It has been known for a long time ( Mestel 1953 ) that the meridional circulation velocity in stars , in the presence of \displaystyle \mu -gradients , is the sum of two terms , one due to the classical thermal imbalance ( \displaystyle \Omega -currents ) and the other one due to the induced horizontal \displaystyle \mu -gradients ( \displaystyle \mu -induced currents , or \displaystyle \mu -currents in short ) .
In the most general cases , \displaystyle \mu -currents are opposite to \displaystyle \Omega -currents .
Vauclair ( 1999 ) has shown that such processes can , in specific cases , lead to a quasi-equilibrium stage in which both the circulation and the helium settling is frozen .
Here we present computations of the circulation currents in halo star models , along the whole evolutionary sequences for four stellar masses with a metallicity of [ Fe/H ] = -2 .
We show that such a self-regulated process can account for the constancy of the lithium abundances and the small dispersion in the Spite plateau .