Our main conclusion is that the Si+ fine structure emission observed in many PDRs is likely to be a consequence of photo-erosion of grain mantles at depths corresponding to a few magnitudes of visual extinction. A small photodesorption yield (), together with the assumption that silicon comes off in atomic form, suffice to explain the weakness of the emission from SiO in the Orion Bar and to account for the observed Si+. Modest amounts of atomic Si (typically 0.001 of the ionized Si) are predicted by such models. An alternative explanation - that the silicon arises from the thermal evaporation of ice mantles - cannot be excluded entirely but appears less likely. The high apparent degree of depletion of Si in dark dust clouds is thus likely to be due to efficient deposition of Si in a mantle which is more refractory than water ice and which has a small photodesorption yield. A by-product of our study is the conclusion that, in regions such as the Orion Bar, direct photodesorption of water ice is the main process causing the transition from solid to gas phase; this occurs at a depth where evaporation of water ice is unimportant.
© European Southern Observatory (ESO) 1999
Online publication: February 22, 1999