2012MNRAS.421.1325L

In 2004, the cosmic background radiation (CBR) temperature T_γ(CN) determined from interstellar CN absorption spectra was found to be 50 ± 20 mK higher than the COBE bolometer measured value. It was proposed that this was due, at least in part, to an error in the evaluation of the rotational components of the oscillator strengths of the rovibronic transitions corresponding to the CN B ²Σ⁺–X ²Σ⁺(0 - 0), R(0), R(1) and P(1) lines. Corrections to the standard Hönl–London (HL) rotational line intensity factors used were determined. New data on interstellar CN absorption show that T_γ(CN) is 29±2 mK greater than the latest value of the cosmological CBR temperature, T_γ(COBE) = 2.72548±0.00057 K. These new results and CN fluorescence lifetime data are shown to give similar derived values for improved HL corrections as well as providing further evidence for the intramolecular coupling between the relevant B ²Σ⁺ state rotational levels and close-lying levels of the A ²Π state which invalidates the standard HL factors. Revised HL factors may be required in future T_γ(CN) measurements, in particular in high-redshift sites, which cannot be studied by bolometric means.