The effect of uncertainties on chemical models of dark clouds

The gas-phase chemistry of dark clouds has been studied with a treatment of uncertainties caused both by errors in individual rate coeffcients and uncertainties in physical conditions. Moreover, a sensitivity analysis has been employed to attempt to determine which reactions are most important in the chemistry of individual species. The degree of overlap between calculated errors in abundances and estimated observational errors has been used as an initial criterion for the goodness of the model and the determination of a best chemical age of the source. For the well-studied sources L134N and TMC-1CP, best agreement is achieved at so-called early times of approximate to 10(5) yr, in agreement with previous calculations but here put on a firmer statistical foundation. A more detailed criterion for agreement, which takes into account the degree of disagreement, is also proposed. Poorly understood but critical classes of reactions are delineated, especially reactions between ions and polar neutrals. Such reactions will have to be understood better before the chemistry can be made more secure. Nevertheless, the level of agreement is low enough to indicate that a static picture of physical conditions without consideration of interactions with grain surfaces is inappropriate for a complete understanding of the chemistry.