We present observations of the \nu _ { 2 } =0 and vibrationally excited \nu _ { 2 } =1 J =9–8 rotational lines of HCN at 797 GHz toward the deeply embedded massive young stellar object GL 2591 , which provide the missing link between the extended envelope traced by lower- J line emission and the small region of hot ( T _ { ex } \geq 300 K ) , abundant HCN seen in 14 \mu m absorption with the Infrared Space Observatory ( ISO ) .
The line ratio yields T _ { ex } = 720 ^ { +135 } _ { -100 } K and the line profiles reveal that the hot gas seen with ISO is at the velocity of the protostar , arguing against a location in the outflow or in shocks .
Radiative transfer calculations using a depth-dependent density and temperature structure show that the data rule out a constant abundance throughout the envelope , but that a model with a jump of the abundance in the inner part by two orders of magnitude matches the observations .
Such a jump is consistent with the sharp increase in HCN abundance at temperatures { { } _ { > } \atop { { } ^ { \sim } } } 230 K predicted by recent chemical models in which atomic oxygen is driven into water at these temperatures .
Together with the evidence for ice evaporation in this source , this result suggests that we may be witnessing the birth of a hot core .
Thus , GL 2591 may represent a rare class of objects at an evolutionary stage just preceding the ‘ hot core ’ stage of massive star formation .