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

Abstract

Abstract 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.