C/2013 R1 (Lovejoy) at IR Wavelengths and the Variability of CO Abundances Among Oort Cloud Comets

We report production rates, rotational temperatures, and related parameters for gases in C/2013 R1 (Lovejoy) using the Near InfraRed SPECtrometer at the Keck Observatory, on six UT dates spanning heliocentric distances (Rh ) that decreased from 1.35 AU to 1.16 AU (pre-perihelion). We quantified nine gaseous species (H2O, OH*, CO, CH4, HCN, C2H6, CH3OH, NH3, and NH2) and obtained upper limits for two others (C2H2 and H2CO). Compared with organics-normal comets, our results reveal highly enriched CO, (at most) slightly enriched CH3OH, C2H6, and HCN, and CH4 consistent with "normal", yet depleted, NH3, C2H2, and H2CO. Rotational temperatures increased from ~50 K to ~70 K with decreasing Rh , following a power law in Rh of –2.0 ± 0.2, while the water production rate increased from 1.0 to 3.9 × 1028 molecules s–1, following a power law in Rh of –4.7 ± 0.9. The ortho-para ratio for H2O was 3.01 ± 0.49, corresponding to spin temperatures (T spin) ≥ 29 K (at the 1σ level). The observed spatial profiles for these emissions showed complex structures, possibly tied to nucleus rotation, although the cadence of our observations limits any definitive conclusions. The retrieved CO abundance in Lovejoy is more than twice the median value for comets in our IR survey, suggesting this comet is enriched in CO. We discuss the enriched value for CO in comet C/2013 R1 in terms of the variability of CO among Oort Cloud comets.