Average widths (FWHM) of the CCH (y-axis) and CH₃OH (x-axis) lines observed in each source of Table 2, using the IRAM-30 m (red) and the Nobeyama-45 m (blue), respectively. The errors are 1σ of the average values. This shows that the two molecules are emitted in a similar and quiescent region (Sect. 5.1). We note that only six sources are reported for the IRAM-30 m observations and seven for the Nobeyama-45 m: this is because the sources FIR6c and FIR2 have been left out from the analysis in both set of observations, the lines being blended with outflows (see Table 2). The source SIMBA-a is contaminated by the wobbler off position only in the IRAM-30 m data set. We could thus perform the analysis with Nobeyama-45 m (see Table 2).