AXISYMMETRICAL MODEL OF THE IONIZED-GAS IN THE ORION NEBULA

Detailed ionization and thermal equilibrium models have been calculated for the ionized gas in the Orion Nebula with an axisymmetric two-dimensional "blister" geometry/density distribution viewed face-on in the foreground of OMC 1. Most of the physical detail of the microphysics and radiative transfer for our earlier spherical modeling is maintained, but the H II region is represented more realistically in view of the overall picture of the Orion Nebula. We compare the predicted surface brightnesses for a large set of lines with observations at different positions to determine the best-fitting physical parameters. Based on dynamical considerations, the plane-parallel density distribution is assumed exponential up to a maximum density and then level. The observed [O II] 3729/3726 angstrom line ratio near the Trapezium constrains this maximum to be 5000 cm-3. The 84 GHz continuum map of Gordon et al. is used with a chi-2 minimization technique to obtain a density at the exciting star of 163 cm-3, a scale height of 0.0483 pc, and the total number of ionizing photons s-1, N(Lyc) = 2.17 x 10(49) photons s-1. Approximately half the ionizing photons from the exciting source escape the nebula in density-bounded directions. As we found in our earlier spherical modeling, the best fit is obtained with a Kurucz stellar atmosphere with T(eff) = 37,000 K and log g = 4. Again, the noon ionization structure calls for a much hotter star. To simulate this, we increase the emergent stellar flux by a factor of 11 above 41 eV. The elemental abundances used in our best-fitting model are He/H = 0.1, C/H = 3.4 X 1O(-4), N/H = 6.8 x 10(-5), O/H = 4 x 10(-4), Ne/H = 8.1 x 10(-5), Si/H = 3.0 x 10(-6), S/H 8.5 x 10(-6), and Ar/H = 4.5 x 10(-6). A significant accomplishment of the present model over earlier work is the ability to explain the strong, singly ionized line emission, such as [O II], along lines of sight near the Trapezium. Our axisymmetric geometry results in roughly a hemisphere in front of the Trapezium that is density-bounded, while the area behind is ionization bounded, thus providing a natural mechanism by which to obtain higher densities in zones with singly ionized species. Strong lines in both singly and doubly ionized species (such as [O II] and [O III]) seen near the Trapezium are explained much more naturally with the present model than with a spherical model.