Development of a cometary fluorescence model of cyanogen in the near-infrared

Among the limited number of space missions dedicated to cometary science, the Rosetta spacecraft launched in 2004 significantly enhanced the understanding of those small bodies, by conducting a two-year study of the comet 67P/Churyumov-Gerasimenko. One of Rosetta’s instruments, the mass spectrometer ROSINA, discovered numerous chemical species in the coma that had never been observed in comets before.

One such species, cyanogen (C₂N₂) is a linear molecule previously only observed in Titan’s atmosphere (Kunde et al. 1981) before its detection in comet 67P (Altwegg et al. 2019). While the CN radical is one of the major species observed in the optical spectra of comets, its origins (i.e., its corresponding parent species) are still poorly understood. In particular, it has been shown that hydrogen cyanide (HCN) can’t be the only source of cometary CN (Hänni et al. 2020; Hänni et al. 2021). As C₂N₂  could also participate in the formation of CN, its detection in 67P must be generalized to other comets to better constrain its abundance in cometary environments.

In this context, we will present the recent analysis of the ν₃  fundamental vibrational band of cyanogen centered around 2158 cm⁻¹  (Fig. 1). From high-resolution spectra recorded at different pathlengths and pressures, ranging from 0.13 to 4 mbar at the LISA facility¹, absorption line positions and intensities were obtained thanks to a multi-spectrum fitting program developed at Université Libre de Bruxelles by Jean Vander Auwera. A fitting example is represented in Fig. 2. Rotational constants were also determined using PGOPHER (Western, 2017).

From our analysis, we have begun developing a cometary fluorescence model by determining the g-factors of cyanogen’s brightest ν₃  lines. As high-resolution databases such as HITRAN (Gordon et al. 2022) currently lack C₂N₂  in the near-infrared, the fluorescence model will open the possibility of a direct spectroscopic detection in the infrared with high-resolution infrared spectrometers, and possibly a quantitative study of this cometary species with ground-based facilities.
This work is part of the COSMIC project (Computation and Spectroscopy of Molecules in the Infrared for Comets), funded by the EIPHI Graduate School².

¹http://www.lisa.u-pec.fr/en
²https://gradschool.eiphi.ubfc.fr/?p=3710

Figure 1: One of the experimental spectra of C₂N₂ recorded at LISA, with a pressure of 1.3404 mbar and a pathlength of 0.849 m. Absorption lines are mainly due to the ν₃ fundamental vibrational band.

Figure 2: Comparison between the experiment and simulation in the region between 2173.15 and 2173.50 cm⁻¹. The two brightest lines (R(53) and R(54)) belong to the ν₃band. Other absorption lines mainly belong to hot bands.

References

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