A cometary fluorescence model of cyanogen in the near-infrared

While the cyano radical (CN) is a well-known and prominent feature in cometary optical spectra, its origin remains poorly understood. In particular, although hydrogen cyanide (HCN) is considered the primary parent molecule of CN, it is not abundant enough to account for the observed amounts of the radical (Fray et al., 2005). As a result, cyanogen (C₂N₂) has been proposed as a secondary potential parent molecule.

Among the major breakthroughs achieved by the Rosetta mission, the discovery of around 40 new molecular species—previously undetected in comets—by the ROSINA mass spectrometer has considerably improved our understanding of cometary chemical diversity. Cyanogen was among the most recently identified molecules in the coma of 67P (Hänni et al., 2021). Although the low derived mixing ratios suggest it is unlikely to produce CN in sufficient amounts on its own, further investigation of its abundance in other comets is essential to better constrain its potential role as a CN precursor and to improve our understanding of cometary chemistry.

In this context, we investigated the presence of cyanogen in near-infrared cometary spectra. In the HITRAN database (Gordon et al., 2022), only the far-infrared ν₅ band of C₂N₂ (Fayt et al., 2012) is currently included, and no spectroscopic data are available above 310 cm^-1. Notably, one of the molecule's fundamental vibrational bands, ν₃, centered at 2158 cm^-1 (4.63 μm), has never been studied at high spectral resolution. This band is particularly interesting as it lies within the M-infrared atmospheric window, a region relatively free from atmospheric emission lines of H₂O and CO₂.

We present here, for the first time, a high-resolution analysis of line positions and intensities in the ν₃ band region of C₂N₂, based on laboratory infrared spectra. From this, we derived molecular parameters for both the ground and excited vibrational states using PGOPHER (Western, 2017).

This spectroscopic analysis enabled the construction of the first line-by-line fluorescence model of cyanogen. Excitation rates for individual lines of the ν₃ band in cometary comae are also presented (Fig. 1).

Finally, we present an upper limit for the abundance of cyanogen in comet C/2022 E3 (ZTF), observed in 2023 with JWST (Milam et al., 2023). We discuss how future instruments such as METIS on the ELT could further improve this limit, or even allow a detection of cyanogen in a bright comet.

 

Figure 1: Emission g-factors of C₂N₂ expressed in photons s^-1 molecule^-1 at T=50 K.

 

This work is part of the COSMIC project (Computation and Spectroscopy of Molecules in the Infrared for Comets), funded by the EIPHI Graduate School. https://gradschool.eiphi.ubfc.fr/?p=3710

 

References

 

Fayt A., Joly A., Benilan Y., Manceron L., Kwabia-Tchana F., Guillemin J.-C., 2012, Frequency and intensity analyses of the far infrared ν₅ band system of cyanogen (C₂N₂) and applications to Titan, Journal of Quantitative Spectroscopy and Radiative Transfer 113, 1195-1219.

Fray N., Bénilan Y., Cottin H., Gazeau M.-C., Crovisier J., 2005, The origin of the CN radical in comets: A review from observations and models. Planetary and Space Sciences 53(12), 1243–1262.

Gordon I.E., Rothman L.S., Hargreaves R.J. et al., 2022, The HITRAN2020 molecular spectroscopic database, Journal of Quantitative Spectroscopy and Radiative Transfer 277.

Hänni N., Altwegg K., Balsiger H. et al., 2021, Cyanogen, cyanoacetylene, and acetonitrile in comet 67P and their relation to the cyano radical, Astronomy And Astrophysics, 647.

Milam S. N., Roth N. X., Villanueva G.L., Wong I., Kelley M. S. P., Bockelée-Morvan D., Hammel H. B., 2023, Asteroids, Comets, Meteors Conference 2023, LPI Contrib. No 2851.

Western C.M., 2017, PGOPHER: A program for simulating rotational, vibrational and electronic spectra, Journal of Quantitative Spectroscopy and Radiative Transfer 186, 221-242.