The distribution of H¹³CN in the circumstellar envelope around IRC+10216

H¹³CN J = 8 → 7 submillimeter line emission produced in the circumstellar envelope around the extreme carbon star IRC+10216 has been imaged at subarcsecond angular resolution using the SMA. Supplemented by a detailed excitation analysis, the average fractional abundance of H¹³CN in the inner wind (≲ 5 × 10¹⁵ cm) is estimated to be about 4 × 10^-7, translating into a total HCN fractional abundance of 2 × 10^-5 using the isotopic ratio ¹²C/¹³C = 50. Multitransitional single-dish observations further require the H ¹³CN fractional abundance to remain more or less constant in the envelope out to a radius of ≈4 × 10¹⁶ cm, where the HCN molecules are effectively destroyed, most probably, by photodissociation. The large amount of HCN present in the inner wind provides effective line cooling that can dominate over that generated from CO line emission. It is also shown that great care needs to be taken in the radiative transfer modeling where nonlocal, and non-LTE, effects are important and where the radiation field from thermal dust grains plays a major role in exciting the HCN molecules. The amount of HCN present in the circumstellar envelope around IRC+10216 is consistent with predicted photospheric values based on equilibrium chemical models and indicates that any nonequilibrium chemistry occurring in the extended pulsating atmosphere has no drastic net effect on the fractional abundance of HCN molecules that enters the outer envelope. It further suggests that few HCN molecules are incorporated into dust grains.