High CO2 Abundance in Interstellar Comet 2I/Borisov Inferred from Oxygen Line Ratio Measurements

The appearance of interstellar comet 2I/Borisov in late 2019 provided an unprecedented opportunity to measure the volatile composition of a cometary object from outside our solar system. Many interesting findings have been discussed in the literature, including a CO abundance higher than most solar system comets, but no direct measurement of CO2 has been reported, due to the necessity of space-based observations to detect CO2 directly. Observations of [OI] emission in the optical can be used as a ground-based proxy for CO2 in comets, but is limited in its accuracy by incomplete knowledge of [OI] photochemistry in cometary comae. Despite this, empirical methods have reproduced observed CO2 abundances in comets with some success. Analysis of the [OI] emission in Borisov was presented by Opitom et al. 2021, and they found a high value for the flux ratio of the [OI]5577 A line to the sum of [OI]6300 and [OI]6364 A lines of ~0.3, whereas most solar system comets have observed ratios between 0.05-0.1. While such a high ratio could be due to the high CO abundance observed for Borisov, we will present new analysis of the [OI] emission that suggests that this high ratio actually reflects a high CO2 abundance for Borisov, with a CO2 production rate on par with its H2O and CO production rate. This finding is supported by as yet unpublished NEOWISE observations, which are sensitive to CO2. This, in conjunction with the previously reported high CO production rate, would imply that Borisov is a hypervolatile-rich object compared to most solar system comets (with the combination of CO and CO2 being more abundant than water), meaning that it perhaps underwent formation in a colder region of its protoplanetary disk compared to solar system comets.