Vibrationally Excited HCN around AFGL 2591: A Probe of Protostellar Structure

Abstract: Vibrationally excited molecules with sub-mm rotational transitions are potentially excellent probes of physical conditions near protostars. This study uses observations of the v=1 and v=2 ro-vibrational modes of HCN (4-3) to probe this environment. The presence or absence and relative strengths of these ro-vibrational lines probe the gas excitation mechanism and physical conditions in warm, dense material associated with protostellar disks. We present pilot observations from the Heinrich Hertz Submillimeter Telescope (HHSMT) and followup observations from the Submillimeter Array (SMA). All vibrationally excited HCN (4-3) v=0, v=1, and v=2 lines were observed. The existence of the three v=2 lines at approximately equal intensity imply collisional excitation with a density of greater than (10^10 cm^-3) and a temperature of >1000 K for the emitting gas. This warm, high density material should directly trace structures formed in the protostellar envelope and disk environment. Further, the line shapes of the v=2 emission may suggest a Keplerian disk. This letter demonstrates the utility of this technique which is of particular interest due to the recent inauguration of the Atacama Large Millimeter Array (ALMA).