Abstract

Contributed Talk - Splinter StarFormation   (MW-1250)

Beyond the resolution limit: How clumpiness alters the molecular and star-formation tracers that bridge different scales

Aditi Sinha(1), Volker Ossenkopf-Okada(1), Markus Röllig(2), Theodoras Topkaras(1), Sebastian Vider(1)
1: Universität zu Köln, 2: Physikalischer Verein, Frankfurt

Understanding the interstellar medium across cosmic environments requires simulations that bridge spatial scales while coupling dynamics, radiation, and chemistry. Hydrodynamic simulations trade resolution against detailed physics and rely on simplified chemical networks, leaving the dense molecular substructure that fuels star formation unresolved. PDR models can extend these networks by post-processing simulation outputs, but most treat each resolved cell as homogeneous, neglecting the sub-cell density and FUV inhomogeneity that govern the chemistry and emergent line emission. We post-process FLASH AMR simulations of an expanding HII region, driven by feedback from a massive star that reshapes the molecular gas from which it formed. Each cell (0.016–0.03 pc) is populated with a clumpy ensemble from kosmatau3d (based on the KOSMA-τ PDR model), spanning clump masses of 0.001–1000 M⊙ down to ~10⁻³ pc cores. Cell density, FUV field, and velocity are inherited from the simulation; dense gas is treated with the kosmatau3d and ionised/diffuse gas with Cloudy. Integrating the radiative transfer equation along sightlines with an adaptive ray scheme yields synthetic [CII] 158 μm, [CI] 609/370 μm, [OI] 63/145 μm, and CO-ladder PPV cubes. Comparing native abundances and temperatures against individual clumps, we find that FUV escaping through low-density channels heats clump surfaces to ~140–600 K, a factor of 3–7 above the ~20–200 K hydrodynamic gas that produces warm, carbon-ionised surfaces over a larger volume than a homogeneous cell would. This substructure shapes line ratios: [OI] 63 μm / [CII] 158 μm tracks clump density, while increasing N_H drives [CII] 158 μm / CO(1–0) down and [CI] 609 μm / [CII] 158 μm up as CO forms in shielded cores. Hence, using the volume filling factor, we quantify how neglecting clumpiness biases the densities, temperatures, and CO-to-H₂ conversion factor inferred from these tracers.