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Arxiv: PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon Published: 8/12/2024 7:41:01 PM Updated: 8/12/2024 7:41:01 PM

Paper abstract: The gas-phase abundance of carbon, x_C = C/H, and its depletion factors areessential parameters for understanding the gas and solid compositions that areultimately incorporated into planets. The majority of protoplanetary disks areborn in clusters and, as a result, are exposed to external FUV radiation. TheseFUV photons potentially affect the disk's evolution, chemical composition, andline excitation. We present the first detection of the [CI]609um fine-structureline of neutral carbon (CI), achieved with ALMA, toward one of these disks,d203-506, in the Orion Nebula Cluster. We also report the detection of CIforbidden and permitted lines (from electronically excited states up to 10 eV)observed with JWST in the IR. These lines trace the irradiated outer disk andphoto-evaporative wind. Contrary to the common belief that these IR lines areC+ recombination lines, we find that they are dominated by FUV-pumping of CIfollowed by fluorescence cascades. They trace the transition from atomic tomolecular gas, and their intensities scale with G0. The lack of outstanding IROI fluorescent emission, however, implies a sharper attenuation of external FUVradiation with E > 12 eV (~Lyman-beta). This is related to a lower effectiveFUV dust absorption cross section compared to that of interstellar grains,implying a more prominent role for FUV shielding by the CI photoionizationcontinuum. The [CI]609um intensity is proportional to N(CI) and can be used toinfer x_C. We derive x_C ~ 1.4E-4. This implies that there is no majordepletion 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 theouter disk and wind by vertically shifting the water freeze-out depth, whichresults in less efficient grain growth and settling. This shift leads to nearlysolar gas-phase C/O abundance ratios in these irradiated layers.