The distribution of HCN, H¹³CN, and CN in IRC +10216

We present aperture synthesis images of millimeter-wavelength molecular emission lines of HCN (J = 1-0 at 88.6 GHz), H¹³CN (J = 1-0 at 86.3 GHz), and CN (N = 1-0, J = 3/2-1/2 at 113.5 GHz) toward the carbon star IRC +10216. The HCN and H¹³CN images were made with the BIMA interferometer, while the CN images were from a combination of interferometer and single-dish data. The HCN and H¹³CN images have an angular resolution of 8″ and a velocity resolution of ∼1.3 km s^-1; the CN images have an angular resolution of 11″ and a velocity resolution of ∼0.9 km s^-1. The images show that HCN and H¹³CN are found concentrated toward the center of the envelope, while CN is found in the shell surrounding the central star. The data for HCN and H¹³CN are compared with the output of a statistical equilibrium code, which takes into account the excitation by IR photons and collisions with H₂, and calculates the radial brightness profiles and spectra. The models are used to determine the abundance distribution of H¹³CN. HCN is too optically thick for accurate modeling; we infer the HCN abundance by scaling H¹³CN up by a factor of 40. For CN, a simple LTE calculation is used to derive the abundance of the molecule as a function of distance from the star. The distributions of HCN and CN are qualitatively consistent with chemical models for AGB star envelopes. Our best estimate of the peak CN/HCN abundance ratio is about 0.12, and the radius of the peak CN abundance is 19″ from the star. Both of these values are significantly less than recent photochemical model predictions. We suggest that CN photodissociates or reacts with other radicals or ions more rapidly than these models assume.