Non-LTE excitation of CN hyperfine line components

The ¹²CN molecule shows hyperfine structure because ¹⁴N has a non-zero nuclear spin. For the ¹²CN N=1→0 transition, this produces nine hyperfine components. The relative intrinsic strengths between hyperfine components are fixed under local thermodynamic equilibrium (LTE). The observed intensity ratios between optically thin components can therefore be used to trace departures from LTE.

Current estimates of the ¹²CN column density and the ¹²CN/¹³CN column density ratio rely strongly on the LTE assumption.

The ¹²CN/¹³CN column density ratio is a key tracer of the Galactic ¹²C/¹³C gradient. Moreover, the ¹²CN column density is widely used in astrochemical studies of UV-irradiated regions. It is therefore important to test the validity of the LTE assumption when deriving the ¹²CN column density.

Our work on this topic includes:
(a) We have found observational evidence for systematic departures from LTE in ¹²CN hyperfine line ratios in Galactic molecular clouds associated with massive star formation. In starless cores, the deviations are smaller, but still present in some sources (to be submitted).
(b) We are modeling the origin of these hyperfine anomalies, including non-LTE collisional excitation, selective photon trapping due to line blending, UV/infrared pumping, and geometric effects, using models such as RADEX, spectralRADEX, pythonRADEX, LIME, and other tools (in preparation).