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tokens: show the rest of the sources detected; in the case of sources 32, 35, and 38 the K = 0, 1 lines were detected, but no valid solution was found for the simultaneous Gaussian fitting of all four components. Integrated line fluxes obtained from the Gaussian fitting are reported in Table 1 and were used to compute Boltzmann plots (i.e., the rotational diagram) relating the column density of the emitters normalized by the level degeneracy and the energy of the levels. Using standard procedures (e.g., Fontani et al. 2002 ), assuming LTE and optically thin conditions that are appropriate for this line and class of sources (e.g., Bergin et al. 1994 ) the temperature and total column density of CH 3 C 2 H can be derived from a linear fit to the rotational diagram (Goldsmith Langer 1999 ). Because our observations are single-pointed toward the 70 μ m peak of the sources, we could only derive beam-averaged column densities and we chose to focus on the analysis of the CH 3 C 2 H rotational temperatures (see Table 1 ). 3. RESULTS 3.1. CH<sub>3</sub>C<sub>2</sub>H Detection Statistics The sources listed in Table 1 are ordered according to their L/M value computed from the Herschel fluxes, augmented with ancillary data at mid-IR wavelengths (D. Elia et al. 2016, in preparation), and it can be immediately seen that the detection rate in CH 3 C 2 H( J = 12–11) is strongly dependent on the L/M values. We can distinguish three ranges of L/M : the rate is 0 (none out of 23 sources) for the sources with L/M 1, 58% (seven out of 12) for sources with 1 L/M 10 and 90% (16 out of 18 sources) for sources with L/M > 10. In Table 1, in sources # 32, 35, and 38, at least the K = 0 line is detected, but the four-component Gaussian fit to the four lines did not converge and so no further analysis could be carried out for these sources. This finding is preliminary confirmation of the working hypothesis that higher values of L/M should be related with a more advanced evolutionary stage. As CH 3 C 2 H is a temperature probe for gas with densities in excess of 10 5 cm −3 (Bergin et al. 1994 ), its values are influenced more by the inner clump regions and relatively less by the outer envelope. Looking at Figure 3 , the range of L/M ≲ 1 is characteristic of “pre-stellar” clumps (in which, contrary to the sources of the present study, 70 μ m emission is not detected); the internal input energy producing the detectable 70 μ m flux would not seem sufficient to drive up considerably the bolometric luminosity as well as the internal clump temperature, resulting in non-detections in CH 3 C 2 H( J = 12–11) for any of the sources with L/M 1. This suggests that in these clumps intermediate or high-mass star formation has not yet started. Above the L/M = 1 threshold, corresponding to the upper envelope of the “pre-stellar” clumps distribution in Figure 3 , the CH 3 C 2 H( J = 12–11) detection rate increases sharply, implying that the internal energy produced by star formation is rising to