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Astron. Astrophys. 321, 465-476 (1997)

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1. Introduction

There are at least three main reasons to reconsider at the present time the problem of the effects of rotation on stellar structure and evolution:

1) The recent finding of Herrero et al. (1992) that all fast rotators among O-stars show surface He-enrichments points towards the existence of very efficient mixing processes induced by rotation. Indeed in these stars, mixing is strong enough to transport to the surface the newly processed elements during a fraction of the main sequence lifetime. The next generation of stellar models should be able to account for such observations.

2) A number of discrepancies between models and observations still exist and they generally point in favor of more mixing processes in the models (Maeder, 1995b, see also the references therein). Without going into the details let us briefly mention some of them below:

Of course we do not claim here that all these differences between theory and observation will be resolved by the inclusion of the effects of rotation into the stellar models. However to go ahead, a first step is to better understand and quantify these effects which can deeply modify stellar evolution.

3) Finally the theory of the transport mechanisms induced by rotation has been considerably improved by Zahn (1992) who studied in details the interaction between meridian circulation and turbulence in rotating stars. He has established in a coherent way the equations governing both the transport of the chemical species and of the angular momentum. In his theory the sole a priori assumption is that the horizontal turbulence is much stronger than the vertical one. This conjecture leads to an internal "shellular" rotation law in agreement with the one deduced in the sun from the study of the acoustic waves (Tomczyk et al. 1995).

With in hands, observations which can serve as a check of our model building, and a theory which for the first time couples in a consistent manner the transport of angular momentum and that of the chemical species, we are now in a good position to address the problem of the effects of rotation on stellar evolution.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998