High-resolution spectroscopy of faint emission lines in the Orion Nebula

We present high-resolution spectrophotometric observations of the Orion Neb ula, made with the Cassegrain echelle spectrograph on the Blanco 4 m telesc ope at Cerro Tololo Inter-American Observatory (CTIO). The resolution and s ignal-to-noise ratio make it possible to identify 444 emission lines in the 3498-7468 Angstrom range, down to 10(4) times fainter than H beta. We pres ent a detailed atlas of these emission lines along with an analysis of the associated errors. This data set is used to study the velocity held in the Orion Nebula. The forbidden lines split into two distinct groups. The low-i onization group has ions with an ionization potential less than 20 eV. Line s of these ions, [O I], [N I], [Ni II], and [Fe II], have recession velocit ies, relative to the hydrogen lines, of +10 to +15 km s(-1). There is a sha rp change to the second, high-ionization group, which includes lines of ion s 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 s light trend of decreasing velocity with the increasing ionization potential . This is consistent with previously proposed dynamical models in which lin es of ions with different ionization potentials originate at different dist ances from the ionizing stars. Significant acceleration appears to take pla ce across the narrow region where Fe2+ exists. Across this region the gas r eceives an acceleration of similar to 2.5 x 10(-5) cm s(-2). This provides a constraint on hydrodynamical models. We set a limit He II 4686/H beta < 7 x 10(-5), which in turn sets a limit to the intensity of the ionizing cont inuum at energies higher than 54 eV. Modern stellar atmospheres predict a c ontinuum that is far stronger than is present in the region near <theta>(1) Ori C.