Excited OH in Absorption Towards W3(OH)

Abstract

There is considerable evidence for clumps or condensations of approximately stellar mass in the neutral gas surrounding compact HII regions. There are a variety of observational approaches to studying such gas and some of these have been discussed at this Conference (see e. g. Bastien et al. (this volume), Forster et al. (this volume)). In this contribution, we discuss recent absorption line observations of excited (300 to 500 K above ground) A-doublet transitions of OH towards the compact HII region W3(OH). Such observations are interesting due to the large spontaneous decay rates of the far infrared transitions which depopulate these excited levels (~ 1 s^-1). This implies either a very large density (> 10⁸ cm^-3) or a very strong far-infrared radiation field in the OH absorption region. In either case, it is hard to avoid the conclusion that what one is observing is a very high density clump close to the HII region ionization front.