The effects of rotation on Wolf–Rayet stars and on the production of primary nitrogen by intermediate mass stars

We use the rotating stellar models described in the paper by A. Maeder & G. Meynet in this volume to consider the effects of rotation on the evolution of the most massive stars into and during the Wolf–Rayet phase, and on the post-Main Sequence evolution of intermediate mass stars. The two main results of this discussion are the following. First, we show that rotating models are able to account for the observed properties of the Wolf–Rayet stellar populations at solar metallicity. Second, at low metallicities, the inclusion of stellar rotation in the calculation of chemical yields can lead to a longer time delay between the release of oxygen and nitrogen into the interstellar medium following an episode of star formation, since stars of lower masses (compared to non-rotating models) can synthesize primary N. Qualitatively, such an effect may be required to explain the relative abundances of N and O in extragalactic metal–poor environments, particularly at high redshifts. 1. The Effects of rotation on the evolution into the Wolf–Rayet phase As is recalled in the paper by P. Eenens in this volume, Wolf–Rayet (WR) stars are the bare cores of initially massive stars, whose H–rich envelope has been removed by strong stellar winds or through Roche lobe overflow in a close binary system. Here we consider the following question: what are the effects of rotation on the evolution of massive single stars into the Wolf–Rayet phase? This subject has been discussed by Maeder (1987), Fliegner and Langer (1995), Maeder & Meynet (2000) and Meynet (2000). We shall briefly summarise the main results presented in those papers and assess their importance in the framework of a new grid of massive star models at solar metallicity (Meynet & Maeder in preparation). As a preamble, let us reconsider the criteria which have been chosen to decide when a stellar model enters into the WR phase. Ideally, of course, the physics of the models should decide when the star is a WR star. However our poor knowledge of the physics involved, as well as the complexity of models coupling the stellar interiors to the winds, are such that this approach is not yet possible. Instead, it is necessary to adopt some empirical criteria for deciding when a star enters the WR phase. In this work the criteria are the following: the star is considered to be a WR star when its temperature is log Teff > 4.0 and the mass fraction of hydrogen at the surface is XS < 0.4. Reasonable changes to these values (for instance adopting XS < 0.3 instead of 0.4) do not affect the results significantly.