SiC₂ emission. Shaded areas indicate the part of the spectrum integrated to obtain the intensities used to construct the rotational diagram in Fig. 4, and vertical red lines correspond to the rest frequency, centred to a velocity of 0 km s⁻¹. We give the rest frequency, quantum numbers, and upper level energy for each transition. We note that (i) the rest frequencies of the 10_6,5−9_6,4 and 10_6,4−9_6,3 lines at 235.7 GHz differ by only 0.1 MHz and (ii) the emission from the 12_0,12 − 11_0,11 transition at 261.991 GHz (last panel) is blended with intense C₂H emission (Fig. 5) and is therefore omitted from the rotational diagram analysis, as is the 11_2,10 − 10_2,9 transition which is tentatively detected in a region of high rms noise.