## 4. Drift velocityTo calculate a typical order of magnitude of the drift velocity between the ions and rest of the plasma, the radiation force will be approximately balanced by the retarding frictional force. The diffusion velocity is Here, is the mass of a proton,
If the mass of species 1 (the stellar plasma - mostly hydrogen and helium) is neglected compared to the mass of species 2 (the diffusing ions), and ion shielding is neglected in the computation of the Debye length, Eq. 40 of Paquette et al. (1986) becomes where where is the density. To obtain a simple expression for the diffusion velocity, an initially chemically homogeneous star is assumed, so that and Eq. 9 are good representations. The diffusion velocity is then This expression will overestimate the drift velocity as soon as a concentration gradient is established, as then in Eq. 11. However, with these points in mind, Eq. 14 is used as an estimate for the velocity in Eq. 4. © European Southern Observatory (ESO) 1999 Online publication: December 4, 1998 |