Large Silicon Abundance in Photodissociation Regions

We have made one-dimensional raster scan observations of the r Oph and s Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the r Oph region, [Si II] 35 km, [O I] 63 km, 146 km, [C II] 158 km, and the H sub(2) pure rotational transition lines S(0) to S(3) are detected, and the photodissociation region (PDR) properties are derived as the radiation field scaled by the solar neighborhood value G sub(0) 6 30-500, the gas density n 6 250-2500 cm super(-3), and the surface temperature T 6 100-400 K. The ratio of [Si II] 35 km to [O I] 146 km indicates that silicon of 10%-20% of the solar abundance must be in the gaseous form in the PDR, suggesting that efficient dust destruction is ongoing even in the PDR and that a fraction of the silicon atoms may be contained in volatile forms in dust grains. The [O I] 63 km and [C II] 158 km emissions are too weak relative to [O I] 146 km to be accounted for by standard PDR models. We propose a simple model, in which overlapping PDR clouds along the line of sight absorb the [O I] 63 km and [C II] 158 km emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the s Sco region, we have detected three fine-structure lines, [O I] 63 km, [N II] 122 km, and [C II] 158 km, and derived that 30%-80% of the [C II] emission comes from the ionized gas.