Cloud fragmentation and proplyd-like features in H II regions imaged by the Hubble Space Telescope 1

Orsola De Marco*, C. R. O'Dell, Pamela Gelfond, R. H. Rubin, S. C O Glover

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

23 Citations (Scopus)


We have analyzed Hubble Space Telescope ACS and WFPC2 new and archival images of eight H II regions to look for new protoplanetary disks (proplyds) similar to those found in the Orion Nebula. We find a wealth of features similar in size (although many are larger) to the bright cusps around the Orion Nebula proplyds. None of them, however, contains a definitive central star. From this, we deduce that the new cusps may not be proplyds but instead fragments of molecular cloud material. Out of all the features found in the eight H il regions examined, only one, an apparent edge-on silhouette in M17, may have a central star. This feature might join the small number of bona fide proplyds found outside the Orion Nebula, in M8, M20, and possibly M16. In line with the results found recently by Smith et al., the paucity of proplyds outside the Orion Nebula can be explained by their transient nature, as well as by the specific environmental conditions under which they can be observed. Several fragments are seen as dark silhouettes against a bright background. We have reanalyzed those found in IC 2944 by Reipurth et al. and found new, similar ones in M16. None of these fragments contains a central star, and we exclude the possibility that they are disks. Reipurth et al. concluded that the IC 2944 silhouettes are not star forming. We argue here that their assumption of a constant optical depth for these fragments is not physical and that it is more likely that these fragments are star forming, a condition that is supported, although not proved, by their shapes and distributions. The process of cloud fragmentation and photoevaporation produces a large number of small fragments, while the size hierarchy expected in a photoevaporative environment would not favor small fragments. The size distributions observed will constrain any future theories of cloud fragmentation. One bright microjet candidate is found in M17, protruding from a large, limb-brightened fragment. A second, larger, jetlike feature, similar in shape a nd size to a Herbig-Haro jet, is found in Pismis 24. No central star appears to be associated with either of these jet candidates.

Original languageEnglish
Pages (from-to)2580-2600
Number of pages21
JournalAstronomical Journal
Issue number5
Publication statusPublished - May 2006
Externally publishedYes


  • H II regions
  • Planetary systems: protoplanetary disks
  • Stars: formation
  • Surveys


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