Abstract
Ultracompact H II regions (UCHIIs) are defined as regions of ionized gas with diameters smaller than ∼0.1 pc. H II regions are expected to expand at velocities of the order of the sound speed (10 km s-1) until reaching equilibrium at dimensions of a few pc. In regions with density of n0 ∼105cm-3, H II regions should remain ultracompact for ∼3000 years and only a few dozen should exist in the Galaxy. However, observations suggest that many more UCHIIs exist and that lifetimes should be one to two orders of magnitude larger. Several models have been proposed to explain this "lifetime paradox"; all have shortcomings. The paradox could be resolved if the molecular gas in which an O star forms is denser and warmer than previously believed, resulting in an initial Strömgren sphere much smaller than originally estimated. This suggestion finds support in observations which show that in dense molecular cloud cores densities of 107cm-3 and temperatures of 100 K are not atypical. The authors compare the expected UCHII radii (using the higher temperatures and densities) with the observed radii of a sample of UCHIIs and find significant agreement between the two.
