Gas phase diagnostics of protoplanetary disc extension

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.