Inferring the CO₂ abundance in Comets 45P/Honda–Mrkos–Pajdušáková and C/1995 O1 (Hale-Bopp) from [OI] Observations

The study of cometary composition is essential to understanding our solar system’s early evolutionary processes. Carbon dioxide (CO₂) is a common hypervolatile in comets that can drive activity. However, CO₂ is more difficult to study than other hypervolatiles due to severe telluric absorption, and, thus, direct observation of CO₂ can only be performed from space-borne assets. A proxy is needed to measure CO₂ abundances in comets with ground-based observations. The flux ratio of the [OI] 5577 Å line to the sum of the [OI] 6300 Å and [OI] 6364 Å lines (hereafter referred to as the oxygen line ratio) has been used in the past as such a proxy with some success. However, the photochemistry responsible for the release of OI into the coma is not yet fully understood, and requires more study in order for the oxygen line ratio to be used as a reliable proxy for CO₂ production in comets. We present an oxygen line ratio analysis of two comets: 45P/Honda–Mrkos–Pajdušáková and C/1995 O1 (Hale-Bopp). 45P is a Jupiter family comet (JFC) and potentially hyperactive. Icy grains driven out by CO₂ sublimation have been proposed as a driver of hyperactivity, but the CO₂ abundance of 45P has not been measured. Inferences about 45P’s CO₂ abundance based on the oxygen line ratio could lead to a better comprehension of what drives its hyperactivity. The CO₂ abundances in JFCs like 45P can also be compared to CO₂ abundances to be measured in Centaurs by the James Webb Space Telescope (JWST) to better understand the evolution of these objects as they transition from the Centaur to the JFC stage. For Hale-Bopp, a long period comet, we will compare our results to the CO₂ abundance measured using the Infrared Space Observatory two weeks prior to our observations, allowing for a better understanding of OI photochemistry. Analysis of our Hale-Bopp observations will also provide context for spectroscopy to be obtained using JWST when the nucleus is inactive, allowing comparison of observed coma abundances to the composition of ices on the surface. This work was supported by the NASA internship program and the NASA Solar System Workings Program through grant 80NSSC20K0140.