X-ray luminosity as a function of the free-free luminosity at 3.5 cm for compact disks (·), substructured disks (⊙), and transition disks (∘). Top: the purple line shows the expected free-free luminosity associated with ionization by X-ray photons estimated following Equation (3) in Pascucci et al. (2012). Bottom: the colored region shows the expected free-free luminosity associated with ionization by EUV photons, as estimated by Pascucci et al. (2012), assuming a stellar EUV luminosity of Φ_EUV = 10⁴¹–10⁴⁴ photons/s. The case where Φ_EUV = 10⁴² photons/s is shown with a purple line.