Emission spectra of a nebula in the regions extracted from a two-dimensional spectral image, corresponding to slit PA = 135° (orientated southeast-northwest; see Table 1 and Fig. 3) at different distances from the star. For clarity, the spectra are shifted vertically, and the original baseline of every spectrum is equal to zero. Spectra are extracted inapertures of 5 pixels in width, corresponding to a spatial resolution of 5.3 pc. The spectra are centered on Hβ (left panel) and Hα (right panel); no other spectral region contains any detectable nebular line.