Cosmic Background Radiation Temperature from CN Absorption

Precise high-resolution measurements of the interstellar lines of CN near 3874 A toward the star Zeta Oph provide a new measurement of the cosmic background radiation temperature of 2.74 + or - 0.05 K at 2.64 mm and 2.75 (+0.24, -0.29) K at 1.32 mm. Using a spectral resolution of 156,000, the intrinsic width of these lines is found to be 19.0 + or - 0.5 mA, which is substantially smaller than previously reported and requires larger saturation corrections than used in previous work to determine the CN excitation temperature.