2025MNRAS.536.1980C

We report a new, reliable determination of the cyano radical (CN) excitation temperature of diffuse molecular clouds in the Milky Way, based on ultrahigh spectral resolution observations. Our determination is based on CN $B^{2}\Sigma ^{+}-X^{2}\Sigma ^{+}$ (0, 0) vibronic band absorption spectra seen along the lines of sight to eight bright Galactic stars. Our analysis is conducted blind, and we account for multiple sources of systematic uncertainty. Like previous studies, our excitation temperature measures exhibit an intrinsic scatter that exceeds the quoted uncertainties. Accounting for this scatter, we derive a 3 per cent determination of the typical CN excitation temperature, $T_{01}=2.769^{+0.084}_{-0.072}~{\rm K}$, which is consistent with the direct determination of the cosmic microwave background (CMB) temperature. We also perform a single joint fit to all sightlines simultaneously, and find that our data can be fit with an excitation temperature $T_{01}=2.725\pm 0.015~{\rm K}$ - a 0.55 per cent measure that is consistent with the CMB temperature. We propose a future observational strategy to reduce systematic uncertainties and firmly test the limitations of using CN as a CMB thermometer.