1999A&A...342..542W

We consider the chemistry of silicon in photo-dissociation regions (PDRs), with special attention to the available observations of the Orion Bar. We focus on the ``paradox'' that fine structure emission of singly ionized silicon (Si<sup>+</sup>) is known to be strong from such regions, whereas silicon in molecular form (and in particular SiO) is absent. We consider a number of possible explanations of this ``paradox'' and conclude that the most likely is that Si is in solid form at depths greater than roughly 3-4 magnitudes of visual extinction, relative to the ionization front, in regions such as the Orion Bar. We consider that direct photodesorption with a small yield is likely to be the process responsible for the ejection of a Si-containing mantle into the gas phase. Other possibilities, such as the Si being present as a component of grain ice mantles, are considered; we believe that they are less likely. We find that grain polar ice mantles are probably also destroyed in PDRs, through photodesorption, whereas thermal evaporation destroys the apolar component of ice mantles deep within the surrounding molecular cloud.