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Astron. Astrophys. 358, 572-574 (2000)

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1. Background

[FORMULA] Ori C is the brightest and hottest member of the Orion Trapezium cluster at the heart of the bright HII nebula M42. Previous optical/UV observations going back many years (Conti 1972; Walborn 1981; Walborn & Panek 1984; Stahl et al. 1993; Walborn & Nichols 1994; Stahl et al. 1996) have shown that [FORMULA] Ori C may be the O-star equivalent of a magnetic oblique rotator. Recent X-ray observations (Gagné et al. 1997; Babel & Montmerle 1997) indicate a modulation in the 15.4 day rotation cycle, with a likely surface magnetic field of several 100 G. In an attempt to detect the magnetic field via Zeeman splitting using high resolution optical spectropolarimetry in circular mode, Donati & Wade (1999) have reported null detections with 250 G 1[FORMULA] error bars for any longitudinal component of a surface magnetic field averaged over the stellar disk. If the field is a dipole, the pole strength must be below some 1800 G. This null result is in agreement with the estimation of an upper threshold for Be by Eversberg et al. (1998).

Donati & Wade (1999) also noted the unexpected discovery of strong, time-variable continuum circular polarisation in the optical spectrum of [FORMULA] Ori C, reaching values as high as 3.8%! As support for the reliability of this result, they note the presence of depolarisation structure associated with the strong nebular emission lines. They conclude that the continuum polarisation must be produced within the immediate (spatially unresolved) circumstellar (CS) environment of the star. Eversberg et al. (1998) failed to detect any depolarisation in the strong H[FORMULA] nebular line, although they possibly could have, had they had higher resolution and S/N.

Among the possible causes of this unusual continuum polarisation, Donati & Wade invoked some kind of CS disk as the most likely explanation. However, the level of linear continuum polarisation should be even larger, contrary to the previous observations of Leroy & Leborgne (1987), despite the latter authors' reporting of variability.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000