2009A&A...508..783L

Agreement between observed and calculated abundances of some molecules in dark clouds is improved when the model gas retains some memory of the assumed initial conditions. We assess the relevance of commonly-assumed intitial conditions giving rise to so-called early-time chemistry in models of dark cloud chemistry. Initial conditions for models of dark cloud chemistry are compared with conditions known to exist in the H₂-bearing diffuse gas whose evolution they are intended to represent. The initial conditions assumed for dark cloud chemistry differ in at least five important regards from those of H₂-bearing diffuse gas. Models typically assume: absolute purity of H₂; absence of polycyclic aromatic hydrocarbons; two times smaller gas-phase abundance of C, N, and O; near-absence of sulfur and some other easily-ionized metals; total absence of the molecules which accompany the presence of H₂ in diffuse gas, for instance, OH, CH, C₂H, HCO⁺. Initial conditions commonly assumed for dark cloud chemical models should be reconsidered and mechanisms for the chemical evolution from diffuse to dark gas should be included in model calculations.