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

Abstract

We present new resolution (22 AU) narrowband Hubble Space T elescope Planetary Camera 0A .05 images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present resolution (9 AU) UV images of seven externally illuminated protostellar environments and the Ðrst 0A .02 UV spectra that cover the spectral range between 1400 and 3000 We discuss 43 objects for which the Ó. 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 di †use 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 105 to 106 cm~3 and the radial intensity proÐles of emission lines decrease roughly as r~3, consistent with an approximately constant-velocity diverging Ñow with an r~2 density proÐle. However, some radial intensity proÐles are better Ðtted with an exponential function. Low ionization fraction near the IF and the heating and acceleration of the photoablation Ñow can explain deviations from the r~3 intensity proÐles. Many young stars located within 30A of h1 Ori C are surrounded by concentric arcs of [O III] and Ha emission located to 3A from the IF facing h1 Ori C. 0A .5 These arcs may trace bow shocks formed by the interaction of the expanding photoablation Ñow with the fast stellar wind from h1 Ori C. The [O III] emission may be enhanced by UV radiation from h1 Ori C, thermal conduction, and/or turbulent mixing of weakly shocked, photoablated gas with the therma-lized shocked stellar wind. About 30% of the bright externally illuminated young stellar objects contain dark regions seen in silhouette against background nebular emission in Ha and the forbidden transitions of common ions that may trace circumstellar protoplanetary disks. In some sources, the regions seen in silhouette in Ha and ionic transitions are bright in the 6300 [O I] line. Most externally illuminated Ó 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 h1 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 105 yr and possibly as short as 104 yr.