The Future of PDR Modelling - Current Challenges and Next Steps

This workshop brings together leading experts in the numerical modelling of
photodissociation regions (PDRs) to assess the current state of the art, identify
critical limitations in existing codes, and chart a course for the next generation of
PDR models. Following the two earlier PDR workshops (2004 and 2013), this meeting
shifts its focus from benchmarking to addressing fundamental modelling challenges,
including microphysical processes such as H₂ formation and grain surface chemistry,
as well as the treatment of cosmic rays, metallicity-dependent physics, and multiscale
geometries. With JWST in full operation, ALMA upgrades underway, and ELT
and CCAT on the horizon, there is a pressing need for PDR models that are not only
more physically complete but also transparent in their uncertainties and predictive
accuracy. The workshop aims to produce a white paper outlining the critical issues
and future priorities for the PDR modelling and ISM community.

Photodissociation regions (PDRs) are critical interfaces in the interstellar medium
where far-ultraviolet photons from young, massive stars govern the thermal and
chemical structure of gas and dust. Since the bulk of molecular material in galaxies
resides within PDRs, understanding their physics is essential for interpreting
observations of star-forming regions and galaxy evolution.
Numerical modelling has been central to PDR research for over 30 years, evolving
from early one-dimensional codes into sophisticated tools that simulate radiative
transfer, complex chemical networks, gas & dust interactions, and thermal balance.
However, significant assumptions remain necessary to keep computations tractable:
simplified H₂ formation prescriptions, steady-state dust temperatures, static grain
charge distributions, and incomplete cosmic ray treatments, among others. These
approximations can introduce systematic uncertainties whose impact on predicted
observables is poorly quantified.
This workshop will bring together developers of major PDR codes, laboratory
astrophysicists, and observers to tackle these challenges across several key topics:
H₂ formation on dust grains and the interplay of Langmuir–Hinshelwood and Eley-
Rideal mechanisms; grain surface chemistry and its coupling to dust physics; charge
distributions of grains and PAHs and their effect on photoelectric heating; stochastic
and multi-photon heating of small grains; cosmic ray propagation and tertiary
ionization processes; and metallicity-dependent physics, particularly relevant for
interpreting observations of low-metallicity environments such as the Small
Magellanic Cloud. A particular emphasis will be placed on the integration of
observational diagnostics from JWST (including PDRs4all), ALMA, and Herschel, and
on the role of machine learning in accelerating PDR modelling and enabling
systematic sensitivity analyses. The workshop will combine invited and contributed
talks with extensive small-group discussion sessions to encourage direct
collaboration and hands-on code comparisons.
The primary aim of the workshop will be a community white paper summarising the
critical shortcomings in current modelling frameworks, identifying areas where
collaboration with adjacent fields (laboratory astrophysics, computational science) is
urgently needed, and setting concrete goals for the coming decade.