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We present observations of the v2=0 and vibrationally excited v2=1 J=9-8rotational lines of HCN at 797 GHz toward the deeply embedded massive youngstellar object GL 2591, which provide the missing link between the extendedenvelope traced by lower-J line emission and the small region of hot (T_ex >=300 K), abundant HCN seen in 14 micron absorption with the Infrared SpaceObservatory (ISO). The line ratio yields T_ex=720^+135_-100 K and the lineprofiles reveal that the hot gas seen with ISO is at the velocity of theprotostar, arguing against a location in the outflow or in shocks. Radiativetransfer calculations using a depth-dependent density and temperature structureshow that the data rule out a constant abundance throughout the envelope, butthat a model with a jump of the abundance in the inner part by two orders ofmagnitude matches the observations. Such a jump is consistent with the sharpincrease in HCN abundance at temperatures >~230 K predicted by recent chemicalmodels in which atomic oxygen is driven into water at these temperatures.Together with the evidence for ice evaporation in this source, this resultsuggests that we may be witnessing the birth of a hot core. Thus, GL 2591 mayrepresent a rare class of objects at an evolutionary stage just preceding the`hot core' stage of massive star formation.

Highly Abundant HCN in the Inner Hot Envelope of GL 2591: Probing the Birth of a Hot Core?