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.
