C2H in the 2 micron infrared spectrum of IRC +10216

Nearly 30 lines of C2H in two bands at 4011 and 4108/cm have been identified in the 2 micron spectrum of IRC +10216. A rotational temperature of about 12.5 + or - 1.5 K and a column density of about 3 X 10 to the 15/sq cm have been found. Modeling the lines with an observer's frame radiative transfer code indicates that the peak radial abundance of C2H relative to H2 is about 0.00001-0.000001, and that the C2H is concentrated about 8 X 10 to the 16 cm away from the central infrared source. A model including photodissociation by interstellar UV demonstrates that all the observed C2H can be formed in the circumstellar shell by photolysis of C2H2. It is suggested that the radial abundance of C2H2 decreases by at least a factor of 5, perhaps by accretion of C2H2 onto grains, from about 100 solar radii to about 1000 solar radii, where photodissociation begins.