The Radiation Temperature of Space at λ 2.6 MM and the Excitation of Interstellar CN

The excitation temperature of interstellar CN molecules is derived from the R(0) and R(1) absorption lines in the spectra of two stars. TE = 3.22 + 0.15 K in Oph and 3.0 + 0.6 K in Per if the lines are not saturated. A reasonable estimate of the upper limit on the degree of saturation implies TE> 2.7 K. Excitation by collisions, fluorescence, and absorption of 2.6-mm radiation by a pure rotational transition is considered. All fail by at least an order of magnitude to account for the observations if the CN is in typical H I clouds, unless a new source of 2 6-mm radiation is postulated. If this source of radiation is identified with the cosmic background discovered by Penxias and Wilson to have TE = 3 5" 1.0" K at X 7.4 cm, the agreement of TE in the two stars can be explained. Furthermore, the background radiation must have a black-body spectrum over a 28:1 wavelength interval, as suggested by Dicke, Peebles, Roll, and Wilkinson for a background component due to black-body emission originating in the early history of the universe. If the CN is in or near an H ii region, its excitation could be due to collisions or to fluorescence, but the agreement of TE in the two stars would then be difficult to explain. Even in this case one can still conclude that the background temperature must be less than 3 4" K, which is sufficient to prove that the radiation observed by Penxias and Wilson cannot be dilute black-body radiation originating at a higher temperature. The detection of the 2.6-mm rotational lines of CN is considered in an Appendix.