Theoretical spectra of photoevaporating protoplanetary discs: An atlas of atomic and low-ionisation emission lines

TL;DR

This paper models the atomic and low-ionisation emission lines from photoevaporating protoplanetary discs, showing X-ray driven winds produce observable forbidden lines with specific profiles and shifts, matching recent observations.

Contribution

It provides the first detailed theoretical spectra of atomic and low-ionisation lines from photoevaporating discs, incorporating radiation-hydrodynamic models with X-ray irradiation.

Findings

X-ray driven winds produce neutral flows emitting the OI 6300A line.

Models match observed line luminosities and profiles of T-Tauri stars.

Forbidden lines can be blue-shifted depending on system parameters.

Abstract

We present a calculation of the atomic and low-ionisation emission line spectra of photoevaporating protoplanetary discs. Line luminosities and profiles are obtained from detailed photoionisation 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 OI 6300A 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 OI 6300A line and other forbidden lines observed by Hartigan et al., which covered a large sample of T-Tauri stars. We find that the OI 6300A and the NeII 12.8um lines are predominantly produced in the X-ray-driven wind and thus appear blue-shifted by a few km/s for some of the systems when observed at non-edge-on inclinations. We note however that blue-shifts 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.