Bistability in interstellar gas-phase chemistry

We present an analysis of ``bistability'' in gas-phase chemical models of dark interstellar clouds. We identify the chemical mechanisms that allow high- and low-ionization solutions to the chemical rate equations to coexist. We derive simple analytic scaling relations for the gas densities and ionization rates for which the chemistry becomes bistable. We explain why bistability is sensitive to the H-3(+) dissociative recombination rate coefficient and why it is damped by gas-grain neutralization.