  We present observations of the v2=0 and vibrationally excited v2=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 >=
300 K), abundant HCN seen in 14 micron 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 >~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.
