The CO2 abundance in Comets C/2012 K1 (PanSTARRS), C/2012 K5 (LINEAR), and 290P/Jäger as measured with Spitzer

Carbon dioxide is one of the most abundant ices present in comets and is therefore important for understanding cometary composition and activity. We present analysis of observations of CO₂ and [O I] emission in three comets to measure the CO₂ abundance and evaluate the possibility of employing observations of [O I] emission in comets as a proxy for CO₂. We obtained NIR imaging sensitive to CO₂ of comets C/2012 K1 (PanSTARRS), C/2012 K5 (LINEAR), and 290P/Jäger with the IRAC instrument on Spitzer. We acquired observations of [O I] emission in these comets with the ARCES echelle spectrometer mounted on the 3.5-m telescope at Apache Point Observatory and observations of OH with the Swift observatory (PanSTARRS) and with Keck HIRES (Jäger). The CO₂/H₂O ratios derived from the Spitzer images are 12.6 ± 1.3% (PanSTARRS), 28.9 ± 3.6% (LINEAR), and 31.3 ± 4.2% (Jäger). These abundances are derived under the assumption that contamination from CO emission is negligible. The CO₂ abundance for PanSTARRS is close to the average abundance measured in comets at similar heliocentric distance to date, while the abundances measured for LINEAR and Jäger are significantly larger than the average abundance. From the coma morphology observed in PanSTARRS and the assumed gas expansion velocity, we derive a rotation period for the nucleus of about 9.2 h. Comparison of H₂O production rates derived from ARCES and Swift data, as well as other observations, suggest the possibility of sublimation from icy grains in the inner coma. We evaluate the possibility that the [O I] emission can be employed as a proxy for CO₂ by comparing CO₂/H₂O ratios inferred from the [O I] lines to those measured directly by Spitzer. We find that for PanSTARRS we can reproduce the observed CO₂ abundance to an accuracy of ∼20%. For LINEAR and Jäger, we were only able to obtain upper limits on the CO₂ abundance inferred from the [O I] lines. These upper limits are consistent with the CO₂ abundances measured by Spitzer.