From lower to upper panel: flux of the line, mass averaged temperature and total species mass in the disk versus X-ray luminosity. See Sect. 3 for the description of the error bars. Left figure: ionized neon fine-structure line emission at 12.8 μm. The line is produced in a high-temperature X-ray heated environment, where the density is low (n_H ~ 10^6 − 7 cm^-3). The line is optically thin and hence very sensitive to the total ionized neon mass, which correlates with L_X. Hence, in our models, [Ne ii] correlates with L_X. Right figure: ionized argon fine-structure line emission at 6.9 μm. [Ar ii] behave very similarly to [Ne ii], and it can be used as a tracer for the same region.