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Paper: Ultracompact HII Regions
Volume: 267, Hot Star Workshop III: the Earliest Stages of Massive Star Birth
Page: 81
Authors: Kurtz, S.
Abstract: The designation ``ultracompact HII region'' first appeared in the astronomical literature over 30 years ago. Since that time, of order 103 actual or candidate ultracompact (UC) HII regions have been identified, and no fewer than seven theoretical models have been proposed to describe them. Here we discuss recent observational evidence that suggests that the conventional ``UC HII'' classification may not be adequate to describe all of the small, dense nebulae surrounding young, massive stars. In particular, it appears that some UC HII regions may be small, high density regions that are integral parts of much larger structures; these we call ``UC HII regions with extended emission''. Other HII regions appear to be an order of magnitude smaller and two orders of magnitude denser than traditional UC HII regions, and hence are deserving of a new classification. We designate these as ``hypercompact HII regions.'' We provide brief scientific and historical perspectives and present the new observational data and arguments for the new classifications. We conclude with a short discussion of the method of spectral index analysis and its application to the study of density structure within HII regions.
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