Fluorescence Modeling of Cometary Cations: A Case Study of CO⁺

Central to advances in characterizing the molecular inventory of comets — and their implications for planetary formation — are remote observations of cometary comae. Produced by sublimation of cometary ices and erosion by the solar wind, the cometary plasma environment is rich in neutrals, molecules, and photochemistry. The spectroscopic signatures of the cometary constituents beget both its formation history and its evolution. Cations produced from the most abundant gases, e.g. CO⁺, H₂O⁺, and CO₂⁺, are promising candidates for tracing the photochemistry of neutral molecules which are difficult to detect via ultraviolet/visible spectroscopy.

The only fluorescence model of oxygen-bearing cometary cations, the CO⁺ fluorescence model of Magnani & A’Hearn [1], is lost to time. We present here a recreation and expansion of the CO⁺ fluorescence model of Magnani & A’Hearn (1986). Building on our atomic fluorescence code “FlorPy” [2], we develop a complete formalism for modeling the spectra of cometary cations in both fluorescence equilibrium, and the time-domain. Our updated CO⁺ model incorporates molecular constants which have improved in accuracy in the thirty-six years since the work of Magnani & A’Hearn, and we expand the number of included rovibronic states substantially. We investigate and elaborate on the necessity and required accuracy of molecular data for understanding the spectroscopic signatures of cometary cations, and their role in understanding cometary comae. Using our “FlorPy” code, we investigate the conditions required to justify common assumptions in fluorescence models, such as e.g. fluorescence equilibrium, and describe a pathway for expanding the FlorPy code to other molecular systems of interest to the atmospheric, astrochemical, and planetary science communities.

[1] Magnani, L, & A’Hearn, M. F. 1986, ApJ, 302, 477

[2] Bromley et al. 2021, PSJ, 2, 228