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Based on astrochemical theory, the complex molecular composition aroundhigh-mass YSOs should evolve from the outer envelope in toward the central hotregion as a sequence of temperature dependent chemical pathways are activatedin ices and in the gas-phase. The resulting complex molecules have been broadlyclassified into three generations dependent on the temperature (<25, >25, and>100 K) required for formation. We combine IRAM 30m and Submillimeter Arrayobservations to explore the spatial distribution of organic molecules aroundthe high-mass young stellar object NGC 7538 IRS9, whose weak complex moleculeemission previously escaped detection, quantifying the emission and abundanceprofiles of key organic molecules as a function of distance from the centralprotostar. We find that emission from N-bearing organics and saturatedO-bearing organics present large increases in emission around 8000 AU andR<3000 AU, while O-bearing molecules and hydrocarbons do not. The increase influx from some complex molecules in the envelope, around 8000 AU or 25 K, isconsistent with recent model predictions of an onset of complex ice chemistryat 20-30 K. The emission increase for some molecules at R<3000 AU suggests thepresence of a weak hot core, where thermal ice evaporation and hot gas-phasechemistry drives the chemistry. Complex organics thus form at all radii andtemperatures around this protostar, but the composition changes dramatically asthe temperature increases, which is used to constrain the chemicalgeneration(s) to which different classes of molecule belong.

The Spatial Distribution of Organics toward the High-Mass YSO NGC 7538 IRS9