Detection of radio recombination-line emission from the Rho Ophiuchi dark cloud.

Recombination-line observations in the direction of the p Oph dark cloud at 21 cm are described in which we have detected the carbon 166a and 167a lines. The excellent agreement between the velocity of these lines and the velocity of molecular lines from the p Oph cloud lead to an unequivocal identification of the regions of recombination-line emission with the dark cloud itself. The velocity width of the recombination lines Av = 2.4 km 1 is comparable with the widths of the CO and OH lines, indicating that the line must arise in cold gas. The emission measure of the line, 0.2 cm -6 pc, is sufficiently large as to require that the line originate in regions with densities n > 630 cm 2 From these observations, together with an analysis of the H I selfabsorption feature seen in the same direction, we infer that the relative electron density in the line-emitting regions of the cloud is ne/n <: 1.6 x 10- . Subject headings: molecules, interstellar - nebulae - radio lines - 21-cm radiation