2015A&A...579A..67S

Photodissociation regions (PDRs) contain a large portion of all of the interstellar matter in galaxies. Classical examples include the boundaries between ionized regions and molecular clouds in regions of massive star-formation, marking the point where all of the photons that are energetic enough to ionize hydrogen have been absorbed. To determine the physical properties of the PDRs associated with the star-forming regions IRAS 23133+6050 and S 106 and present them in the context of other Galactic PDRs associated with massive star-forming regions. We employ Herschel PACS and SPIRE spectroscopic observations to construct a full 55-650µm spectrum of each object from which we measure the PDR cooling lines, other fine- structure lines, CO lines, and the total far-infrared flux. These measurements (and combinations thereof) are then compared to standard PDR models. Subsequently, detailed numerical PDR models are compared to these predictions, yielding additional insight into the dominant thermal processes in the PDRs and their structures. We find that the PDRs of each object are very similar and can be characterized by a two-phase PDR model with a very dense, highly UV irradiated phase (n∼10⁶cm^–3, G₀∼10⁵) interspersed within a lower density, weaker radiation field phase (n∼10⁴cm^–3, G₀∼10⁴). We employed two different numerical models to investigate the data. We first used RADEX models to fit the peak of the ¹²CO ladder, which in conjunction with the properties derived, yielded a temperature of around 300 K. Subsequent numerical modeling with a full PDR model revealed that the dense phase has a filling factor of around 0.6 in both objects. The shape of the ¹²CO ladder was consistent with these components, with heating dominated by grain photoelectric heating. An extra excitation component for the hightest-J lines (J>20) is required for S 106.