Presentation #210.08 in the session “Comets”. Cross-listed as presentation #105.04.
Comets are primitive leftovers from the formation of the solar system, meaning their volatile composition reflects the physical and chemical conditions where they formed. However, they are not pristine, as evolutionary effects may have influenced the coma composition we observe today. CO2 is one of the most abundant species in comets and its abundance is important for answering key questions in cometary science concerning their composition, activity, and evolution. Because direct studies of CO2 are limited to space-borne assets, due to severe telluric absorption, proxy observations accessible from the ground are a potentially powerful tool for studying CO2 in comets. The flux ratio of the [OI]5577Å line to the sum of the [OI]6300 and [OI]6364Å lines (hereafter the oxygen line ratio) has been proposed as one of these proxies, but the photochemistry responsible for OI release in cometary comae that results in these emissions is still not fully understood.
Comet C/2016 R2 (PanSTARRS) has a peculiar composition that deviates significantly from the sample of comets observed to date, with CO and CO2 being the dominant oxygen-bearing molecules rather than H2O. Therefore, C/2016 R2 provides a rare case where the OI photochemistry can be studied without H2O photodissociation being a major source of OI, shedding light on how the photodissociation of CO and CO2 releases OI into the coma. In this presentation we will present analysis of oxygen line ratio measurements in C/2016 R2 from previously published observations near-perihelion (2.6-3.0 au), as well as unpublished observations when the comet was at a heliocentric distance of 4.7 au post-perihelion. We interpret the measured oxygen line ratios in terms of what they reveal about our understanding of the photochemistry of OI release from CO and CO2 photodissociation. This will in turn allow for the oxygen line ratio to serve as a more accurate proxy for CO2 abundances in the cometary population as a whole. This work was supported by the NASA Solar System Workings Program through grant 80NSSC20K0140.