Interstellar cyanogen and the temperature of the cosmic microwave background radiation

We present the results of a recently completed effort to determine the amount of CN rotational excitation in five diffuse interstellar clouds for the purpose of accurately measuring the temperature of the cosmic microwave background radiation (CMBR). In addition, we report a new detection of emission from the strongest hyperfine component of the 2.64 mm CN rotational transition (N = 1 → 0) in the direction toward HD 21483. We have used this result in combination with existing emission measurements toward our other stars to correct for local excitation effects within diffuse clouds which raise the measured CN rotational temperature above that of the CMBR. After making this correction, we find a weighted mean value of T_CMBR = 2.729 (+ 0.023, -0.031) K. This temperature is in excellent agreement with the new COBE measurement of 2.726 ± 0.010 K (Mather et al. 1993). Our result, which samples the CMBR far from the near-Earth environment, attests to the accuracy of the COBE measurement and reaffirms the cosmic nature of this background radiation. From the observed agreement between our CMBR temperature and the COBE result, we conclude that corrections for local CN excitation based on millimeter emission measurements provide an accurate adjustment to the measured rotational excitation. This is in contrast to the recent results of a similar study (Palazzi et al. 1992) in which the authors report statistically significant excess CN rotational excitation above that implied by the COBE direct CMBR measurement.