ULTRACOMPACT H-II REGIONS - ARE THEIR LIFETIMES EXTENDED BY DENSE, WARM ENVIRONMENTS

Ultracompact H II regions (UCHIIs) are de fined as regions of ionized gas with diameters smaller than similar to 0.1 pc (Wood and Churchwell 1989a). H II regions are expected to expand at velocities on the orde r of the sound speed (10 km s(-1)) until reaching equilibrium at dimen sions of a few pc. In regions with density of no similar to 10(5) cm(- 3), H II regions should remain ultracompact for similar to 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 t o explain this ''lifetime paradox''; all have shortcomings. The parado x could be resolved if the molecular gas in which an O star forms is d enser and warmer than previously believed, resulting in an initial Str omgren sphere much smaller than originally estimated. This suggestion finds support in recent observations, which show that in dense molecul ar cloud cores densities of 10(7) cm(-3) and temperatures of 100 K are not atypical. We compare the expected UCHII radii (using the higher t emperatures and densities) with the observed radii of a sample of UCHI I regions and find significant agreement between the two.