'Metals' (any atom with equal or more than three protons) are heavier then hydrogen or helium. So 'metals' are slower then H or He with the same kinetic energy. That means that over time the heat density becomes less along the expansion vector. This effect spreads the heat over a bigger volume and lowers the heat(energy) density, making that area cooler.
GSte2022 t1_j6551bq wrote
Reply to Why are "metals" more effective at cooling molecular clouds than hydrogen and helium? by Thomas_Bonk
'Metals' (any atom with equal or more than three protons) are heavier then hydrogen or helium. So 'metals' are slower then H or He with the same kinetic energy. That means that over time the heat density becomes less along the expansion vector. This effect spreads the heat over a bigger volume and lowers the heat(energy) density, making that area cooler.