We present a calculation of the atomic and low-ionization emission line spectra of photoevaporating protoplanetary discs. Line luminosities and profiles are obtained from detailed photoionization calculations of the disc and wind structures surrounding young active solar-type stars. The disc and wind density and velocity fields were obtained from the recently developed radiation-hydrodynamic models of Owen et al. that include stellar X-ray and EUV irradiation of protoplanetary discs at various stages of clearing, from primordial sources to inner hole sources of various hole sizes.
Our models compare favourably with currently available observations, lending support to an X-ray driven photoevaporation model for disc dispersal. In particular, we find that X-rays drive a warm, predominantly neutral flow where the O i 6300 Å line can be produced by neutral hydrogen collisional excitation. Our models can, for the first time, provide a very good match to both luminosities and profiles of the low-velocity component of the O i 6300 Å line and other forbidden lines observed by Hartigan et al., which covered a large sample of T-Tauri stars.
We find that the O i 6300 Å and the Ne ii 12.8 μm lines are predominantly produced in the X-ray-driven wind and thus appear blueshifted by a few km s−1 for some of the systems when observed at non-edge-on inclinations. We note, however, that blueshifts are only produced under certain conditions: X-ray luminosity, spectral shape and inner hole size all affect the location of the emitting region and the physical conditions in the wind. We caution therefore that while a blueshifted line is a tell-tale sign of an outflow, the lack of a blueshift should not be necessarily interpreted as a lack of outflow. Comparison of spectrally resolved observations of multiple emission lines with detailed model sets, like the one presented here, should provide useful diagnostics of the clearing of gaseous discs.