Nicholas J. Wright, Jeremy J. Drake, Janet E. Drew, Mario G. Guarcello, Robert A. Gutermuth, Joseph L. Hora, Kathleen E. Kraemer
We report the discovery of ten proplyd-like objects in the vicinity of the
massive OB association Cygnus OB2. They were discovered in IPHAS H-Alpha images
and are clearly resolved in broad-band HST/ACS, near-IR and Spitzer mid-IR
images. All exhibit the familiar tadpole shape seen in photoevaporating objects
such as the Orion proplyds, with a bright ionization front at the head facing
the central cluster of massive stars, and a tail stretching in the opposite
direction. Many also show secondary ionization fronts, complex tail
morphologies or multiple heads. We consider the evidence that these are either
proplyds or `evaporating gaseous globules' (EGGs) left over from a fragmenting
molecular cloud, but find that neither scenario fully explains the
observations. Typical sizes are 50,000--100,000 AU, larger than the Orion
proplyds, but in agreement with the theoretical scaling of proplyd size with
distance from the ionizing source. These objects are located at projected
separations of 6-14pc from the OB association, compared to 0.1pc for the Orion
proplyds, but are clearly being photoionized by the 65 O-type stars in Cyg OB2.
Central star candidates are identified in near- and mid-IR images, supporting
the proplyd scenario, though their large sizes and notable asymmetries is more
consistent with the EGG scenario. A third possibility is therefore considered,
that these are a unique class of photoevaporating partially-embedded young
stellar objects that have survived the destruction of their natal molecular
cloud. This has implications for the properties of stars that form in the
vicinity of massive stars.
View original:
http://arxiv.org/abs/1201.2404
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