Physical conditions in the ISM towards HD185418

Abstract

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 towards HD185418, a line-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements predicted by MHD simulations.

arXiv comment31 pages, accepted for publication in ApJ

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• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds
• The origin of molecular hydrogen emission in cooling-flow filaments
• CO-dark gas and molecular filaments in Milky Way-type galaxies - II: The temperature distribution of the gas
• On The Gas Temperature of Molecular Cloud Cores
• On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
• A study of low-metallicity DLAs at high redshift and C II* as a probe of their physical conditions
• Modelling the spinning dust emission from dense interstellar clouds
• Herschel/HIFI Spectral Mapping of C${}^{+}$, CH${}^{+}$, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH${}^{+}$ Formation
• Modeling of diffuse molecular gas applied to HD 102065 observations
• Physical conditions in three high-z H2-bearing DLAs: implications for grain size
• Multiwavelength investigation of a near-solar metallicity sub-DLA at z =1.3647 towards PKS 0237-233
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• A recent update of gas-phase chemical reactions and molecular lines in CLOUDY: its effects on millimeter and sub-millimeter molecular line predictions
• Probing the tidal disruption event iPTF16axa with CLOUDY and disc-wind models
• Physical conditions in two high-redshift H${}_2$-bearing GRB-DLAs, 120815A and 121024A
• Spectroscopic modelling of two high-mass X-ray binaries, Cyg X--3 and 4U 1538--522
• Rate Constants for Fine-Structure Excitations in O-H Collisions with Error Bars Obtained by Machine Learning