ar X iv : a st ro - p h / 03 11 45 0 v 1 1 9 N ov 2 00 3 On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling

With the chemical reaction rate database UMIST95 (Millar etal. 1997) we analyze how uncertainties in rate constants of gas-phase chemical reactions influence the modelling of the molecular abundances in the inter-stellar medium. Random variations are introduced into the rate constants to estimate the scatter in theoretical abundances. Calculations were performed for the dark and translucent molecular clouds where gas phase chemistry is adequate (Terzieva & Herbst 1998). Similar approach was used by Pineau des Forets & Roueff (2000) for the study of chemical bistability. All the species are divided into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. It is shown that the distribution of species within these groups depends on the number of atoms in them and on the adopted physical conditions. The simple method is suggested which allows to single out reactions that are most important for the evolution of a given species. 1 Scatter in the model abundances To study the influence of uncertainties in reaction rates on the model abundances we calculated 10 4 variants of each species abundance. Figure 1 shows the scatter for some representative species. We divided all species into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. Distribution of the species within these groups depends on adopted physical conditions (see fig. 1). Scatter in logarithmic abundances of simple molecules lies within 0.5 − 1 dex. Figure 2 shows how the species are distributed by the number of atoms in different sensitivity groups. It is clear that the scatter in abundances significantly increases with the number of atoms in the molecule.