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P.D. Gensheimer (Steward-SMTO), J.H. Bieging (Steward Obs.)
We have mapped emission from the circumstellar envelope of the nearby carbon star IRC+10216 in the J=4-3 (\lambda0.85 mm) pure rotational lines of HCN in both the ground and the (0,11c,0) vibrationally excited states. We used the On The Fly (OTF) mapping technique with the Heinrich Hertz Telescope of the Submillimeter Telescope Observatory (SMTO) on Mt. Graham, AZ. The angular resolution was 22\arcsec (HPBW). The (0,11c,0) state is the lowest-lying bending mode of HCN, at an energy, E/k, of ~1050 K above ground. The two observed HCN lines are separated by only 45 MHz and so could be observed simultaneously, thereby minimizing relative calibration uncertainties. The data allow us to constrain the angular size distributions of the two transitions. We compare our data with results of statistical equilibrium models for the excitation of HCN in the IRC+10216 circumstellar envelope to constrain the physical conditions in the inner part of the circumstellar envelope.