Detection of HC₃N emission. Top three rows: spectra extracted around all frequencies of HC₃N rotational transitions in the vibrational ground state v = 0 covered by our observations. Shaded areas indicate a velocity range [−25, 25] km s⁻¹ around the rest frequency of the transition, and vertical red lines correspond to the rest frequency centred to a velocity of 0 km s⁻¹. All panels are labelled with the transition and whether the line is considered tentatively detected (T) or not detected (ND). Bottom row: result of stacking the emission for selected transitions, aligned in velocity space: left: tentatively identified transitions (marked T), middle: transitions not detected (marked ND), right: all twelve transitions shown in the top three rows. We note that the intensity scale is normalised to the peak and has no physical meaning as such.