Observations of [O I] emission in Comets C/2014 Q2 (Lovejoy) and C/2007 N3 (Lulin): Possible Influence of Solar Activity on Oxygen Line Ratios

Abstract

Observing [O I] emission to calculate an "oxygen line ratio" has been proposed as a potential proxy for direct CO$_2$ measurement in comets. However, the photochemistry governing [O I] release into the coma is not well understood, and using theoretical release rates often yields different results than using empirical release rates determined in conjunction with direct space-based measurements of CO$_2$. We hypothesize that the accuracy of the release rates could depend on the level of solar activity at the time the comet is observed, which will be influenced by the solar cycle. We present observations and analysis of [O I] emission in one comet observed near solar maximum, C/2014 Q2 (Lovejoy), and one near solar minimum, C/2007 N3 (Lulin). Our [O I] measurements were obtained using two high spectral resolution optical spectrographs: the Tull Coud\'e spectrometer at McDonald Observatory and the ARCES spectrometer at Apache Point Observatory. We use empirical and theoretical models for [O I] emission from the literature to derive multiple sets of inferred CO$_2$ abundances for these comets and compare to contemporaneous space-based measurements of CO$_2$. We find that the empirical model, which was developed based on comet observations obtained near solar maximum, reproduces the directly measured CO$_2$ abundances better for Lovejoy. Neither model accurately reproduces the direct measurement for Lulin. We discuss the implications of our findings for the accuracy and dependencies of the oxygen line ratio method for inferring CO$_2$ abundances in cometary comae.