Helium absorption and emission towards Theta^1 ORI C

The Hei absorption and emission systems towards Theta^1Ori C, the exciting star of the Orion Nebula, are investigated. To this end, high-resolution near-infrared long-slit spectra centred on the Hei 1.083-μm and Brgamma lines and an ultra-high-resolution (R~10^6) spectrum of the optical Hei 3889-Angstroms line have been obtained. These data are supplemented by blue high spectral resolution echelle observations of Theta^1Ori C and the other members of the Trapezium. Even at R~10^6, the Hei absorption profile, associated with foreground gas at a heliocentric velocity of +3km s^-1, is very smooth, suggesting a simple broadening mechanism and homogeneity. The combination of the Hei 2^3S column density, deduced from the 3889-Angstroms line, with the non-detection of Brgamma emission at the same velocities sets an upper limit on the electron density in this medium of 10^10 m^-3. The Hei 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 Theta^1Ori 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 ^1Ori C.