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 mm, [O I] 63 mm, 146 mm, [C II] 158 mm, and the H2 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 G0 ~ 30-500, the gas density n ~ 250-2500 cm-3, and the surface temperature T ~ 100-400 K. The ratio of [Si II] 35 mm to [O I] 146 mm 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 mm and [C II] 158 mm emissions are too weak relative to [O I] 146 mm 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 mm and [C II] 158 mm 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 mm, [N II] 122 mm, and [C II] 158 mm, and derived that 30%-80% of the [C II] emission comes from the ionized gas. The upper limit of the [Si II] 35 mm is compatible with the solar abundance relative to nitrogen, and no useful constraint on the gaseous Si is obtained for the s Sco region.