Precise measurement of the cosmic microwave background temperature from optical observations of interstellar CN

We present very precise (S/N > 2000) observations of the 3874 A band of interstellar CN toward zeta Per and o Per. In the zeta Oph, zeta Per, and o Per lines of sight, the saturation-corrected CN line strengths yield respective excitation temperatures of 2.72 +- 0.05 K, 2.76 +- 0.05 K, and 2.78 +- 0.07 K for the J = 0--1 rotational transition at 2.64 mm. The excellent agreement among these temperatures confirms the expectation that the cosmic microwave background radiation (CMB) is primarily responsible for populating the excited rotational levels of interstellar CN. With small corrections for the local CN excitation due to electron impact, the J = 0--1 excitation temperatures toward zeta Oph, zeta Per, and o Per are all consistent with a CMB brightness temperature of 2.70 +- 0.04 K at 2.64 mm. This value represents the most precise determination to date of the CMB intensity at any wavelength. In addition, the CN J = 1--2 excitation temperatures toward these objects indicate a CMB temperature of 2.76 +- 0.20 K at 1.32 mm. Our temperatures at 2.64 mm and 1.32 mm are thus consistent with a 2.7 K blackbody spectrum for the CMB and do not support the spectral distortions observed near these wavelengths by Woody and Richards. Indeed, by confirming the blackbody character of the CMB spectrum at wavelengths near the peak of its flux, we have reinforced the simplest explanation of the CMB as primeval fireball radiation from a hot big bang