Modeling far-infrared line emission from the HII region S125

We present ISO-LWS spectral line observations of the [OI] 63 and 146 mum lines, the [CII] 158 mum line, and the [NII] 122 mum line at 17 positions in the HII region S125. We model this emission by a two-dimensional geometrical blister model, utilizing the parameters derived from our previous modeling of the HII, HI and FIR dust emission in this source, thereby deriving an interpretation of the region that takes account of the thermal continuum radiation and the line emission self-consistently. We show very good agreement with the observed line emission in the central region, but in order to fit the spatial profile, it is necessary to allow for a systematic increase in the gas temperature along the PDR boundary with decreasing distance from the ionizing star. This would not be predicted by PDR models, hence we suggest that the size distribution of small grains responsible for the photoelectric heating may be changing along the boundary, depending on the distance from the star. The model shows that the HII region, the PDR region, as well as H-2 molecular region all make contributions to the emission observed in these fine structure lines and that accurate modeling of the PDR region needs to include the radiation coupling between the ionized region and the neutral molecular cloud. Comparison with values derived from one-dimensional, constant density slab models shows the present blister modeling giving higher G(0) values for the radiation field at the PDR boundary.