A. Maeder et G. Meynet, Stellar evolution with rotation. VII. Low metallicity models and the blue to red supergiant ratio in the SMC, ASTRON ASTR, 373(2), 2001, pp. 555-571
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.