Resumen:
We have carried out a molecular survey of the Class 0 IM protostar NGC 7129 - FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkHα 234 with the aim of studying the chemical evolution of the envelopes of intermediate-mass (IM) young stellar objects (YSOs). The two objects have similar luminosities (∼500 L⊙) and are located in the same molecular cloud which minimizes the chemical differences due to different stellar masses or initial cloud conditions. Moreover, since they are located at the same distance, we have the same spatial resolution in both objects. A total of 17 molecular species (including rare isotopes) have been observed in both objects and the structure of their envelopes and outflows has been determined with unprecedent detail. Our results show that the protostellar envelopes are dispersed and warmed up during the evolution of the YSO into a pre-main sequence star. In fact, the envelope mass decreases by a factor >5 from FIRS 2 to LkHα 234, while the kinetic temperature increases from ∼13 K to 28 K. On the other hand, there is no molecular outflow associated with LkHα 234. The molecular outflow seems to stop before the star becomes visible. These physical changes strongly affect the ...