Inferring the CO2 Abundance in Comet 45P/Honda-Mrkos-Pajdušáková from [O i] Observations: Implications for the Source of Icy Grains in Cometary Comae

Abstract

The study of cometary composition is important for understanding our solar system's early evolutionary processes. Carbon dioxide (CO _2 ) is a common hypervolatile in comets that can drive activity but is more difficult to study than other hypervolatiles owing to severe telluric absorption. CO _2 can only be directly observed from space-borne assets. Therefore, a proxy is needed to measure CO _2 abundances in comets using ground-based observations. The flux ratio of the [O i ] λ 5577 line to the sum of the [O i ] λ 6300 and [O i ] λ 6364 lines (hereafter referred to as the [O i ] line ratio) has, with some success, been used in the past as such a proxy. We present an [O i ] line ratio analysis of comet 45P/Honda-Mrkos-Pajdušáková (HMP), using data obtained with the Tull Coudé Spectrograph on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory, taken from UT 2017 February 21–23, when the comet was at heliocentric distances of 1.12–1.15 au. HMP is a hyperactive Jupiter-family comet (JFC). Icy grains driven out by CO _2 sublimation have been proposed as a driver of hyperactivity, but the CO _2 abundance of HMP has not been measured. From our [O i ] line ratio measurements, we find a CO _2 /H _2 O ratio for HMP of 22.9% ± 1.4%. We compare the CO _2 /H _2 O ratios to the active fractions of the nine comets (including HMP) in the literature that have data for both values. We find no correlation. These findings imply that CO _2 sublimation driving out icy grains is not the only factor influencing active fractions for cometary nuclei.

Keywords

• Comets
• Comae
• Carbon dioxide
• Comet volatiles
• Short period comets