We have carried out an observational study of the Class 0 IM protostar NGC 7129-FIRS 2 and compared it with the Type I Herbig Be star LkH alpha 234 using the 30m IRAM telescope. These two stars are located in the same molecular cloud and have similar luminosities. Thus, most of their physical and chemical differences are expected to be due to their different evolutionary stage instead of to different stellar masses or initial conditions. Our results suggest an evolutionary sequence in which as the protostar evolves to become a visible star, the protostellar envelope is being dispersed and warmed up. In addition, the bipolar molecular outflow fades, even dissapear, before the star becomes visible. These two physical changes produce important chemical changes in the enviroment of the IM YSOs. While the abundances of some molecules like N2H+ and NH3 remain constant, the abundance of others like SiO decreases by a factor similar to 10 from the Class 0 protostar to the HBe star. Significant changes are also found in the abundances of CN,HCN, CH3OH, H2CO and CH3CN. Based on our results in FIRS 2 and LkH alpha 234, we propose some abundance ratios that can be used as chemical clocks for the envelopes of IM YSOs. The SiO/CS, CN/N2H+ (or HCN/N2H+), DCO+/HCO+ and D2CO/DCO+ ratios seem to be goof diagnostics of the protostellar evolutionary stage (Paper I). In addition to single-dish observations, we have obtained interferometric images of the Class 0 protostar NGC 7129-FIRS 2 using the Plateau de Bure Interferometer. Our results show the existence of a hot core in this IM Class 0 object. This is the first hot core detected in an IM protostar. The size of the hot core (similar to 600AUx800AU) is similar to those of the hot cores associated with massive stars. Our interferometric data also reveal a chemistry very rich and complex chemistry in this IM hot core.
