1998A&A...334.1047L

The nature of bistability in large gas-phase chemical networks of dense interstellar clouds at 10K is examined. The dependence of bistability on the parameter ζ/n_H, the cosmic ray ionization rate divided by the total hydrogen density, for a wide range of elemental depletions is investigated in detail. We confirm that bistability can exist at steady-state for a range of ζ/n_H, but we also confirm that the range of bistability is very dependent on elemental depletions, and also dependent on which network is utilized. In particular, bistability is a more salient feature in the new neutral-neutral model than it is in the new standard model. With the former model, we find that for some gas-phase elemental abundances, the bistability range is non-existent while for others the bistability range includes gas densities as high as 1x10⁵cm^–3 assuming a standard value for ζ. When all of our new neutral-neutral model results are plotted on one diagram with the fractional electron abundance as ordinate and the parameter ζ/n_H as abscissa, it is found that bistability is confined to a vertical band which is narrower at small ζ/n_H (high densities). Above and below the band lie the so-called ``high metal'' and ``low metal'' single-phase results. The intermediate electron abundances at which bistability occurs are best obtained by relatively high abundances of the element sulfur because this element, unlike real metals, is a ``soft'' ionizer; i.e., its abundance is not totally in ionized forms. We present newly-obtained steady-state results for a variety of molecules in both the HIP (high ionization phase) and LIP (low ionization phase) solutions for a bistable model at a rather high density near 3x10⁴cm^–3 with a standard cosmic ray ionization rate. Both the steady-state results as well as a variety of early-time results are compared with observations in TMC-1 and L134N.