Expansion of W3(OH)

A direct measurement of the expansion of W3(OH) is made by comparing Very Large Array images taken ~10 yr apart. The expansion is anisotropic with a typical speed of 3-5 km s -1 , indicating a dynamical age of only 2300 yr. These observations are inconsistent with either the freely expanding shell model or a simple bow shock model. The most favored model is a slowly expanding shell-like H II region, with either a fast rarefied flow or another less massive, diffuse, ionized region moving toward the observer. There is also a rapidly evolving source near the projected center of emission, perhaps related to the central star.