The star-formation history of molecular cores is largely determined by gas phase chemical processes that are greatly modified by gas-dust interactions. Substantial elemental depletions may result from the efficient formation of refractory grain material. The depletion of carbon and of refractory elements such as S, Si, Mg, etc. in molecular clouds is well known, but is very poorly constrained. Star forming regions are cold, dark and chemically quiescent, so that in addition to the initial elemental depletions, an ongoing dynamical depletion of molecular material occurs as gas-phase material freezes out onto the surface of dust grains. Observational evidence for anomalous depletions in low mass star forming clumps became apparent in the early 1980s when the narrowness of molecular emission lines (especially those of NH3) suggested that high velocity infalling material is being depleted from the gas phase. This prompted further studies into the chemical effects of differential depletion and together with radiative transfer models has established molecular diagnostics of infall/depletion sources. Recent observations at high spatial resolution show direct evidence for gas-phase depletions in the central regions of protostellar cores.
In addition to the diagnostic and purely chemical implications for collapsing cores, depletion plays a very active role in the physical evolution of star-forming regions. Most gravitationally unstable low mass cores are believed to be magnetically sub-critical. Recent polarimetric studies of elongated cores are also providing evidence of magnetic alignment, giving strong support to the idea that magnetic fields play a dominant role in core evolution. The relaxation of magnetic pressure (whether through ambipolar diffusion, or by the damping of MHD waves) is critically dependent on the ionization structure which, in turn, is highly sensitive to the elemental and molecular depletions.