We report observations with the IRAM 30-m telescope of SiO(2-1), (3-2), and (5-4) towards the Orion Bar, S 140, NCC 2023 and NGC 7023. We detect SiO towards the Orion Bar and the S 140 photon dominated region (PDR) with an abundance of order 10 −11 relative to H2. Towards the Orion Bar, SiO is extended and is not confined to a layer close to the ionization front. We put limits of 2-4 × 10 −11 on [SiO]/[H2] towards several positions in NGC 2023 and NGC 7023. In order to interpret these data, we

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... resent model calculations of the SiO abundance in PDRs as a function of the incident radiation field and consider also the expected Si + 35 µm fine structure line intensity. Our data suggest that silicon is very highly depleted onto dust grains within the molecular cloud but that a fraction of the order of ten percent of the silicon comes back into the gas phase in a layer close to the ionization front. We attempted to estimate whether the observed silicon fine structure line emission could originate in fully ionized gas and conclude that in the case of the Orion Bar, it is quite likely. Finally, a by-product of our observations was the detection of the 86773.43 MHz J = 1/2 → 1/2 F = 1 → 1 component of the 101 − 000 line of HCO in all four sources confirming that this species is relatively abundant in PDRs.