2009AJ....137..367O

The three-dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and newly established data. We find that the widely accepted model of a concave blister of ionized material needs to be altered in the southwest direction from the Trapezium, where we find that the Orion-S feature is a separate cloud of very optically thick molecules within the body of ionized gas, which is probably the location of the multiple embedded sources that produce the optical and molecular outflows that define the Orion-S star formation region. Evidence for this cloud comes from the presence of H₂ CO lines in absorption in the radio continuum and discrepancies in the extinction derived from radio-optical and optical-only emission. We present an equilibrium Cloudy model of the Orion-S Cloud, which successfully reproduces many observed properties of this feature, including the presence of gas-phase H₂ CO in absorption. We also report the discovery of an open-sided shell of [O III] surrounding the Trapezium stars, revealed through emission-line ratio images and the onset of radiation shadows beyond some proplyds. We show that the observed properties of the shell are consistent with it being a stationary structure, produced by shock interactions between the ambient nebular gas and the high-velocity wind from θ¹ Ori C. We examine the implications of the recently published evidence for a large blueshifted velocity of θ¹ Ori C with respect to the Orion molecular cloud, which could mean that this star has only recently begun to photoionize the Orion Nebula. We show that current observations of the nebula do not rule out such a possibility, so long as the ionization front has propagated into a pre-existing low-density region. In addition, a young age for the nebula would help explain the presence of nearby proplyds with a short mass-loss timescale to photoablation.