IRAM 30 m ¹³CO and C¹⁸O J = 2 → 1 spectra towards the C1-N, F1, and F2 cores. The red lines show the best fits to the data. The cores are labelled in the top left of each spectral window and the isotopologue in the top right. The vertical, dashed, dark purple lines give the central velocity of the N₂D⁺J = 3 → 2 line detected towards the cores (Tan et al. 2013) while the dashed, light blue lines give the central velocities of the parent IRDCs (Simon et al. 2006).