Top: stacking of PACS spectra at 63 μm for objects with no detection in Taurus and Cha II. The blue line shows a Gaussian fit to the emission features. The red dashed vertical line marks the position of the rest frame wavelength for the [OI] transition at 63.185 μm. Bottom: continuum noise in the resulting stacked spectra for different numbers of co-added spectra in Taurus (in red) and Cha II (in blue). The average noise and noise error are computed as the average and standard deviation of the N realisations of the bootstrap test (see text).