Detected ammonia inversion transitions in the central beams (black histograms). Shown from left to right are NH₃ (1,1), NH₃ (2,2) (central main hyperfine group only), and ¹⁵NH₃ (1,1). In each panel the coloured histograms show the best-fit results. Top row: spectra extracted in central beam at dust peak of L1544. Centre row: same as the top row, but for IRAS4A. The spectra are averages over the central 90″. The best-fit models obtained using a single velocity-component approach are shown with red histograms. Bottom row: left and central panels show the results of the two-component fit performed on the NH₃ lines in IRAS4A. The two velocity-components are shown in green (warmer component) and blue (colder component), whilst the total spectrum is shown in red. In the right panel the red curve shows the one-component fit to the ¹⁵NH₃ line, using the warmer component fit results as input for the temperature values.