Clearing the Air: Updated CS Fluorescence Efficiencies Don't Solve the NUV/Sub-mm Cometary CS Abundance Discrepancy

Observations of carbon monosulfide (CS), a possible photodissociation fragment of CS₂, has a long history serving as a remote proxy for atomic sulfur, and more broadly, one of the sulfur reservoirs in cometary bodies. Interpretations of CS fluorescence emissions in ultraviolet observations of comets have, to-date, relied on a murky and ill-documented lineage of calculations for CS fluorescence efficiencies that can be difficult to assess. We report new fluorescence efficiencies of the CS radical, utilizing a rovibrational structure with vibrational states up to v = 8 and rotational states up to N = 100. A new set of band transition rates is produced through benchmarking comparisons to laboratory electron impact experiments, and subsequently utilized in the fluorescence calculations. We confirm historical reports of the ultraviolet (0, 0) band fluorescence efficiency, and conclude that blends and an expanded rovibrational structure have little impact on the fluorescence properties. Combined, the present results affirm the accuracy of the historical record of sulfur abundances derived via ultraviolet CS emissions in cometary observations with e. g. , IUE and HST. Benchmark comparisons to IUE observations of comet Bradfield show favorable agreement with the theoretical models. We expand the model to the time-domain, and find that thermalization of the ground state and/or `hot' initial distributions of CS lead to noticable changes in the shape of the (0, 0) band. The impact on existing sulfur abundances derived via ultraviolet CS observations, and new connections to sulfur reservoirs in protoplanetary disks are discussed. The model code and computed fluorescence efficiencies are made publicly available.