Gas-Phase Chemical and Spectral Modeling of N2+ in Cometary Atmospheres

Comets are regarded as primitive remnants left over from the formation of the solar system.  As comets approach the sun, the ices in their nuclei sublimate to form their comae.  Therefore, understanding the chemical composition of cometary comae is vital for understanding the formation and evolution of our solar system.  The relative abundances of cometary volatile species, often measured with respect to H₂O or other abundant volatiles, provides critical information about a comet’s thermal history and evolutionary pathway.  These volatiles sublimate under slightly different conditions and have different polarizabilities [1], making their abundance ratios sensitive probes of the temperature and environment of ice formation. 

The chemical composition of cometary comae is obtained through analysis of emission spectra at optical, IR, and/or near-UV wavelengths.  However, obtaining these data for neutral chemical species that lack permanent dipole moments and/or have transitions that conflict with atmospheric absorption features is particularly challenging.  One promising approach is to take advantage of spectral transitions of their corresponding cations in the UV/optical regimes accessible by ground-based observatories such as SOAR and the VLT.  To determine chemical abundances of these ions in the coma we need to understand both ionization and excitation processes.  Here, we present the results of our study of the production and emission mechanisms of N₂⁺, which has been observed previously in comets [2, 3].

Using quantum chemical methods, we have determined transition rates (Einstein A coefficients) for N₂⁺, as well as the full rovibronic structure of this diatomic cation at wavelengths from 200 nm and above.  

Fluorescence emission models of the A-X and B-X bands of N₂⁺ have been constructed from a combination of new theoretical data and existing experimental data. Our synthetic spectra agree with observed cometary spectra containing the N₂⁺ B-X band. Further, we investigate the level of complexity required to fully and accurately model the band luminosities of the strong (0,0) band typically used to derive N₂⁺ column densities from cometary spectra.

Dominant production pathways for N₂⁺ have been investigated using a large-scale chemical reaction network and suggest that photochemical production mechanisms in the coma dominate for N₂⁺.

These studies are part of a larger effort to understand and predict the spatial distribution of several notable cometary cations and their spectra across a variety of dynamical classes and cometary compositions. The modeled band luminosities are expected to impact the interpretation of cationic emission features and their connections to their presumed parent molecules (e.g. N₂).

This material is based on work supported by the National Science Foundation under grant no. AST-2407815 at Auburn University.  Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

 

[1]  Rubin, M., Altwegg, K., van Dishoeck, E. F., and Schwehm, G., 2015, ApJL, 815, 11.

[2]  Cochran, A. L, and McKay, A. J., 2018, ApJL, 854, 10.

[3] Opitom, C. et al., 2019, A&A, 624, 64.