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PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions
Thomas G. Bisbas, Ewine F. van Dishoeck, Chia-Yu Hu, Andreas Schruba
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Determining the atomic and molecular content of the interstellar medium (ISM) as a function of environmental parameters is of fundamental importance to understand the star-formation process across the epochs. Although there exist various three-dimensional hydro-chemical codes modelling the ISM at different scales and redshifts, they are computationally expensive and inefficient for studies over a large parameter space. Building on our earlier approach, we present PDFchem, a novel algorithm that models the cold ISM at moderate and large scales using functions connecting the quantities of the local (A_(V,eff)) and the observed (A_(V,obs)) visual extinctions, and the local number density, n_H, with probability density functions (PDF) of A_(V,obs) on cloud scales typically tens-to-hundreds of pc as an input. For any given A_(V,obs)-PDF, sampled with thousands of clouds, the algorithm instantly computes the average abundances of the most important species (HI, H₂, CII, CI, CO, OH, OH^+, H₂O^+, CH, HCO^+) and performs radiative transfer calculations to estimate the average emission of the most commonly observed lines ([CII]~$158\mu$m, both [CI] fine-structure lines and the first five rotational transitions of ¹²CO). We examine two A_(V,obs)-PDFs corresponding to a non star-forming and a star-forming ISM region, under a variety of environmental parameters combinations. These cover FUV intensities in the range of χ/χ₀=10⁻¹-10³, cosmic-ray ionization rates in the range of ζ_(CR)=10⁻¹⁷-10⁻¹³ s⁻¹ and metallicities in the range of Z=0.1-2 Z_⊙. PDFchem is fast, easy to use, reproduces the PDR quantities of the time-consuming hydrodynamical models and can be used directly with observed data to understand the evolution of the cold ISM chemistry.
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