Project A6

Project A6 plans to improve our understanding of the physics of photon-dominated regions (PDRs), this means interstellar clouds where the UV radiation from young massive stars creates a characteristic chemistry and excitation in the medium. Based on models we will try to derive the properties of  the usually deeply embedded young stars from the observable radiative signatures at infrared and radio wavelengths. Understanding the PDR physics allows us to interpret the observations of the main cooling and additional diagnostics lines as a tool to measure the impinging UV field produced by short-living O- and B-type stars.

Major questions to be solved are the role and distribution of PAHs and very small dust grains and the details of the H₂ formation and excitation in the ISM. For both aspects we expect revolutionising new observational constraints from JWST spectroscopy. At the same time a better understanding of the dust physics in PDRs is a prerequisite to interpret the spatially unresolved observations of PDR tracers at high redshifts.

Based on the long-term experience in Cologne with clumpy PDR modelling, we plan to implement a versatile framework to fit observations of any highly complex UV-dominated structure by clump ensembles using the KOSMA-tau-3D approach. A major work package covering the whole CRC time frame is the development of efficient and specific tools for the comparison of the large observational data sets, with the model predictions taking the full set of lines and continuum data, the spatial structure, and the velocity profiles into account.

By answering the PDR-specific question: Which observational signatures, which lines and chemical tracers are reliable measures of the local UV radiation across different galaxies and cosmic times? we provide tools so that CRC 1601 can address the question: How can we actually measure the star-formation rate across the universe?