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During star formation, the dense gas undergoes significant chemical evolutionleading to the emergence of a rich variety of molecules associated with hotcores and hot corinos. The physical and chemical conditions are poorlyconstrained; the early phases of emerging hot cores in particular represent anunexplored territory. We provide here a full molecular inventory of a massiveprotostellar core that is proposed to be a precursor of a hot core. Weperformed an unbiased spectral survey towards the hot core precursor associatedwith clump G328.2551-0.5321 between 159GHz and 374GHz. To identify the spectrallines, we used rotational diagrams and radiative transfer modelling assumingLTE. We detected 39 species and 26 isotopologues, and were able to distinguisha warm and compact inner region, a colder more extended envelope, and thekinematic signatures of the accretion shocks that have previously been observedwith ALMA. We associate most of the emission of the small molecules with thecold gas, while the molecular emission of the warm gas is enriched by complexorganic molecules (COMs). We find a high abundance of S-bearing molecules inthe cold gas phase suggesting a low sulphur depletion, with a factor of > 1%.We identify nine COMs in the warm gas, four in the cold gas, and four towardsthe accretion shocks. The high abundances of S-bearing species originating fromthe undisturbed gas may suggest a contribution from shocked gas at the outflowcavity walls. The molecular composition of the warm gas is similar to that ofboth hot cores and hot corinos, but the molecular abundances are closer to thevalues found towards hot corinos than to values found towards hot cores.Considering the compactness of the warm region and its moderate temperature, wesuggest that thermal desorption has not been completed towards this object yet,representing an early phase of the emergence of hot cores.

astronomy and astrophysics

Sulphur-rich cold gas around the hot core precursor G328.2551-0.5321. An  APEX unbiased spectral survey of the 2 mm, 1.2 mm, and 0.8 mm atmospheric  windows