Externally Illuminated Young Stellar Environments in the Orion Nebula: Hubble Space Telescope Planetary Camera and Ultraviolet Observations

We present new 0.05" resolution (22 AU) narrowband Hubble Space Telescope Planetary Camera images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present 0.02" resolution (9 AU) UV images of seven externally illuminated protostellar environments and the first UV spectra that cover the spectral range between 1400 and 3000 Å. We discuss 43 objects for which the angular resolution has been improved over previous data by more than a factor of 2. These young stellar objects are either embedded inside the Orion Nebula and externally illuminated by the Trapezium stars or located in front of the nebula and seen in silhouette. The visibility of young stars surrounded by diffuse matter is dominated by intense line emission from ionization fronts (IFs) propagating into material photoablated from circumstellar disks by soft UV radiation. Near the Trapezium stars, the electron density at these IFs is around 10^5 to 10^6 cm^-3 and the radial intensity profiles of emission lines decrease roughly as r^-3, consistent with an approximately constant-velocity diverging flow with an r^-2 density profile. However, some radial intensity profiles are better fitted with an exponential function. Low ionization fraction near the IF and the heating and acceleration of the photoablation flow can explain deviations from the r^-3 intensity profiles. Many young stars located within 30" of theta^1 Ori C are surrounded by concentric arcs of [O iii] and Hα emission located 0.5" to 3" from the IF facing theta^1 Ori C. These arcs may trace bow shocks formed by the interaction of the expanding photoablation flow with the fast stellar wind from theta^1 Ori C. The [O iii] emission may be enhanced by UV radiation from theta^1 Ori C, thermal conduction, and/or turbulent mixing of weakly shocked, photoablated gas with the thermalized shocked stellar wind. About 30% of the bright externally illuminated young stellar objects contain dark regions seen in silhouette against background nebular emission in Hα and the forbidden transitions of common ions that may trace circumstellar protoplanetary disks. In some sources, the regions seen in silhouette in Hα and ionic transitions are bright in the 6300 Å [O i] line. Most externally illuminated young stellar objects have dusty tails pointing radially away from the source of ionizing photons. Tails have an average length of 500 AU, independent of the projected distance from theta^1 Ori C, are limb-brightened in emission lines, and are sometimes seen in silhouette against background nebular light, indicating that they contain large column densities of gas and dust. We discuss a variety of tail formation mechanisms and conclude that initial conditions probably play a key role in their formation. The large fraction of young stellar objects with extended circumstellar structure, the mass limits on this structure, and the estimated mass-loss rates are combined to produce an estimate for the photoionization age of the Orion Nebula. The derived photoionization age of the Orion Nebula is less than 10^5 yr and possibly as short as 10^4 yr.