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Mid-infrared atomic and ionic line ratios measured in spectra of pre-mainsequence stars are sensitive indicators of the hardness of the radiation fieldimpinging on the disk surface. We present a low-resolution Spitzer IRS searchfor [Ar II] at 6.98 $\mu$m, [Ne II] at 12.81 $\mu$m, and [Ne III] 15.55 $\mu$mlines in 56 transitional disks. These objects, characterized by reducednear-infrared but strong far-infrared excess emission, are ideal targets to setconstraints on the stellar radiation field onto the disk because their spectraare not contaminated by shock emission from jets/outflows or by molecularemission lines. After demonstrating that we can detect [Ne II] lines andrecover their fluxes from the low-resolution spectra, here we report the firstdetections of [Ar II] lines towards protoplanetary disks. We did not detect [NeIII] emission in any of our sources. Our [Ne II]/[Ne III] line flux ratioscombined with literature data suggest that a soft-EUV or X-ray spectrumproduces these gas lines. Furthermore, the [Ar II]/[Ne II] line flux ratiospoint to a soft X-ray and/or soft-EUV stellar spectrum as the ionization sourceof the [Ar II] and [Ne II] emitting layer of the disk. If the soft X-raycomponent dominates over the EUV than we would expect larger photoevaporationrates hence a reduction of the time available to form planets.

Observational Constraints on the Stellar Radiation Field Impinging on Transitional Disk Atmospheres