Circumstellar Disks in Young Nebulous Clusters
Karl Stapelfeldt, Raghvendra Sahai, Michael Werner, and John Trauger
Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA
More than 100 circumstellar gas clumps with ionized rims have been detected by HST studies of the Orion Nebula H II region (O'Dell & Wong 1996). Seven additional objects appear as dark oval silhouettes seen against background nebula light (McCaughrean & O'Dell 1996); these are very likely to be protoplanetary disks seen in absorption. To take advantage of the enhanced visibility of circumstellar matter externally illuminated in a nebular environment, and to determine if the Orion "proplyds" are unique, we observed four other young nebulous clusters with HST/WFPC2 during Cycle 5. Eight fields were observed in H-alpha and I-band continuum in the nearby H II regions NGC 2024, NGC 2264, and S155. Two fields were observed in V-band in the reflection nebula NGC 7023. In all these fields, only two circumstellar ionized rims were found and no dark ovals. These results suggest that the visibility of proplyds in the Orion Nebula arises from the unique circumstances and geometry of this region. A very young "blister" H II region, with an associated embedded cluster actively being excavated from its parent molecular cloud, appears necessary to show circumstellar ionization and absorption regions before the harsh nebular environment can sweep the disks away. The absence of circumstellar absorption in NGC 7023 suggests that the disks dissipate via endogenic processes less than 40 million years after their formation.