2022A&A...662A..32B

Context. During the process of star formation, the dense gas undergoes significant chemical evolution leading to the emergence of a rich variety of molecules associated with hot cores and hot corinos. However, the physical conditions and the chemical processes involved in this evolution are poorly constrained; the early phases of emerging hot cores in particular represent an unexplored territory.
Aims. We provide here a full molecular inventory of a massive protostellar core that is proposed to represent a precursor of a hot core. We investigate the conditions for the molecular richness of hot cores.
Methods. We performed an unbiased spectral survey towards the hot core precursor associated with clump G328.2551-0.5321 between 159 GHz and 374 GHz, covering the entire atmospheric windows at 2 mm, 1.2 mm, and 0.8 mm. To identify the spectral lines, we used rotational diagrams and radiative transfer modelling assuming local thermodynamical equilibrium.
Results. We detected 39 species plus 26 isotopologues, and were able to distinguish a compact (∼2''), warm inner region with a temperature, T, of ∼100 K, a colder, more extended envelope with T ∼ 20 K, and the kinematic signatures of the accretion shocks that have previously been observed with ALMA. We associate most of the emission of the small molecules with the cold component of the envelope, while the molecular emission of the warm gas is enriched by complex organic molecules (COMs). We find a high abundance of S-bearing molecules in the cold gas phase, including the molecular ions HCS⁺ and SO⁺. The abundance of sulphur-bearing species suggests a low sulphur depletion, with a factor of >=1%, in contrast to low-mass protostars, where the sulphur depletion is found to be stronger. Similarly to other hot cores, the deuterium fractionation of small molecules is low, showing a significant difference compared to low-mass protostars. We find a low isotopic ratio in particular for ¹²C/¹³C of ∼30, and ³²S/³⁴S of ∼12, which are about two times lower than the values expected at the galactocentric distance of G328.2551-0.5321. We identify nine COMs (CH₃OH, CH₃OCH₃, CH₃OCHO, CH₃CHO, HC(O)NH₂, CH₃CN, C₂H₅CN, C₂H₃CN, and CH₃SH) in the warm component of the envelope, four in the cold gas, and four towards the accretion shocks.
Conclusions. The presence of numerous molecular ions and high abundance of sulphur-bearing species originating from the undisturbed gas may suggest a contribution from shocked gas at the outflow cavity walls. The molecular composition of the cold component of the envelope is rich in small molecules, while a high abundance in numerous species of COMs suggests an increasing molecular complexity towards the warmer regions. The molecular composition of the warm gas is similar to that of both hot cores and hot corinos, but the molecular abundances are closer to the values found towards hot corinos than to values found towards hot cores. Considering the compactness of the warm region and its moderate temperature, we suggest that thermal desorption has not been completed towards this object yet, representing an early phase of the emergence of hot cores.