Physical conditions in the interstellar medium toward HD 185418

We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H₂ absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H₂ is intricate, and it is important to validate these numerical simulations in better understood environments. This paper studies a well-defined line of sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM toward HD 185418, a line of sight with many observables. We deduce density, temperature, local radiation field, cosmic-ray ionization rate, and chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density and similar abundances, and we require a cosmic-ray flux enhanced over the Galactic background value, consistent with enhancements predicted by MHD simulations.