Convection; Significance for Stellar Structure and Evolution

Å. Nordlund [1,2] and R. .F. Stein [3]
  1. Theoretical Astrophysics Center, Copenhagen
  2. Astronomical Observatory / NBIfAFG, University of Copenhagen
  3. Dept. of Phys.~and Astronomy, Michigan State University, East Lansing

32 Liege Int. Astroph. Coll., (1995) (gzip postscript).

Abstract

We present our current understanding of stellar convection based on the lessons that can be drawn from numerical convection simulations. We discuss properties of convection in the surface layers and in the bulk of the convection zone and we consider indirect effects of convection on stellar atmospheres. The average thermal properties of convection obtained from numerical simulations are very robust and are essentially unchanged as the resolution is increased. These thermal properties change extremely rapidly near the surface, on scales much smaller than the size of the convective cells transporting the energy. The interconnected downflow lanes visible as intergranule lanes at the solar surface quickly decompose into isolated downdrafts and the separation scale between these downdrafts gradually increases with increasing depth. A distinct asymmetry develops between the upflow and downflow regions because the ascending fluid flow is diverging and the descending flow converging in order to conserve mass. Most of the entropy fluctuations occur, most of the buoyancy work is performed, and most of the flux is transported, in the downdrafts. Penetration of downflows into the stable subconvective layers produce some entropy fluctuations, but these are smoothed out in the ascending gas over a few scale heights due the flow's divergence. Stellar convection is primarily driven by entropy fluctuations produced by radiative cooling at the surface. The lower part of the convection zone is slightly stably stratified, due to heating of the fluid by the radiative flux as energy transport gradually switches from radiative to convective.

Aside from its direct affect on stellar structure, convection also plays a significant role in controlling the structure of stellar atmospheres by driving the atmospheric dynamics. Convection excites the p-mode oscillations, generate waves that give rise to the enhanced chromospheric emission and stresses the magnetic field to supply energy to the corona.

Bob Stein home page or stein@pa.msu.edu