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

Abstract

The authors present very precise (S/N > 2000) observations of the 3874 Å band of interstellar CN toward ζ Per and ο Per. In the ζ Oph, ζ Per, and ο Per lines of sight, the saturation-corrected CN line strengths yield respective excitation temperatures of 2.72±0.05K, 2.76±0.05K, and 2.78±0.07K 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 excitation temperatures toward ζ Oph, ζ Per, and ο Per are all consistent with a CMB brightness temperature of 2.70±0.04K at 2.64 mm.