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        Helium absorption and emission towards Theta^1 ORI C

        Author
        Oudmaijer, R.D.
        Drew, J.E.
        Barlow, M.J.
        Crawford, I.A.
        Proga, D.
        Attention
        2299/4036
        Abstract
        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.
        Publication date
        1997
        Published in
        Monthly Notices of the Royal Astronomical Society
        Other links
        http://hdl.handle.net/2299/4036
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