2024A&A...689L...4G

The gas-phase abundance of carbon, x_C = [C/H]_gas = x_C+ + x_C0 + x_CO + … , and its depletion factors are essential parameters for understanding the gas and solid compositions that are ultimately incorporated into (exo)planets. The majority of protoplanetary disks are born in clusters and, as a result, are exposed to external far-ultraviolet (FUV) radiation. These FUV photons potentially affect the disk's evolution, chemical composition, and line excitation. We present the first detection of the [C I] 609 μm fine-structure (³P₁-³P₀) line of neutral carbon (C⁰), achieved with ALMA, toward one of these disks, d203-506, in the Orion Nebula Cluster. We also report the detection of [C I] forbidden and C I permitted lines (from electronically excited states up to ∼10 eV) observed with JWST in the near-infrared (NIR). These lines trace the irradiated outer disk and photo-evaporative wind. Contrary to the common belief that these NIR lines are C⁺ recombination lines, we find that they are dominated by FUV-pumping of C⁰ followed by fluorescence cascades. They trace the transition from atomic to molecular gas, and their intensities scale with G₀. The lack of outstanding NIR O I fluorescent emission, however, implies a sharper attenuation of external FUV radiation with E ≳ 12 eV (λ ≲ Lyman-β). This is related to a lower effective FUV dust absorption cross section compared to that of interstellar grains, implying a more prominent role for FUV shielding by the C⁰ photoionization continuum. The [C I] 609 μm line intensity is proportional to N(C⁰) and can be used to infer x_C. We derive x_C ≃ 1.4 × 10^–4. This implies that there is no major depletion of volatile carbon compared to x_C measured in the natal cloud, hinting at a young disk. We also show that external FUV radiation impacts the outer disk and wind by vertically shifting the water freeze-out depth, which likely results in less efficient grain growth and settling. This shift leads to nearly solar gas-phase C/O abundance ratios in these irradiated layers.