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). Both objects have similar luminosities (~500
Lsun) 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 rarer
isotopes) have been observed in both objects and the structure of their
envelopes and outflows is determined with unprecedent detail.
Our results show that the protostellar envelopes are dispersed and warmed up
during the evolution to become a pre-main sequence star. In fact, the envelope
mass decreases by a factor >5 from FIRS 2 to LkHα234, while the kinetic
temperature increases from ~13K to 28K. On the other hand, there is no
molecular outflow associated with LkHα234. The molecular outflow seems
to stop before the star becomes visible. These physical changes strongly affect
the chemistry of their envelopes.
Based on our results in FIRS2 and LkHα 234, we propose some abundance
ratios that can be used as chemical clocks for the envelopes of IM YSOs. The
SiO/CS, CN/N2H+, HCN/N2H+, DCO+/HCO+ and D2CO/DCO+ ratios are good diagnostics
of the protostellar evolutionary stage.Comment: 24 pages, 17 figure