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 rotatio n for massive stars in the range of 9 to 60 M-. and metallicity Z = 0.004 a ppropriate for the SMC. Remarkably, the ratios Omega/Omega (crit) of the an gular velocity to the break-up angular velocity grow strongly during the ev olution of high mass stars, contrary to the situation at Z = 0.020. The rea son is that at low Z, mass loss is smaller and the removal of angular momen tum during evolution much weaker, also there is an efficient outward transp ort of angular momentum by meridional circulation. Thus, a much larger frac tion of the stars at lower Z reach break-up velocities and rotation may thu s 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 r ed supergiants. We discuss in detail the physical effects of rotation which favour a redwards evolution in the HR diagram. The models also predict lar ge N enrichments during the evolution of high mass stars. The predicted rel ative N-enrichments are larger at Z lower than solar and this is in very go od agreement with the observations for A-type supergiants in the SMC.