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Session 42 - Molecular Clouds: Models and Observations.
Display session, Tuesday, June 11
Great Hall,
We have measured the angular expansion of the ultra-compact HII region W3(OH) from VLA observations separated by 9.3 years. The difference maps are produced from high sensitivity 2 cm continuum maps , and the proper motion sensitivity is 1 km/s. The difference map shows anisotropic expansion of the ionized region with a typical rate of 4 km/s. This expansion rate agrees with that of OH masers surrounding the region. The measurements do not agree with a simple bow-shock model, but a consistent with a so-called champagne flow model, in which an HII region expands in an inhomogeneous environment. The dynamical age of the region implied by the expansion is 2500 years. We have also detected a rapidly evolving or moving source near the projected center of the HII region.
