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 Tele scope Planetary Camera images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present 0.''02 resolutio n (9 AU) UV images of seven externally illuminated protostellar enviro nments and the first UV spectra that cover the spectral range between 1400 and 3000 Angstrom. We discuss 43 objects for which the angular re solution has been improved over previous data by more than a factor of 2. These young stellar objects are either embedded inside the Orion N ebula and externally illuminated by the Trapezium stars or located in front of the nebula and seen in silhouette. The visibility of young st ars surrounded by diffuse matter is dominated by intense line emission from ionization fronts (IFs) propagating into material photoablated f rom 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) an d the radial intensity profiles of emission lines decrease roughly as r(-3), consistent with an approximately constant-velocity diverging fl ow with an r(-2) density profile. However, some radial intensity profi les are better fitted with an exponential function. Low ionization fra ction near the IF and the heating and acceleration of the photoablatio n flow can explain deviations from the r(-3) intensity profiles. Many young stars located within 30 '' of theta(1) Ori C are surrounded by c oncentric arcs of [O III] and H alpha emission located 0.'' 5 to 3 '' from the IF facing theta(1) Ori C. These arcs may trace bow shocks for med by the interaction of the expanding photoablation flow with the fa st stellar wind from theta(1) Ori C. The [O III] emission may be enhan ced by UV radiation from theta(1) Ori C, thermal conduction, and/or tu rbulent mixing of weakly shocked, photoablated gas with the thermalize d 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 alpha and the forbidden transitions of common ions that may trace circumstellar protoplanetary disks. In s ome sources, the regions seen in silhouette in H alpha and ionic trans itions are bright in the 6300 Angstrom [O I] line. Most externally ill uminated 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 silhouet te against background nebular light, indicating that they contain larg e column densities of gas and dust. We discuss a variety of tail forma tion mechanisms and conclude that initial conditions probably play a k ey role in their formation. The large fraction of young stellar object s with extended circumstellar structure, the mass limits on this struc ture, and the estimated mass-loss rates are combined to produce an est imate for the photoionization age of the Orion Nebula. The derived pho toionization age of the Orion Nebula is less than 10(5) yr and possibl y as short as 10(4) yr.