CI Traces the Disk Atmosphere in the IM Lup Protoplanetary Disk

Abstract

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.