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Issue A&A Volume 373, Number 2, July II 2001 Page(s) 555 - 571 Section Formation, structure and evolution of stars DOI 10.1051/0004-6361:20010596

A&A 373, 555-571 (2001)
DOI: 10.1051/0004-6361:20010596

Stellar evolution with rotation. VII.

Low metallicity models and the blue to red supergiant ratio in the SMC

A. Maeder and G. Meynet

Geneva Observatory 1290 Sauverny, Switzerland
    e-mail: Georges.Meynet@obs.unige.ch

(Received 15 March 2001 / Accepted 12 April 2001)

Abstract
We calculate a grid of models with and without the effects of axial rotation for massive stars in the range of 9 to and metallicity Z = 0.004 appropriate for the SMC. Remarkably, the ratios 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.

Key words: stars: evolution -- stars: rotation -- supergiant -- magellanic clouds

Offprint request: A. Maeder, Andre.Maeder@obs.unige.ch

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