Multicomponent radiatively driven stellar winds
IV. On the helium decoupling in the wind of sigma Ori E

Jiri Krticka$^1$, Jiri Kubat$^2$, Detlef Groote$^3$

1 - Ustav teoreticke fyziky a astrofyziky PrF MU, CZ-611 37 Brno, Czech Republic
2 - Astronomicky ustav, Akademie ved Ceske republiky, CZ-251 65 Ondrejov, Czech Republic
3 - Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany

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.

Reference: A&A
Status: Manuscript has been accepted