ON THE UNIQUENESS OF THE SOLUTIONS TO THE CHEMICAL RATE-EQUATIONS IN INTERSTELLAR CLOUDS - THE GAS-DUST INTERFACE

We have computed the chemical equilibrium state of dark interstellar c louds allowing for the influence of the grains. We find that the solut ions of the chemical rate equations are bistable, as reported previous ly. If it is assumed that hydrogenation reactions occur rapidly on gra in surfaces and saturated species are returned to the gas phase, the z one of bistability is restricted to low gas density n(H) approximate t o 10(2) cm(-3). However, in this case, the fractional abundances of sa turated species in the gas phase are unrealistically large. We point o ut that, if desorption occurs preferentially from the outer layers of grain mantles, the final (equilibrium) state of the gas must depend on the initial conditions.