Highly abundant HCN in the inner hot envelope of GL 2591:: Probing the birth of a hot core?

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 greater than or equal to 300 K), abundant HCN seen in 14 mum absorption with the Infrared Space Observatory (ISO). The Line ratio yields T-ex = 720(-100)(+135), 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 2 orders of magnitude matches the observations. Such a jump is consistent with the sharp increase in HCN abundance at temperatures greater than or similar to 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.