Effects of X-ray irradiation and disc flaring on the [Ne II] 12.8 μm emission from young stellar objects

The [Ne II] fine-structure emission line at 12.8 mu m has been detected in several young stellar objects spectra. This line is thought to be produced by X-ray irradiation of the warm protoplanetary disc atmospheres; however, the observational correlation between [Ne II] luminosities and measured X-ray luminosities shows a large scatter. Such spread limits the utility of this line as a probe of the gaseous phase of discs, as several authors have suggested pollution by outflows as a probable cause of the observed scatter. In this work, we explore the possibility that the large variations in the observed [Ne II] luminosity may be caused instead by different star-disc parameters. In particular we study the effects that the hardness of the irradiating source and the structure (flaring) of the disc have on the luminosity and spectral profile of the [Ne II] 12.8 mu m line. We find that varying these parameters can indeed cause up to an order of magnitude variation in the emission luminosities which may explain the scatter observed, although our models predict somewhat smaller luminosities than those recently reported by other authors who observed the line with the Spitzer Space Telescope. Our models also show that the hardness of the spectrum has only a limited (undetectable) effect on the line profiles, while changes in the flaring power of the disc significantly affect the size of the [Ne II] emission region and, as a consequence, its line profile. In particular, we suggest that broad-line profiles centred on the stellar radial velocity may be indicative of flat discs seen at large inclination angles.