Determining the photodissociation region (PDR) properties of the interstellar medium (ISM), such as the atomic and molecular mass content, is of fundamental importance to understand the star-formation process. The advent of ALMA has made possible to observe molecular lines in the distant Universe. In addition, modern observations of the ISM in galaxies detect a variety of atomic and molecular species. Scientific groups worldwide are constantly working on connecting these observations to the astrochemical properties of the ISM.
Modern 3D hydro-chemical simulations have now made possible to simultaneously calculate the dynamical and the chemical evolution of the ISM in galaxies. However, this modelling comes with a high computational cost and it limits the possibilities of exploring a large parameter space of environmental parameters, such as the cosmic-ray ionization rate, the strength of the far-UV radiation field and the metallicity.
To overcome the above computational difficulties, I have developed PDFchem, a novel algorithm that models the cold ISM at moderate and large scales using functions connecting the quantities of the local (or effective) and the observed visual extinctions, and the local number density, with probability density functions (PDF) on cloud scales typically tens-to-hundreds of pc as an input. PDFchem is fast, easy to use, reproduces the PDR quantities of the hydrodynamical models, and can be used directly with observed data of the cold ISM.