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

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1. Introduction

Periodic line profile variability is a common feature in magnetic early-type stars. In most of these objects, the generally accepted interpretation of these variations is rotational modulation caused by inhomogeneities in the element abundances at the surface. The abundance patterns usually follow magnetic surface structures, e.g. the magnetic poles, which then gives rise to rotational modulation, if the magnetic and the rotational axis are inclined. This is the oblique magnetic rotator model. One of the hottest of the established magnetic stars is [FORMULA] Ori E (=HD 37479, HR 1932, spectral type B2Vp), a He-variable star. [FORMULA] Ori E is also one of the best-studied of the variable chemically peculiar B stars (see e.g. Groote & Hunger 1997).

Periodic spectral variations in some emission lines in the well-known young O star [FORMULA] Ori C (=HD 37022, HR 1895, spectral type O6pe) were first detected by Stahl et al. (1993). Later Stahl et al. (1996) found that the photospheric absorption lines are also variable, with the same period of about 15 days. The stellar wind lines in the UV (Walborn & Nichols 1994) and the X ray emission (Gagné et al. 1997) are also variable with the same period.

Although [FORMULA] Ori C is much hotter than any of the classical magnetic A and B stars, the observed variations are reminiscent of these stars. Attempts to measure a magnetic field in [FORMULA] Ori C were unsuccessful (Donati & Wade 1999). The derived upper limits are so high, however, that this does not exclude a magnetic origin of the variations. Babel & Montmerle (1997) could quantitively explain the X-ray flux and variability with a model of a magnetically confined wind shock. This gives strong support to a magnetic origin for the variability of [FORMULA] Ori C. This star is of particular interest, since it is by far the hottest object known which shows such strictly periodic variations. It could represent an extension of magnetic stars to the O star range.

In this paper we model the variations of the photospheric lines of [FORMULA] Ori C with surface spots of different element abundances. As a comparison object we also model the magnetic star [FORMULA] Ori E. In this star, a magnetic origin of the variations is undisputed. The aim of the present paper is to investigate whether the variations observed for [FORMULA] Ori C can be explained, in analogy to [FORMULA] Ori E, by chemical abundance variations at the surface or whether an other mechanism is needed.

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000