PDF_CHEM: Fast simulations of the chemical ISM using probability distributions

We present PDFCHEM, a new numerical method able to compute the photodissociation region (PDR) chemistry of large-scale inhomogeneous ISM regions using probability distributions of physical parameters as an input. We distinguish between two visual extinctions, namely the ‘effective’ (AV,eff) referring to the local extinction and the ‘observed’ (AV,obs) referring to the extinction taken from observations. Using 1,200 pre-calculated PDR simulations, we apply PDFCHEM to two hypothetical AV,obs−PDFs, representing a diffuse region and a Giant Molecular Cloud. PDFchem is fast and can replace the computationally expensive hydrodynamical models in understanding the chemistry of the ISM in different environments, including at low metallicities and high cosmic-ray ionization rates.