Compact H II Regions and OB Star Formation

Abstract

In 1967 radio observations led to the discovery of very small (d 104 cm -3) H II regions. Ryle & Downes (1967) found that the obscured object DR 2 1 has a very high emission measure E.M. ( = n/d) = 5 x 107 pc cm-6, while Mezger, Schraml & Terzian (1967a) deduced from the measured radio spectrum that the similarly obscured galactic source W49A contains a high density component with E.M. = 108 pc cm 6. Subsequently, Mezger et al. (1967b) coined the term com­ pact H II regions, showed the close relationship to OH main-line masers, and proposed that compact H II regions/OH masers should be recognized as a new class of galactic sources. The recombination rate of the compact H II regions requires ionization by one or more early 0 type star; from the observed high turbulent velocities (V;:;:: 25 km s -1 ) Mezger et al. (1967b) concluded that the objects must be in a state of rapid expansion, and must represent a very early stage in the life of an OB star. In fact, Vandervoort (1963) and W. G. Mathews (1965) had already pointed out that H II regions with such densities must of necessity undergo rapid expansion and therefore have short lifetimes. The discovery of compact H II regions coincided with their theoretical prediction by Davidson & Harwit (1967), who also drew attention to the strong infnired emission expected from the (predicted) dust cocoon (see also Davidson 1970). The first compact H II regions to be recognized as strong sources of near-infrared emission were W 51-IRS2 and K3-50