Stellar evolution with rotation VII: Low metallicity models and the blue to red supergiant ratio in the SMC

We calculate a grid of models with and without the effects of axial rotation for massive stars in the range of 9 to 60 M⊙ and metallicity Z = 0.004 appropriate for the SMC. Remarkably, the ratios Ω/Ωcrit of the angular velocity to the break–up angular velocity grow strongly during the evolution of high mass stars, contrary to the situation at Z = 0.020. The reason is that at low Z, mass loss is smaller and the removal of angular momentum during evolution much weaker, also there is an efficient outward transport of angular momentum by meridional circulation. Thus, a much larger fraction of the stars at lower Z reach break–up velocities and rotation may thus be a dominant effect at low Z. The models with rotation well account for the long standing problem of the large numbers of red supergiants observed in low Z galaxies, while current models with mass loss were predicting no red supergiants. We discuss in detail the physical effects of rotation which favour a redwards evolution in the HR diagram. The models also predict large N enrichments during the evolution of high mass stars. The predicted relative N–enrichments are larger at Z lower than solar and this is in very good agreement with the observations for A–type supergiants in the SMC.