The HII region/PDR connection: Self-consistent calculations of physical conditions in star-forming regions

We have performed a series of calculations designed to reproduce infrared diagnostics used to determine physical conditions in star-forming regions. We self-consistently calculate the thermal and chemical structure of an H II region and photodissociation region (PDR) that are in pressure equilibrium. This differs from previous work, which used separate calculations for each gas phase. Our calculations span a wide range of stellar temperatures, gas densities, and ionization parameters. We describe improvements made to the spectral synthesis code Cloudy that made these calculations possible. These include the addition of a molecular network with similar to 1000 reactions involving 68 molecular species and improved treatment of the grain physics. Data from the Spitzer First Look Survey, along with other archives, are used to derive important physical characteristics of the H II region and PDR. These include stellar temperatures, electron densities, ionization parameters, UV radiation flux (G(0)), and PDR density. Finally, we calculate the contribution of the H II region to PDR emission line diagnostics, which allows for a more accurate determination of physical conditions in the PDR.