  The [Ne II] fine-structure emission line at 12.8 micron has been detected in
several young stellar objects (YSO) spectra. This line is thought to be
produced by X-ray irradiation of the warm protoplanetary disk 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 disks, 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-disk parameters. In
particular we study the effects that the hardness of the irradiating source and
the structure (flaring) of the disk have on the luminosity and spectral profile
of the [Ne II] 12.8 micron line. We find that varying these parameter 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 disk 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 disks seen at large
inclination angles.
