Fractionated stellar wind – the example of σOrionis E

In main-sequence stars, at effective temperatures below ' 20 000 K, radiation ceases to be able to drive a stellar wind. In a rather narrow transition regime, elemental segregation seems possible due to the decoupling of He and, under special circumstances, also of H from the absorbing metal ions in the wind. This effect may even be more pronounced, if the wind is surface modulated, say by a magnetic field and rotation. A first example of the latter kind is shown to be the Hevariable HD 37479 (σ Ori E) which has two axes of symmetry, one characterized by the depletion of metals and one by the enrichment of He. The former is oriented along the magnetic dipole axis, while the latter is set off by roughly 45◦. Theory of diffusion is unable to explain these symmetries. In order to locate the the wind bases, phase variations of the wind are extracted from IUE-spectrograms. It is shown that there exist two windmodes: a weak and variable inner (photospheric) wind that feeds the corotating clouds, and a phase independent outer (coronal) wind which has its origin in a hot corona outside the clouds. The resulting phase variations of the photospheric wind are in good agreement with the predictions of a semi-quantitative wind model. The wind bases further turn out to be (practically) congruent with the observed He-patches. Hence it is concluded that the surface enrichment of He indeed is due to decoupling of He from themetals in the wind. Themetal poor polar caps, however, cannot be explained in this way. H-decoupling, evidently, does not play a role in σ Ori E. Instead, arguments are brought forward that point to accretion caps on top of themagnetic poles, possibly of reaccreted H (and He) that veils the UV-metal lines. σ Ori E appears to be the first stellar object with a fractionated wind, i.e. a wind whose chemical composition is distinctly different from the composition of its base.