2018AJ....156..258D

We obtained high-resolution (λ/Δλ ∼ 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 H₂O for eight trace molecules: CO, H₂CO, CH₃OH, CH₄, C₂H₂, C₂H₆, HCN, and NH₃. 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 H₂O 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 NH₃, which was consistent with its median abundance. Most surprising were pronounced increases in abundance ratios for CH₃OH (by 51% relative to simultaneously measured H₂O) and especially C₂H₆ (by 87%) between September 5 and 6. On September 5, C₂H₆ was severely depleted, consistent with its lowest abundance yet measured for any comet. It also tracked the spatial profile of H₂O, suggesting C₂H₆ was associated with a polar ice phase dominating gas production. On September 6, C₂H₆ was only moderately depleted and was spatially distinct from H₂O, 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.