High‐Resolution Spectroscopy of Faint Emission Lines in the Orion Nebula

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Abstract
We present high-resolution spectrophotometric observations of the Orion Nebula, made with the Cassegrain echelle spectrograph on the Blanco 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO). The resolution and signal-to-noise ratio make it possible to identify 444 emission lines in the 3498-7468 Å range, down to 104 times fainter than Hβ. We present a detailed atlas of these emission lines along with an analysis of the associated errors. This data set is used to study the velocity field in the Orion Nebula. The forbidden lines split into two distinct groups. The low-ionization group has ions with an ionization potential less than 20 eV. Lines of these ions, [O I], [N I], [Ni II], and [Fe II], have recession velocities, relative to the hydrogen lines, of +10 to +15 km s-1. There is a sharp change to the second, high-ionization group, which includes lines of ions with ionization potentials larger than 20 eV, namely, [S II], [O II], [N II], and [Fe III]. These lines have velocities around +3 km s-1, with a slight trend of decreasing velocity with the increasing ionization potential. This is consistent with previously proposed dynamical models in which lines of ions with different ionization potentials originate at different distances from the ionizing stars. Significant acceleration appears to take place across the narrow region where Fe2+ exists. Across this region the gas receives an acceleration of ~ 2.5 × 10-5 cm s-2. This provides a constraint on hydrodynamical models. We set a limit He II 4686/Hβ < 7 × 10-5, which in turn sets a limit to the intensity of the ionizing continuum at energies higher than 54 eV. Modern stellar atmospheres predict a continuum that is far stronger than is present in the region near θ1 Ori C.

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