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 clouds allowing for the influence of the grains. We find that the solutions of the chemical rate equations are bistable, as reported previously. If it is assumed that hydrogenation reactions occur rapidly on grain surfaces and saturated species are returned to the gas phase, the zone of bistability is restricted to low gas density n(H) approximate to 10(2) cm(-3). However, in this case, the fractional abundances of saturated species in the gas phase are unrealistically large. We point out 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.