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Astron. Astrophys. 363, 585-592 (2000)

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6. Discussion

In Sect. 4, model calculations of the stellar atmosphere of [FORMULA] Ori E have been presented. A good fit for the data has been obtained. Notably, the line profile variations versus rotational phase have been reproduced. The oblique rotator model presented by Groote & Hunger (1997) is confirmed for [FORMULA] Ori E. We found an offset of [FORMULA] between He- and metal-abundance spots on the stellar surface. This shows that our method for modeling stellar surfaces with local abundance variations actually works.

With this background, model calculations of [FORMULA] Ori C have been attempted. Low abundance spots at the pole and high abundance spots near the equator have been modeled. Both give a reasonable fit to the observations, but the equatorial spots give a better overall agreement with the observations.

For the classical Ap stars and He-variable stars two mechanisms for chemical fractionation are discussed: Diffusion (Michaud et al. 1976) in the atmosphere and stellar wind fractionation (Michaud et al. 1987). Diffusion requires a very stable atmosphere and therefore mass-loss is a process which competes with diffusion. According to calculations by Michaud (1992), diffusion does not operate for mass-loss rates above about 10-13[FORMULA] yr-1. This is much lower than the mass-loss rates of [FORMULA] Ori E and [FORMULA] Ori C. Diffusion therefore is unlikely to contribute in these stars.

Stellar wind fractionation, on the other hand, seems to be able to explain the abundance pattern observed for He-rich and He-poor stars (Hunger & Groote 1999). The theory leads to predictions about enrichment and depletion of helium and metals at the poles, depending on the ionization of H and He in the stellar wind. Computations by Porter & Skouza (1999) indicate that stellar wind fractionation can operate up to maximum mass-loss rates of the order of 10-9[FORMULA] yr-1.

[FORMULA] Ori E fits in this picture. He-rich and metal-poor caps are found to be located at nearly the same position. This behaviour is expected for stars with [FORMULA]K. Hence we can identify the spots as poles of a magnetic dipole. This dipole coincides with the spectropolarimetric measurements of the magnetic field within the errors (Bohlender et al. 1987). A deviation from a centered dipole has been found, but we made no further efforts to investigate the detailed surface structure of [FORMULA] Ori E.

For [FORMULA] Ori C we tried the same model geometry, assuming two abundance spots near the pole. He and metals are depleted in the spots. The spots do not lie opposite each other. If they are interpreted as magnetic poles, the magnetic field is decentered. This model could be interpreted in a similar way as for [FORMULA] Ori E. Additionally, a model with a different abundance distribution has been investigated. In this model, we assume overabundant spots near the equator. The fit with observation is better in this case.

However, none of the known mechanisms for chemical fractionation is expected to work in the case of [FORMULA] Ori C. According to Howarth & Prinja (1989), the mass-loss rate of [FORMULA] Ori C is about 4[FORMULA][FORMULA] yr-1. This makes diffusion an even more unlikely process for [FORMULA] Ori C than for [FORMULA] Ori E. Also stellar wind fractionation is not expected to operate. This mechanism does not predict any fractionation if the temperature is higher than about 25 000 K (Hunger & Groote 1999). Also, the stellar wind is strong enough to prevent decoupling (Porter & Skouza 1999).

Therefore, another interpretation seems more plausible: Stahl et al. (1996) discussed the optical emission, UV absorption and photospheric absorption lines. They expected the UV absorption maximum at [FORMULA] to originate from the magnetic pole, while the photospheric absorption and emission maxima are due to a region of enhanced density situated near the magnetic equator. This region may be related to the circumstellar disk predicted by Babel & Montmerle (1997).

The region of excess absorption in the high-velocity stellar wind in the UV is supposed to be located at the pole. It is shifted by [FORMULA] with respect to the density enhanced region at the magnetic equator. In this scenario the shift can be explained: due to the inclination of the star and the obliquity of the magnetic field, we look straight at the magnetic equator with its density enhanced region at phase 0.0. At phase 0.5 one magnetic pole points directly in the direction of the observer.

The line profile variations of the optical absorption lines are interpreted in this model as variable excess absorption in the circumstellar environment and not as variations of the chemical abundance. The region where the excess absorption in [FORMULA] Ori C originates is a region of enhanced density on the magnetic equator. The emission lines have maximum strength at the same phase. This can naturally be explained, when the emission originates in the same region. A possible location for this region of enhanced density at the equator could be intersection points of the rotational and magnetic equator. [FORMULA] Ori E also seems to have circumstellar matter in this region (Groote & Hunger 1997). The fact that we see the effects of this circumstellar matter in [FORMULA] Ori C even in the "photospheric" lines indicates a quite high density of circumstellar matter. This is also supported by the fact that the absorption line profiles are distorted (see e.g. Fig. 5 and the red-shift of the absorption feature in Fig. 8), indicating significant absorption from cirumstellar matter.

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Online publication: December 11, 2000