Synthetic spectra of CCH obtained with LOC (top rows: L1544, bottom row: HH211). Each hyperfine line is plotted separately, numbered from low to high frequency. For L1544, the results are splitted between using the Keto-Caselli model (top) and the HDCRT model (centre) as approximation for the physical structure of the core. Towards L1544, the best-fitting abundances and timesteps are 2 × 10⁻⁸ (KC), 1e5 yr (STKC), 3 × 10⁻⁸ (HD), 1e5 yr (STHD). Towards HH211, the best-fitting abundances and timesteps are 2 × 10⁻⁹ (const), 1e6 yr (2-phase).