Detection and significance of carbon recombination lines in diffuse interstellar clouds.

The carbon 166-alpha recombination line has been detected in the diffuse clouds which produce optical interstellar lines in the spectra of Zeta Oph and Omicron Per. The diffuse clouds must have densities in excess of 1000 per cu cm to produce the observed recombination lines. It is shown that the average carbon ionization rate, Gamma, in diffuse clouds can also be derived from these data. After correction for extinction and H2 absorption, the result for both clouds is that Gamma is several times greater than the value appropriate for the unattenuated average galactic radiation field. The clouds must therefore be within about 25 pc of the stars.