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Title:
Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
Authors:
De Marco, Orsola; O'Dell, C. R.; Gelfond, Pamela; Rubin, R. H.; Glover, S. C. O.
Affiliation:
AA(Astrophysics Department, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024; .), AB(Department of Physics and Astronomy, Vanderbilt University, Box 1807-B, Nashville, TN 37235; .), AC(Department of Physics, Yale University, 217 Prospect Street, New Haven, CT 06511; .), AD(NASA Ames Research Center, Moffett Field, CA 94035-1000 .; Orion Enterprises, MS 245-6, Moffett Field, CA 94035-1000.), AE(Astrophysics Department, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024; .)
Publication:
The Astronomical Journal, Volume 131, Issue 5, pp. 2580-2600. (AJ Homepage)
Publication Date:
05/2006
Origin:
UCP
AJ Keywords:
ISM: H II Regions, Stars: Planetary Systems: Protoplanetary Disks, Stars: Formation, Surveys
DOI:
10.1086/501528
Bibliographic Code:
2006AJ....131.2580D

Abstract

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 II 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 and size to a Herbig-Haro jet, is found in Pismis 24. No central star appears to be associated with either of these jet candidates.

Based on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555.

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