ISSN 1063-7729, Astronomy Reports, 2009, Vol. 53, No. 7, pp. 611–633. c  Pleiades Publishing, Ltd., 2009. Original Russian Text c  M.S. Kirsanova, D.S. Wiebe, A.M. Sobolev, 2009, published in Astronomicheski˘ ı Zhurnal, 2009, Vol. 86, No. 7, pp. 661–683. Chemodynamical Evolution of Gas Near an Expanding HII Region M. S. Kirsanova 1 , D. S. Wiebe 1 , and A. M. Sobolev 2 1 Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya ul. 48, Moscow, 109017 Russia 2 Ural State University, pr. Lenina 51, Yekaterinburg, 620083 Russia Received December 4, 2008; in final form, December 30, 2008 Abstract—A self-consistent model for the chemical–dynamical evolution of a region of ionized hydrogen around a massive young star and of the surrounding molecular gas is presented. The model includes all main chemical and physical processes, namely the photoionization of atomic hydrogen, photodissociation of molecular hydrogen and other molecules, and the evaporation of molecules from the mantles of dust particles. Heating and cooling processes are taken into account in the temperature calculations, including cooling in molecular and atomic lines. The hydrodynamical equations were solved using the Zeus2D hy- drodynamical software package. This model is used to analyze the expansion of a region of ionized hydrogen around massive stars (effective temperature of 30 000 and 40 000 K) in a medium with various initial density distributions. The competition between evaporation from dust mantles and the photodissociation of molecules results in the formation of a transition layer between the hot HII region and cool quiescent medium, characterized by high abundances of molecules in the gas phase. The thickness of the transition layer is different for different molecules. Since there is a velocity gradient along the transition layer, and the maxima in the distributions of different molecules are at different distances from the star, observations of molecular emission lines should reveal distinction in shifts of lines of different molecules relative to the velocity of the quiescent gas. Such shifts have indeed been detected during molecular observations of the region of ionized hydrogen Sh2-235. For an initial gas density of 10 3 cm −3 , the increase in the abundances of H 2 O and H 2 CO in the transition layer after desorption from dust occurs gradually rather than in a jump-like fashion; therefore, the concept of a “evaporation front” can be used only formally. In addition, the distances between the evaporation fronts for different molecules are significant. At higher initial gas densities (10 4 cm −3 ), sharp evaporation fronts are formed for the different molecules, which are close to each other and to the shock front. In this case, it is possible to speak of a single evaporation front for CO, H 2 O, and H 2 CO. PACS numbers: 98.38.Hv, 97.21.+a, 97.10.Bt, 95.30.Wi DOI: 10.1134/S106377290907004X 1. INTRODUCTION Ionized gas is an important component of the galactic interstellar medium. According to modern concepts, the warm ionized medium (WIM) occupies about 25% of the volume of the Galaxy and has a mass of the order of 10 9 M ⊙ ; this is only a factor of a few less than the total mass of neutral material [1]. A considerable fraction of the ionized gas in the Galaxy is included in regions of ionized hydrogen (HII regions) surrounding young massive stars and their clusters. The range of parameters of HII regions is very broad. “Classical” regions have sizes of several par- secs and electron densities of the order of 10 2 cm −3 . However, there also exist ultracompact and hyper- compact HII regions (with sizes less than 0.1 pc and densities >10 4 cm −3 ) and giant HII regions (sizes of the order of 100 pc, densities <30 cm −3 ). The evolutionary relations between these different types of regions of ionized hydrogen are not clear. It cannot be excluded that all of them represent stages of a single process, during which an expanding hyper- compact HII region around a young massive star is successively transformed into an ultracompact and classical HII region, then merges with the regions of neighboring stars to produce a giant HII region around an OB association [2]. Classical HII regions are currently best studied, both observationally and theoretically; the earliest studies of these objects began more than 60 years ago. During this time, the expansion of a region of ionized hydrogen has become a classical astro- physical problem (see, e.g., the books of Kaplan and Pikel’ner [3] and Spitzer [4]). Various analytical 611