THE ABUNDANCE DISTRIBUTION OF C4H IN IRC +10216

We have observed the N = 9-8 spin doublet emission line of C4H at 85.6 GHz in the carbon star IRC + 10216 with the BIMA millimeter interferometer. The aperture synthesis images have an angular resolution of approximately 10'' and a velocity resolution of approximately 1.3 km s-1. The data clearly show that C4H is distributed in a shell surrounding the central star, with little emission from the inner envelope. The peak of the average radial brightness profile in a map centered at the systemic velocity and with a width of 10 km s-1 is approximately 0.88 K at a projected radius of 15''. From the brightness temperature profile we infer that the thickness of the shell at half-peak intensity is approximately 15''. We derive the abundance of the molecule as a function of distance from the star using an LTE model. The peak abundance is calculated to be 1.8 x 10(-6) (with respect to the H-2 number density) at a radius of 16.7'' or 2.5 x 10(16) cm assuming a distance of 100 pc, a mass-loss rate of 2 x 10(-5) M. yr-1, and an outflow velocity of 13.8 km s-1. We compare our derived C4H distribution with the predictions of a photochemical model and find that the model underestimates the observed abundance of C4H by a factor of 5.