Panel a: Class I YSOs marked by plus symbols overlaid on the C¹⁸O (1–0) emission. Panel b: corresponding overlay of Class I YSOs surface density contours on the C¹⁸ O (1–0) emission. The contour levels start from 0.5 to 1.7 pc⁻² (magenta contours) in steps of 0.13 pc⁻². The different symbols represent the clumps in different stages, similar to those in Fig. 5a. In addition, the clumps marked with the red colour are in the velocity range of 38.5–42 km s⁻¹.