 1 tabl