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The chemical composition of high-mass protostars reflects the physicalevolution associated with different stages of star formation. In this study,the molecular inventory of the forming high-mass star AFGL 4176 is studied indetail at high angular resolution (~0.35 arcsec) using ALMA. This highresolution makes it possible to separate the emission associated with the innerhot envelope and disk around the forming star from that of its cool outerenvelope. In addition, the high sensitivity of ALMA makes it possible toidentify weak and optically thin lines and allows for many isotopologues to bedetected, providing a more complete and accurate inventory of the source. Atotal of 23 different molecular species and their isotopologues are detected inthe spectrum towards AFGL 4176. The most abundant species is methanol (CH3OH),remaining species are present at levels between 0.003 % and 15 % with respectto CH3OH. Hints that N-bearing species peak slightly closer to the location ofthe peak continuum emission than the O-bearing species are seen. AFGL 4176comprises a rich chemical inventory including many complex species present ondisk-scales. On average, the derived column density ratios with respect tomethanol of O-bearing species are higher than those derived for N-bearingspecies by a factor of three. This may indicate that AFGL 4176 is a relativelyyoung source since nitrogen chemistry generally takes longer to evolve in thegas-phase. Taking methanol as a reference, the composition of AFGL 4176 moreclosely resembles that of the low-mass protostar IRAS 16293-2422B than that ofhigh-mass star-forming regions located near the Galactic centre. Thissimilarity hints that the chemical composition of complex species is alreadyset in the cold cloud stage and implies that AFGL 4176 is a young source whosechemical composition has not yet been strongly processed by the centralprotostar.

Molecular complexity on disk-scales uncovered by ALMA: The chemical composition of the high-mass protostar AFGL 4176