Modeling far-infrared emission from the HII region S125

We present ISO-LWS 45-180 mum spectral observations at 17 positions in the HII region S125. We model, as one integrated region, the far-IR continuum emission as well as the observed emissions in the radio continuum and in the 21 cm line, comparing two different model density distributions and several silicate-carbon dust models. The dust in the HII region is found to be severely depleted, while the dust in the PDR region has a normal dust/gas ratio, but with a major fraction of the mass in the form of the very small dust grains. Silicate-carbon dust must have strongly enhanced far-IR emissivities to be consistent with the observed spectra. It is found that the ionizing star alone does not power the infrared luminosity, but that additional sources of far-IR radiation must be present in a ring-like region surrounding the HII region. In this region the star accounts for about 50% of the observed emission. For most positions, the additional emission peaks at a wavelength around 70 mum.