SUB-MILLIMETER MOLECULAR LINES IN THE CIRCUMSTELLAR ENVELOPE IRC+10216

Observations are presented of the circumstellar envelope IRC+10216 obtained with the 15m James Clerk Maxwell Telescope. Maps with the 15" FWHM beam have been made in the CO J=3-2, CS J=7-6 and HCN J=4-3 transitions. The shape of the radial CO distribution agrees well with the Kwan-Linke outflow model. The HCN J=4-3 intensity is significantly greater than that of CO J=3-2, but drops off almost twice as rapidly with distance from the source centre. This rapid fall-off of the HCN intensity is indicative of the dominance of radiative excitation, which contrasts with the mainly collisional mechanism driving CO excitation. Single spectra at or near the source centre are also presented for the isotopes (CO)-C-13, (HCN)-C-13, (CS)-S-34, for the J=3-2 line of HCN and its isotope (HCN)-C-13 (with 23" FWHM beam) and for CO J=4-3 (11" FWHM beam). Upper limits are also obtained for the (CO)-O-18 J=3-2 isotopic line and for the neutral carbon 3P1-3P0 fine structure transition at 492.16 GHz. Simple modelling leads to estimates of relative abundances and excitation temperatures of CO, HCN and CS and an upper limit to the neutral carbon abundance.