Abstract
Abstract The 2018 perihelion passage of comet 46P/Wirtanen afforded an opportunity to measure the abundances and spatial distributions of coma volatiles in a Jupiter-family comet with exceptional spatial resolution for several weeks surrounding its closest approach to Earth (Δmin ∼0.078 au on UT December 16). We conducted near-infrared spectroscopic observations of 46P/Wirtanen using iSHELL at the NASA Infrared Telescope Facility on UT 2018 December 18 in direct coordination with observations using the newly upgraded NIRSPEC-2 instrument at the W. M. Keck Observatory, and securely detected fluorescent emission from CH3OH, C2H6, and H2O. This coordinated campaign utilizing the two premier near-infrared facilities in the northern hemisphere enabled us to sample distinct projections of the coma into the plane of the sky simultaneously, and provided an unprecedented view into the inner coma of 46P/Wirtanen near closest approach. We report rotational temperatures, production rates, and abundance ratios (i.e., mixing ratios) for all sampled species and compare our iSHELL results to simultaneous (or near-simultaneous) measurements taken with NIRSPEC-2. We demonstrate the extraordinary synergy of coordinated measurements using iSHELL and NIRSPEC-2, and advocate for future cometary studies that jointly leverage the capabilities of these two facilities.
Highlights
The study of comets affords a unique window into the birth, infancy, and subsequent evolution of the solar system
Our results indicate that Wirtanen was enriched in both C2H6 and CH3OH compared with their respective mean abundances in Jupiter-family comet (JFC) characterized to date at near-IR wavelengths (Dello Russo et al 2016a)
The Trot measured for H2O and C2H6 are in formal agreement, and the measured iSHELL CH3OH Trot is consistent with all of these within 1σ uncertainty
Summary
The study of comets affords a unique window into the birth, infancy, and subsequent evolution of the solar system. Long-slit, high-resolution nearinfrared spectroscopy samples the chemistry of the coma within a few thousand kilometers of the nucleus, providing information in two dimensions, spectral and spatial With their wide spectral grasp, modern state-of-the-art near-infrared echelle spectrometers, such as iSHELL at the NASA Infrared Telescope Facility (IRTF) and NIRSPEC-2 at the W. M. Keck Observatory, provide compositional information by sampling multiple strong transitions of targeted coma molecules, and spatial information via measurement of molecular column densities along their slits (of length 15′′ for iSHELL and 24′′ for NIRSPEC-2). Keck Observatory, provide compositional information by sampling multiple strong transitions of targeted coma molecules, and spatial information via measurement of molecular column densities along their slits (of length 15′′ for iSHELL and 24′′ for NIRSPEC-2) These measurements provide abundances of trace species (e.g., ethane (C2H6), methanol (CH3OH), hydrogen cyanide (HCN), ammonia (NH3)) relative to water (H2O), the dominant volatile in the comae of most measured comets, and reveal how volatiles (ices) are associated or segregated in the nucleus by searching.
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