Excitation temperature vs. the size of the emission region for each molecule whose emission arises from the envelope. The black curve represents the power-law fit of the data. The grey area indicates power-law temperature profiles with an exponent between –0.3 and –0.9. This profile does not include the emission originating from the accretion shocks that correspond to a local increase in temperature. It also excludes species for which no strong constraints on the size of the emitting region could be derived (see Sect. 3.4.2).