Comet C/2013 V5 (Oukaimeden): Evidence for Depleted Organic Volatiles and Compositional Heterogeneity as Revealed through Infrared Spectroscopy

Abstract

We obtained high-resolution ( λ /Δ λ  ∼ 25,000) pre-perihelion spectra of Comet C/2013 V5 (Oukaimeden) using NIRSPEC at Keck II on UT 2014 September 5–6, and CSHELL at the NASA-Infrared Telescope Facility on September 11–13, altogether spanning a range in heliocentric distance R h  = 0.789–0.698 au. We report water production rates, and production rates and abundance ratios relative to co-measured H 2 O for eight trace molecules: CO, H 2 CO, CH 3 OH, CH 4 , C 2 H 2 , C 2 H 6 , HCN, and NH 3 . Our measured water production rates from NIRSPEC and CSHELL observations remained relatively constant and were close to those from SOHO /Solar Wind ANisotropies observations that encompassed our dates, suggesting H 2 O production dominated by release directly from or within approximately 2000 km of the nucleus. All trace volatiles were depleted relative to their respective median abundances among comets, excepting NH 3 , which was consistent with its median abundance. Most surprising were pronounced increases in abundance ratios for CH 3 OH (by 51% relative to simultaneously measured H 2 O) and especially C 2 H 6 (by 87%) between September 5 and 6. On September 5, C 2 H 6 was severely depleted, consistent with its lowest abundance yet measured for any comet. It also tracked the spatial profile of H 2 O, suggesting C 2 H 6 was associated with a polar ice phase dominating gas production. On September 6, C 2 H 6 was only moderately depleted and was spatially distinct from H 2 O, suggesting both polar- and nonpolar-dominated ice phases contributed to the activity then. Our results are consistent with a nonhomogeneous volatile composition for C/2013 V5, implying differential processing of its constituent ices.