Sensitivity analyses of dense cloud chemical models
  Because of new telescopes that will dramatically improve our knowledge of the
interstellar medium, chemical models will have to be used to simulate the
chemistry of many regions with diverse properties. To make these models more
robust, it is important to understand their sensitivity to a variety of
parameters. In this article, we report a study of the sensitivity of a chemical
model of a cold dense core, with homogeneous and time-independent physical
conditions, to variations in the following parameters: initial chemical
inventory, gas temperature and density, cosmic-ray ionization rate, chemical
reaction rate coefficients, and elemental abundances. From the results of the
parameter variations, we can quantify the sensitivity of the model to each
parameter as a function of time. Our results can be used in principle with
observations to constrain some parameters for different cold clouds. We also
attempted to use the Monte Carlo approach with all parameters varied
collectively. Within the parameter ranges studied, the most critical parameters
turn out to be the reaction rate coefficients at times up to 4e5 yr and
elemental abundances at later times. At typical times of best agreement with
observation, models are sensitive to both of these parameters. The models are
less sensitive to other parameters such as the gas density and temperature. The
improvement of models will require that the uncertainties in rate coefficients
of important reactions be reduced. As the chemistry becomes better understood
and more robust, it should be possible to use model sensitivities concerning
other parameters, such as the elemental abundances and the cosmic ray
ionization rate, to yield detailed information on cloud properties and history.
Nevertheless, at the current stage, we cannot determine the best values of all
the parameters simultaneously based on purely observational constraints.
