The Influence of Variations of Elemental Composition of a Gas on the Molecular Kinetics

The low efficiency of metal mixing in the interstellar gas preserves their spatial inhomogeneities for longer times. In this work, we study how the variations in the elemental composition affects the chemical evolution of the molecular gas, that cools down behind the supernovae shock fronts. We perform numerical simulations of the non-equilibrium chemical kinetics of a Lagrangian gas element experiencing the shockwaves travelling at 5–15 km/s velocities. We vary the chemical abundances of carbon and oxygen in range ±0.3 dex from the values typical for the ISM. We find significant differences in fractions of several molecular species, disproportionate with the variations in the abundances of the corresponding chemical elements. While these differences are remarkably higher than the variations of the given element abundances. We investigate how these changes in the molecular fractions depend on the properties of the gas, as well as on the external effects: gas density, ultraviolet flux, cosmic ray ionization rate and extinction. We find, for instance, that variations of oxygen abundance for δ[O/H] ~ ±0.3 dex of the Solar value, modify the water molecule fraction already by ~ 3–50 times. The differences are the most prominent for extinction values A ~ 0.3–3. We discuss the importance of these findings for chemical kinetics in star-forming regions.