14 - Mixing in stellar radiation zones

Summary

Stars undergo some mild mixing in their radiation zones, which is due to a thermally driven large scale circulation, and presumably to turbulence caused by shear instabilities. It is the rotation of the star which is responsible for these motions, and therefore the transport of angular momentum must be described in time and space when modeling stellar evolution. We review the present state of the problem and discuss briefly the open questions.

The observational evidence

At first sight, there seems to be no mixing at all in stellar radiation zones, since a thoroughly mixed and therefore homogeneous star would not evolve to the red giant stage. This is why such mixing is ignored in the standard modeling of stellar evolution. However there are several signs that at least some partial mixing occurs in radiative interiors, and that this may have an impact on the later phases of stellar evolution.

Let us start by reviewing briefly the observational evidence pointing to such mixing.

• Models of built by pretending that there is no mixing in the radiation zones do not agree well with the observed global properties of stars, such as their luminosity and radius (or effective temperature). This is apparent when comparing theoretical isochrones with their observed counterpart in the Hertzsprung-Russel diagram, for stars with more than about 2 solar masses. The situation improves if one allows for some extra mixing beyond the convective core.