A Precise CN Measurement of the Microwave Background at 1.32 mm

We present very high signal-to-noise observations of the (1,0) and (0,0) vibrational bands of interstellar CN near 3580 and 3874 Å toward HD 21483. Corrected for saturation with a derived b-value of 1.29±0.05 km s-1, these CN line strengths yield excitation temperatures of 2.83±0.09 and 2.76±0.07 K for the J=1-2 and J=0-1 rotational transitions at 1.32 and 2.64 mm. In the absence of local CN excitation, these values represent the brightness temperature of the cosmic microwave background (CMB) radiation at these wavelengths. Millimeter observations have revealed no CN emission at 2.64 mm toward HD 21483 and allow us to set a 2σ upper limit of 0.11 K on the contribution of local processes to the J=0-1 excitation. Considering this limit and the lesser likelihood of local J=1-2 excitation, our observations indicate a CMB temperature of 2.83±0.09 K at 1.32 mm.