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 cloud s for the purpose of accurately measuring the temperature of the cosmi c 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 H D 21483. We have used this result in combination with existing emissio n measurements toward our other stars to correct for local excitation effects within diffuse clouds which raise the measured CN rotational t emperature above that of the CMBR. After making this correction, we fi nd a weighted mean value Of T(CMBR) = 2.729 (+0.023, -0.031) K. This t emperature 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 t he COBE measurement and reaffirms the cosmic nature of this background radiation. From the observed agreement between our CMBR temperature a nd the COBE result, we conclude that corrections for local CN excitati on based on millimeter emission measurements provide an accurate adjus tment to the measured rotational excitation. This is in contrast to th e recent results of a similar study (Palazzi et al. 1992) in which the authors report statistically significant excess CN rotational excitat ion above that implied by the COBE direct CMBR measurement.