Reading the message in the light: understanding STIS and COS spectra, preparing for the Webb Era

UV spectroscopy reveals the chemical composition, star formation history, kinematic state, and excitation conditions of a vast variety of astronomical objects. The gas producing the spectrum is usually far from equilibrium and its properties set by a host of microphysical processes. I developed the plasma simulation code Cloudy to model just such environments. The code is widely used across the astronomical community, with nearly 200 papers citing its documentation each year, many of these studies involving HST observations. I request support for continued development of Cloudy with an emphasis on low-ionization regions detected in absorption in the UV, and in emission in the UV through IR. This complements my previous Theory grant, now ending, which developed a framework for using large atomic databases and creating models of high-ionization species. The proposed new work has several parts: A) the incorporation of recently developed theories for the formation of molecular hydrogen on grain surfaces, B) the expansion of the atomic database to low-ionization species, complementing the previous work, and C) upgrade the low-ionization atomic and molecular emission models. These more robust simulations of lower-ionization species formed near atomic / molecular transition regions have obvious implications for observations of intergalactic or associated absorbers. The atoms which absorb in the UV also emit in the optical / IR. Cloudy does a full simulation of the microphysics of the gas and dust, so the IR emission is also predicted. The work proposed here, while focusing on the UV, will, as a no-cost spinoff, improve emission models for wavelengths detected by Webb.