A precise CN measurement of the cosmic microwave background temperature at 1.32 millimeters
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Abstract
We present very high signal-to-noise ratio observations of the (1,0) and 0,0) vibrational bands of interstellar CN near 3580 and 3874 A toward HD 21483. These spectra provide the most accurate measurements of the CN (0,0) R(2), P(2), and (1,0) R(0) absorption made in any line of sight to date and the first confident detections of the (1,0) R(1) and P(1) lines. 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 mm and 2.64 mm. In the absence of local CN excitation, these values represent the brightness temperature of the cosmic microwave background radiation (CMB) at these wavelengths. We have obtained millimeter observations which reveal no CN emission at 2.64 mm in the HD 21483 line of sight and allow us to set a 2 σ upper limit of 0.11 K on the contribution of local processes to the J = 0-1 excitation temperature (T_01_). Considering this limit, the agreement of T_01_ with previous CN measurements of the CMB, 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. This value is consistent with the recent Matsumoto et al. rocket measurement of 2.799 +/- 0.018 K at 1.16 mm.