Abundances with respect to H₂ for selected molecules. Results for core #3 (red squares) and core #6 (blue lines) considering different source sizes (see legend) are from LTE modelling. The temperature T_ex = 190 K is fixed for core #3, it varies from 10 to 200 K for core #6. The blue area represents an error of 50% on the estimation of the abundance, which includes the error on the line area and on the value of N_{H_2}. The grey area is the temperature range estimated for core #6 from the rotational diagrams. For a given size, a molecule becomes optically thick below the black line. The sources listed in Table 4 are represented by a dot for prestellar cores or a star for protostellar cores. The colour code is white for low-mass, grey for intermediate-mass, and black for high-mass cores. The names refer to Col. 2 of Table 4.