Multicomponent radiatively driven stellar winds IV. On the helium   decoupling in the wind of sigma Ori E
  We study the possibility of the helium decoupling in the stellar wind of sigma Ori E. To obtain reliable wind parameters for this star we first calculate an NLTE wind model and derive wind mass-loss rate and terminal velocity. Using corresponding force multipliers we study the possibility of helium decoupling. We find that helium decoupling is not possible for realistic values of helium charge (calculated from NLTE wind models). Helium decoupling seems only possible for a very low helium charge. The reason for this behaviour is the strong coupling between helium and hydrogen. We also find that frictional heating becomes important in the outer parts of the wind of sigma Ori E due to the collisions between some heavier elements and the passive components -- hydrogen and helium. For a metallicity ten times lower than the solar one both hydrogen and helium decouple from the metals and may fall back onto the stellar surface. However, this does not explain the observed chemical peculiarity since both these components decouple together from the absorbing ions. Although we do not include the effects of the magnetic field into our models, we argue that the presence of a magnetic field will likely not significantly modify the derived results because in such case model equations describe the motion parallel to the magnetic field. 
Jiri Krticka, Jiri Kubat, Detlef Groote