Sub-millimetre molecular lines in the circumstellar envelope IRC+10216

The study reports observations of the circumstellar envelope IRC+10216 obtained with the 15-m James Clerk Maxwell Telescope. Maps with the 15" FWHM beam were made in the CO J = 3-2, CS J = 7-6, and HCN J = 4-3 transitions. 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 center. 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 center are also presented for the isotopes 13CO, H13CN, C34S, for the J = 3-2 line of HCN and its isotope H13CN and for CO J = 4-3. Simple modeling results in estimates of relative abundances and excitation temperatures of CO, HCN, and CS, and an upper limit to the neutral carbon abundance