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Paper: A Continuum of Extragalactic UCHII Clusters
Volume: 267, Hot Star Workshop III: the Earliest Stages of Massive Star Birth
Page: 307
Authors: Johnson, K. E.
Abstract: A number of extragalactic star clusters have been discovered which are still deeply enshrouded in their natal cocoons. These clusters appear to have properties which are analogous to those of ultracompact HII regions in the Galaxy, albeit vastly scaled up in total mass and luminosity. Because of their similarity to ultracompact HII regions in the Galaxy, Kobulnicky & Johnson dubbed these objects ``ultradense HII regions'' (UDHII's). Generally identified in the radio regime as having an optically thick free-free spectral energy distribution at wavelengths < 6 cm (alpha > 0 for Snu propto nualpha), some of these UDHII's have also recently been confirmed as luminous mid-infrared sources. Energy requirements indicate that the current sample of UDHII's host a range of a few to 1000 equivalent O stars, implying that the most massive and dense of these UDHII's are plausibly proto-globular clusters. The luminosities and radio spectral energy distributions of UDHII's are consistent with model HeII regions having average electron densities from ne approx 103 to 105 cm-3 and radii of a few parsecs. Here I will overview the properties of the currently known sample of UDHII's and examine their relation to Galactic UCHII's.
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