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.Comment: 11 pages; accepted for publication in A&
