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

We present new 0.05 00 (22 AU) resolution narrow-band Hubble Space Telescope Planetary Camera images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present 0.02 00 (9 AU) resolution UV images of 7 externally illuminated proto-stellar environments and the rst UV spectra that cover the spectral range between 1400 and 3000 A. We discuss 43 objects for which the angular resolution has been improved over previous data by more than a factor of two. 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 diiuse matter is dominated by intense line emission from ionization fronts propagating into material photo{ablated from circumstellar disks by soft UV radiation. Near the Trapezium stars, the electron density at these ionization fronts is around 10 5 to 10 6 cm ?3 and the radial intensity prooles of emission lines decrease roughly as r ?3 , consistent with an approximately constant velocity diverging ow with an r ?2 density proole. However, some radial intensity prooles are better t with an exponential function. Low ionization fraction near the ionization front and the heating and acceleration of the photo{ablation ow by the conduction of heat from the shocked 1 Orionis C stellar wind bubble into the photo{ablation ow can explain deviations from the r ?3 intensity prooles. Many young stars located within 30 00 of 1 Orionis C are surrounded by concentric arcs of Oiii] and H emission located 0.5 00 to 3 00 from the ionization-front facing 1 Orionis C. These arcs may trace bow shocks formed by the interaction of the expanding photo{ablation ow with the fast stellar wind from 1 Orionis C. The Oiii] emission may be enhanced by UV radiation from 1 Orionis C, thermal conduction, and/or turbulent mixing of weakly-shocked photo-ablated 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 H and the forbidden transitions of common ions that may trace circumstellar proto-planetary disks. In some sources, the region seen in silhouette in H and ionic transitions, are bright in the 6300 A Oi] line. Most externally illuminated young stellar objects have dusty tails pointing radially away from the source of ionizing photons. …