Baseline of the [C ii] profiles showing the hyperfine components of the [¹³C ii] transitions. The upper plot shows the sum spectrum from all spectra with integrated intensities above 100 K km s^-1 from the IRS 1-3 region, the lower plot from the interface region. The blue lines represent the same spectra scaled by an abundance ratio of 1/67 and the corresponding line ratio and frequency of the three hyperfine transitions, i.e. they represent the spectrum that we expect in the case of an optically thin [C ii] line.