The Microphysics of Hydrodynamical Flows: Conditions in Starburst

Ultraviolet, optical, and infrared emission line regions of galactic H II regions like Orion, giant extragalactic HII regions, and the luminous Starburst Galaxies, are most likely ablating layers of photoionized gas on the surface of a molecular cloud. There is a large body of work that calibrates the observed emission line spectrum in terms of the fundamental parameters of the stellar system, including the star formation rate, the mass budget, and chemical composition. This work relies on large-scale numerical simulations of the microphysics of the plasma, including the ionization and excitation state of the plasma, molecular processes, and interactions between grains and the gas. At the same time hydrodynamicists have made great progress in describing the nature of the flows, but without a complete treatment of the microphysics. This project brings together the ingredients necessary for a complete description of both the microphysics and the hydrodynamics, allowing a self-consistent treatment of the emission line regions, and an insight into the nature of the system. We will incorporate the necessary hydrodynamics into Cloudy, a plasma emission code that includes the physics needed to simulate the ionized, atomic, and molecular phases of the environment. The hydrodynamical flow, from the molecular cloud through the PDR into the H II region, brings advection terms into the thermal and ionization balance, a complication, but allows both the H II region and PDR to be treated self-consistently, a great simplification. This will allow us to fully exploit the mid to far IR spectrum to understand the chemical evolution of the host system as well as star formation on many scales.