LARGE SILICON ABUNDANCE IN PHOTODISSOCIATION REGIONS

摘要

We have made one-dimensional raster scan observations of the ρ Oph and σ Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the ρ Oph region, [Si Ⅱ] 35 μm, [O Ⅰ] 63 μm, 146 μm, [C Ⅱ] 15 8 μm, and the H_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_0 ～ 30-500, the gas density n ～ 250-2500 cm~(-3), and the surface temperature T ～ 100-400 K. The ratio of [Si Ⅱ] 35 μm to [O Ⅰ] 146 μm 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 μm and [C Ⅱ] 158 μm emissions are too weak relative to [O Ⅰ] 146 μm 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 [OⅠ] 63 μm and [C Ⅱ] 158 μm emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the σ Sco region, we have detected three fine-structure lines, [O Ⅰ] 63 μm, [N Ⅱ] 122 μm, and [C Ⅱ] 158 μm, and derived that 30%-80% of the [C Ⅱ] emission comes from the ionized gas. The upper limit of the [Si Ⅱ] 35 μm is compatible with the solar abundance relative to nitrogen, and no useful constraint on the gaseous Si is obtained for the a Sco region.