The He I absorption and emission systems towards Θ1 Ori C, the exciting star of the Orion Nebula, are investigated. To this end, high-resolution near-infrared long-slit spectra centred on the He I 1.083-μm and Brγ lines and an ultra-high-resolution (R ~ 106) spectrum of the optical He I 3889-Å line have been obtained. These data are supplemented by blue high spectral resolution echelle observations of Θ1 Ori C and the other members of the Trapezium. Even at R ~ 106, the He I absorption profile, associated with foreground gas at a heliocentric velocity of +3 km s−1, is very smooth, suggesting a simple broadening mechanism and homogeneity. The combination of the He I 23 S column density, deduced from the 3889-Å line, with the non-detection of Brγ emission at the same velocities sets an upper limit on the election density in this medium of 1010 m−3. The He I 1.083- μm long-slit spectrum shows the familiar background nebular emission, while a second blueshifted component is visible off-star at the same velocities as the absorption on-star. Several mechanisms to explain this emission are explored. We conclude that it is most probably emission formed behind, and leaking through, the absorbing gas layer. A clue to the origin of this emission is found in its spatial distribution: unlike the bright background nebular emission, the blueshifted component peaks symmetrically around the position of Θ1 Ori C, suggesting a physical association with the star. A possible model for the blueshifted emission is that it arises in a large-scale, dense shock front, provoked in some way by the wind of Θ1 Ori C.
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Oudmaijer et al. (1997) studied this question.