BISTABILITY IN LARGE CHEMICAL NETWORKS - A GLOBAL VIEW

The nature of bistability in large gas-phase chemical networks of dens e interstellar clouds at 10 K is examined. The dependence of bistabili ty on the parameter zeta/n(H), the cosmic ray ionization rate divided by the total hydrogen density, for a wide range of elemental depletion s is investigated in detail. We confirm that bistability can exist at steady-state for a range of zeta/n(H), but we also confirm that the ra nge 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 1 10(5) cm(-3) assuming a standard value for zeta. When all of our new neutral-neutral model results are plotted on one diagram with the fractional electron abundance as ordinate and the parameter zeta/n(H) as abscissa, it is found that bistability is confined to a vertical ba nd which is narrower at small zeta/n(H) (high densities). Above and be low the band lie the so-called ''high metal'' and ''low metal'' single -phase results. The intermediate electron abundances at which bistabil ity occurs are best obtained by relatively high abundances of the elem ent sulfur because this element, unlike real metals, is a ''soft'' ion izer; 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) soluti ons for a bistable model at a rather high density near 3 10(4) cm(-3) with a standard cosmic ray ionization rate. Both the steady-state resu lts as well as a variety of early-time results are compared with obser vations in TMC-1 and L134N.