The evolution of HII regions

Here we review some recent results on the observed properties of UCHII regi ons, and on the new class of Super-ultra-compact HII regions, and describe the theoretical scheme for HII region expansion, from star-forming cloud co res to the more diffuse external medium. High-pressure cores can stop the e xpansion on relatively short time scales, of the order of 3x 10(4) yr, and the resulting pressure equilibrium UCHII regions have small sizes and large ion densities. If the ionization front enters into the decreasing density gradient of the cloud core, the expansion is accelerated and the HII region "pops'' out of the core and becomes a "blister''-like region. The properti es of the outflows depend on the cloud density structure, and a variety of different shocks can be formed. Density inhomogeneities inside photoionized regions are smoothed out on short time scales, but new clumps are continuo usly created at the cool shell generated by the shock front. Instabilities in both the ionization and shock fronts cause the fragmentation of the shel l, maintaining the presence of clumps during the whole evolution.