Panel a: ALMA spectrum of ortho c-C₃H₂ 2_1,2 − 1_0,1 at 85338.896 MHz towards the UCHII region K4 (Gaume et al. 1995). The spectrum in magenta is the synthetic spectrum obtained with MCWeeds. The blue lines show the central velocities of the fitted components. The dotted line represents the continuum level and the dashed line marks the 4σ line detection threshold. The grey area marks the velocity range of the envelope of Sgr B2 (v_LSR > 42 km s⁻¹). Panel b: residuals, that is the difference between the observed and synthetic spectra. The dashed lines indicate the − 4σ level. The spectrum is corrected for the primary beam attenuation.