2023ApJ...959L..27L

The central star and its energetic radiation fields play a vital role in setting the vertical and radial chemical structure of planet-forming disks. We present observations that, for the first time, clearly reveal the UV-irradiated surface of a protoplanetary disk. Specifically, we spatially resolve the atomic-to-molecular (C I-to-CO) transition in the IM Lup disk with Atacama Large Millimeter/submillimeter Array archival observations of [C I] ³P₁-³P₀. We derive a C I emitting height of z/r ≳ 0.5 with emission detected out to a radius of ≈600 au. Compared to other systems with C I heights inferred from unresolved observations or models, the C I layer in the IM Lup disk is at scale heights almost double that of other disks, confirming its highly flared nature. C I arises from a narrow, optically thin layer that is substantially more elevated than that of ¹²CO (z/r ≈ 0.3-0.4), which allows us to directly constrain the physical gas conditions across the C I-to-CO transition zone. We also compute a radially resolved C I column density profile and find a disk-averaged C I column density of 2 × 10¹⁶ cm^–2, which is ≈3-20× lower than that of other disks with spatially resolved C I detections. We do not find evidence for vertical substructures or spatially localized deviations in C I due, e.g., to either an embedded giant planet or a photoevaporative wind that have been proposed in the IM Lup disk, but emphasize that deeper observations are required for robust constraints.