Multi-line Observations, Models, and Data Needed to Understand the Nature of UV-irradiated Interstellar Matter

Far-ultraviolet photons from OB-type massive stars regulate the heating, ionization, and chemistry of much of the neutral interstellar gas in star-forming galaxies. The interaction of FUV radiation and interstellar matter takes place in environments broadly known as photodissociation regions (PDRs). PDR line diagnostics are the smoking gun of the radiative feedback from massive stars. Improving our understanding of stellar feedback in the ISM requires quantifying the energy budget, gas dynamics, and chemical composition of PDR environments. This goal demands astronomical instrumentation able to deliver multi-line spectroscopic images of the ISM (of the Milky Way and nearby galaxies). It also requires interdisciplinary collaborations to obtain the rate coefficients and cross sections of the many microphysical processes that occur in the ISM and that are included in models such as the Meudon PDR code.Comment: To appear in "The European Physical Journal Web of Conferences". Invited paper at "Multi-line Diagnostics of the Interstellar Medium", Nice 202