HCN spectrum depending on kinetic temperature, density and radiation field for a slab model. The HCN column density is fixed to 2 × 10¹⁵ cm^-2 and the line width to Δv = 1 km s^-1. The intensity scales of the IR (2–18 μm) and far-IR/submillimeter portions of the spectrum are different. The blue dashed line shows the typical detection limit of CRIRES and Spitzer assuming an emitting region with radius R = 1 au. For Herschel-PACS, the detection limit within 50 au is shown.