High Density Condensations in H II Regions

The state of the hydrogen gas in high density condensations in H II regions, such as the globules in Orion Nebula, is concluded to be molecular. The well known assumption that the conditions at the bright rims of these condensations are D -critical is examined. It is shown that the assumption is valid only when the colour temperature of the exciting star is comparatively low. With higher stellar temperatures the conditions at the front are strong D -type. In the latter case, the gas speed behind the front can be determined uniquely when the colour temperature of the exciting star is known. With stellar temperatures around $${5.\,10}^{4^{\circ}}\,\text{K}$$ these speeds are about twice the isothermal sonic speed in the ionized region.