Spectra of the J = 1  →  0 and 4  →  3 lines of H¹³CO⁺ and HN¹³C, and the J = 2  →  1 and 8  →  7 lines of SiO towards the sources NGC 1333, L1448-IRS3, VLA 1623 and IRAS 20353, for the positions selected in Sect. 4 (given at the top of each column). To account for the different beam sizes, the spectra have been averaged by assuming a Gaussian taper, whose FWHM corresponds to the largest beam size of the observations (~29′′). The vertical axis is line intensity in Kelvin on the scale. The spectra have been smoothed to a velocity resolution of  ~0.1 km s^-1, apart from SiO 2  →  1 towards VLA 1623, which has been smoothed to  ~0.5 km s^-1 in order to see the line more clearly. The insets where plotted show the full width of the spectrum.