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Paper: Molecular-Line Study of the Isolated Ultracompact HII-Region Onsala 1 (ON1)
Volume: 267, Hot Star Workshop III: the Earliest Stages of Massive Star Birth
Page: 385
Authors: Lim, J.; Choi, M.; Ho, P. T. P.
Abstract: The very small ultracompact (UC) HII region ON1, located at an inferred distance of 1.4 kpc, is embedded in a dense molecular core with size of 0.8 pc and mass 200 Msun. Here, we show that the center of this core is closely coincident with ON1, and exhibits a secondary enhancement extending as far as 0.1 pc north of center. The core is elongated and rotating about its minor axis, and may be radially contracting. A mostly one-sided molecular outflow, with an upper limit in dynamical age of 50,000 yrs, emerges perpendicular to the major axis of this core. The central part of the core with radius 0.04 pc is spinning faster than its outer region, mapped to a radial distance of 0.6 pc. This central region exhibits rigid-body rotation. We suggest that the central part of the core may have fragmented and decoupled from its surroundings, and may be evolving quasi-statically by slowly contracting while conserving at least some of its angular momentum. The region that must have collapsed to form the central high-mass star may be smaller still than our highest spatial resolution of 0.02pc (4000 AU).
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