Continuum subtracted LWS grating spectrum of IRC +10216 from 75 to 197 microns (thick line). An offset has been introduced to the continuum to show simultaneously the computed emission of the CO v = 0,1, ¹³CO v = 0, HCN and H¹³CN v = 0, and HCN v2 = 1, 2 v1,3 = 1 (thin line). See text for the adopted model parameters. the rotational transitions of CO, HCN, and HCN v2 = 1, 2 and v1,3 = 1 are indicated by arrows at the top and bottom of each panel. The weak lines in the model correspond to H¹³CN, ¹³CO and to the stretching modes of HCN. (image x2)