Bolometric luminosity as a function of the bolometric temperature, highlighting the regions where specific molecular transitions are detected towards the source position, and well-known Class 0 and I sources from Dunham et al. (2015) and Harsono et al. (2014), respectively. This plot only covers the lines listed in Table 2 plus SO₂ 11_1,11 – 10_0,10 from Harsono et al. (2014). The Class 0 covers T_bol ≤ 70 K (Dunham et al. 2014b), and the Class I/II region includes the sources where no line emission is detected.