Comparison of the [Ne III]/[Ne II] (top panel) and [Ne II]/[Ar II] (bottom panel) line ratios for HK Tau A and B (orange shaded region) and the other published detections. Binary systems are marked with a black contour. The dashed coloured lines show the line ratio thresholds for determining the dominant irradiation source. Sz Cha is taken from Espaillat et al. (2023), T Cha is taken from Bajaj et al. (2024), RW Aur B and VW Cha BC are taken from Kurtovic et al. (2026), and all others are taken from Arulanantham et al. (2025).