Numerical modelling of chemical and dynamic evolution of colliding HI clouds

Interstellar clouds collisions have a strong influence on star formation, dissipation of kinetic energy and gaseous structure of galaxies. In addition, clouds collisions are one of the most important physical processes that are responsible for transitions between different phases of the interstellar medium and observed mass spectrum of diffuse HI clouds. The main aim of the article is high resolution numerical simulations of diffusion clouds collisions in two dimensions in the frame of the self-consistent model. This approach implies concurrent consideration of dynamical, thermal and chemical processes in the interstellar medium with particular attention to formation of H2 and CO during clouds inelastic collisions. The approach of Nelson and Langer (1997) for modeling of chemistry of the interstellar medium was chosen. The thermal model of the interstellar medium includes the most relevant cooling and heating processes for temperature interval between 10 and 2·104 К. We assume that clouds are characterized by temperature Tcl = 80 К and number density ncl = 10 sm-3, radius of the clouds are 1 pc before collision. Initially clouds are in pressure equilibrium with the warm surrounding intercloud medium with temperature Twim = 8 000 К, nwim = 0,1 sm-3. The chemical composition of the clouds and intercloud medium is basically the same, except for H2. The abundance of H2 in clouds are x(Н2) = 0,1, all gas in intercloud medium is in the form of HI. We have found that in radiative regime of head-on collisions of equal mass clouds the destruction of clouds takes place with formations of cold clouds filaments. The fractional abundance of H2 in filaments does not exceeds 0,1, CO fraction is very small. In adiabatic regime of interaction clouds are destroyed completely, so all the clouds material is converted in warm phase of the ISM.