The sulphur depletion problem

From observations of sulphur-bearing and other molecular species and chemical models it has been established that elemental sulphur is roughly two orders of magnitude more depleted in the detectable parts of such regions than are elemental carbon, nitrogen and oxygen. It seems surprising that sulphur is so depleted but not entirely depleted. We suggest that the fact that much of the sulphur is in S(+) in translucent clumps with hydrogen number densities of less than 10(3) cm(-3) plays a significant role in determining why it is so depleted in denser sources. Ions collide more rapidly with grains and may stick more efficiently to them than neutrals; so, as a clump collapses, sulphur may become depleted in it more rapidly than elements that are not primarily ionized in translucent material. Eventually in the collapse, gas-phase sulphur will become contained mostly in neutral species, which in our picture leads to a large decrease in its depletion rate and a remnant gas-phase elemental fractional abundance high enough for sulphur-bearing species in dense cores to be detectable.