Helium absorption and emission towards Θ1 Ori C

Oudmaijer, R.D. and Drew, J.E. and Barlow, M.J. and Crawford, Ian and Proga, D. (1997) Helium absorption and emission towards Θ1 Ori C. Monthly Notices of the Royal Astronomical Society 291 (1), pp. 110-120. ISSN 0035-8711.

Abstract

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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