A precise CN measurement of the cosmic microwave background temperature at 1.32 millimeters

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High-SNR observations of the (1,0) and (0,0) vibrational bands of interstellar CN near 3580 and 3874 A toward HD 21483 are reported. The CN line strengths yield excitation temperatures of 2.83 + or - 0.09 and 2.76 + or - 0.07 K for the J = 1-2 and J = 0-1 rotational transitions at 1.32 mm and 2.64 mm. It is noted that in the absence of local CN excitation, these values represent the brightness temperatures of the cosmic background radiation (CMB) at these wavelengths. No CN emission is noted at 2.64 mm in the HD 21483 line of sight, setting a 2-sigma upper limit of 0.11 K on the contribution of local processes to the J = 0-1 excitation temperature. 24 refs.