Gas phase diagnostics of protoplanetary disc extension (Research Note)

Aims. We investigate the potential of using ratios of fine structure and near-infrared forbidden-line transitions of atomic carbon to diagnose protoplanetary disc extension. Methods. Using results from 2D photoionisation and radiative transfer modelling of a realistic protoplanetary disc structure irradiated by X-rays from a T Tauri star, we obtain theoretical emission maps from which we construct radial distributions of the strongest emission lines produced in the disc. Results. We show that ratios of fine structure to near-infrared forbidden-line emission of atomic carbon are especially promising for constraining the minimum size of gaseous protoplanetary discs. While theoretically viable, the method presents a number of observational difficulties that are also discussed here.