Average IRS spectra obtained with the high spectral resolution mode for three objects of our sample (L1721, California, and N7023E). In each panel, the line spectrum is shown with the local rms noise (for clarity, an arbitrary offset value has been added to the noise). The black and green lines represent the line spectrum and the noise, respectively. With increasing wavelength, the panels show H₂ pure rotational lines at 12.29, 17.03, and 28.2 μm.