tokens: 12 CO(2–1) transition at 230.53 GHz with a high spectral resolution of 0.485 MHz (or 0.63 km s −1 ). Each of our nine sources was observed for ~1.4 min in total. The system temperature has a mean value of 91 K. The data was pipeline-calibrated using standard procedures of CASA 4.5.2. The continuum was imaged concatenating together the 128 channels of the spectral window dedicated to the continuum emission, which is a valid approach for pre-stellar structures for which no complex organic molecule emission lines are expected ( Motte et al. 2018a ). The 12 CO(2–1) emission line was slightly smoothed to a spectral resolution of 0.7 km s −1 . Both the continuum and 12 CO line emissions were imaged with the Clark method using the weighting Briggs with a robust parameter of 0.5 1 . The mean achieved root mean square (rms) noise level is of ~0.34 mJy/beam in continuum and of 16.6 mJy beam −1 per channel in 12 CO(2–1), at a resolution of 0.7 km s −1 . Table A.1 details the rms for each source. Fig. 1 Herschel -SPIRE-250 μ m emission of the NGC 6334 region. The 9 MDCs of the present work are located by the blue stars together with their number, following the nomenclature by Tigé et al. (2017) . Red arrows and Roman numerals (I, V) denote bright far-infrared sources from Kraemer Jackson (1999) . Credits: ESA, Herschel Science Archive. 2.2. MOPRA observations These observations were performed in September 2008 with the ATNF MOPRA 22-m telescope with the 3 mm receiver and the MOPRA spectrometer (MOPS) in the “zoom mode” that allows to observe simultaneously up to 16 different frequencies, among them the HCO + (1–0) at 89.18 GHz, the H 13 CO + (1–0) at 86.75 GHz, and the N 2 H + (1–0) at 93.17 GHz emission lines. Small maps (between 3′ × 3′ and 5′ × 5′) were observed in the direction of the 40 dense cores of Russeil et al. (2010) . These maps cover seven of our nine MDCs. These maps were done in the OTF mode and the data reduction was performed with Livedata and Gridzilla tools 2 producing data-cubes with a spectral and spatial resolution of 0.11 km s −1 and 30′′, respectively. The MOPRA dataset is complemented with observations from the MALT90 survey ( Jackson et al. 2013 ) to cover one additional MDC. To sum up, we gather a coherent sample of spectral lines from the MOPRA telescope for all our nine MDCs but the MDC #44. 3. Analysis 3.1. Large-scale view of the MDCs Figure 1 shows the Herschel 250 μ m emission of NGC 6334, overshowing the column density map plotted with the positions of the nine MDCs studied here. Figures 2 – 4 show the column density map over each MDC (left columns) and the molecular line emissions in the MDC (right columns). All observed MDCs are detected in HCO + . Five MDCs are detected in H 13 CO + , and seven in N 2 H + (see Table 1 ). Only the MDC #11 was undetected in both N 2 H + and H 13 CO + . We fitted the lines in CLASS 3 using the hfs procedure to fit the multiplet N 2 H + molecular emission line and Gaussian functions to fit
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