The spatial distribution of organics toward the high-mass YSO NGC 7538 IRS9

Complex molecules have been broadly classified into three generations dependent on the mode of formation and the required formation temperature (<25, 25-100 K, and >100 K). Around massive young stellar objects (MYSOs), icy grain mantles and gas are exposed to increasingly higher temperatures as material accretes from the outer envelope in toward the central hot region. The combination of this temperature profile and the generational chemistry should result in a changing complex molecular composition with radius around MYSOs. We combine IRAM 30 m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection. We find that emission from N-bearing organics and CH3OH present substantial increases in emission around 8000 AU and R <3000 AU, while unsaturated O-bearing molecules and hydrocarbons do not. The increase in line flux for some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for many of the same molecules at R <3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase reactions drive the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used together with an adapted gas-grain astrochemical model to constrain the chemical generation(s) to which different classes of molecules belong.