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1. Comprehensive Study of the Chemical Composition and Spatial Outgassing Behavior of Hyperactive Comet 46P/Wirtanen Using Near-IR Spectroscopy during its Historic 2018 Apparition

   https://doi.org/10.3847/1538-3881/acc074  

   Khan, Younas ; Gibb, Erika L. ; Roth, Nathan X. ; DiSanti, Michael A. ; Dello Russo, Neil ; Bonev, Boncho P. ; Ejeta, Chemeda T. ; Saki, Mohammad ; Vervack, Jr., Ronald J. ; McKay, Adam J. ; et al ( May 2023 , The Astronomical Journal)

   We present a comprehensive analysis of the chemical composition of the Jupiter-family comet and potential spacecraft target 46P/Wirtanen, in the near-IR wavelength range. We used iSHELL at the NASA Infrared Telescope Facility to observe the comet on 11 pre-, near-, and postperihelion dates in 2018 December and 2019 January and February during its historic apparition. We report rotational temperatures, production rates, and mixing ratios with respect to H₂O and C₂H₆or 3σupper limits of the primary volatiles H₂O, HCN, CH₄, C₂H₆, CH₃OH, H₂CO, NH₃, CO, C₂H₂, and HC₃N. We also discuss the spatial outgassing of the primary volatiles, to understand their sources and the spatial associations between them. The spatial profiles of H₂O in 46P/Wirtanen suggest the presence of extended H₂O outgassing sources in the coma, similar to the EPOXI target comet 103P/Hartley 2. 46P/Wirtanen is among the few known hyperactive comets, and we note that its composition and outgassing behavior are similar to those of other hyperactive comets in many ways. We note that the analyzed parent volatiles showed different variations (relative mixing ratios) during the apparition. We compared the chemical composition of 46P/Wirtanen with the mean abundances in Jupiter-family comets and the comet population as measured with ground-based near-IR facilities to date. The molecular abundances in 46P/Wirtanen suggest that although they were changing, the variations were small compared to the range in the comet population, with CH₃OH showing notably more variation as compared to the other molecules.

    

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2. The Return of the Rosetta Target: Keck Near-infrared Observations of Comet 67P/Churyumov–Gerasimenko in 2021

   https://doi.org/10.3847/1538-3881/acee59  

   Bonev, Boncho P. ; Dello Russo, Neil ; Kawakita, Hideyo ; Vervack Jr., Ronald J. ; DiSanti, Michael A. ; Shinnaka, Yoshiharu ; Ootsubo, Takafumi ; Gibb, Erika L. ; Combi, Michael R. ; Altwegg, Kathrin ; et al ( November 2023 , The Astronomical Journal)

   High-resolution near-infrared ground-based spectroscopic observations of comet 67P/Churyumov–Gerasimenko near its maximum activity in 2021 were conducted from the W. M. Keck Observatory, using the facility spectrograph NIRSPEC. 67P is the best-studied comet to date because of the unprecedented detail and insights provided by the Rosetta mission during 2014–2016. Because 67P is the only comet where the detailed abundances of many coma volatiles were measured in situ, determining its composition from the ground provides a unique opportunity to interpret Rosetta results within the context of the large database of ground-based compositional measurements of comets. However, previous apparitions, including in 2015, have been unfavorable for in-depth ground-based studies of parent volatiles in 67P. The 2021 apparition of 67P was thus the first-ever opportunity for such observations. We report gas spatial distributions, rotational temperatures, production rates, and relative abundances (or stringent upper limits) among seven volatile species: C₂H₂, C₂H₆, HCN, NH₃, CH₃OH, H₂CO, and H₂O. The measured abundances of trace species relative to water reveal near average or below average values compared to previous comets studied at infrared wavelengths. Both gas rotational temperatures and the spatial distributions of H₂O, C₂H₆, and HCN measured with Keck-NIRSPEC in 2021 are consistent with the outgassing patterns revealed by Rosetta in 2015 at very similar heliocentric distance  (post-perihelion). These results can be integrated with both Rosetta mission findings and ground-based cometary studies of the overall comet population, for which we encourage a wide-scale collaboration across measurement techniques.

    

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3. Coma Abundances of Volatiles at Small Heliocentric Distances: Compositional Measurements of Long-period Comet C/2020 S3 (Erasmus)

   https://doi.org/10.3847/1538-3881/ad0e02  

   Ejeta, Chemeda ; Gibb, Erika ; Roth, Nathan ; DiSanti, Michael A. ; Dello Russo, Neil ; Saki, Mohammad ; McKay, Adam J. ; Kawakita, Hideyo ; Khan, Younas ; Bonev, Boncho P. ; et al ( December 2023 , The Astronomical Journal)

   We report production rates of H₂O and nine trace molecules (C₂H₆, CH₄, H₂CO, CH₃OH, HCN, NH₃, C₂H₂, OCS, and CO) in long-period comet C/2020 S3 (Erasmus) using the high-resolution, cross-dispersed infrared spectrograph (iSHELL) at the NASA Infrared Telescope Facility, on two pre-perihelion dates at heliocentric distancesR_h= 0.49 and 0.52 au. Our molecular abundances with respect to simultaneously or contemporaneously measured H₂O indicate that S3 is depleted in CH₃OH compared to its mean abundance relative to H₂O among the overall comet population (Oort Cloud comets and Jupiter-family comets combined), whereas the eight other measured species have near-average abundances relative to H₂O. In addition, compared to comets observed atR_h< 0.80 au at near-infrared wavelengths, S3 showed enhancement in the abundances of volatile species H₂CO, NH₃, and C₂H₂, indicating possible additional (distributed) sources in the coma for these volatile species. The spatial profiles of volatile species in S3 in different instrumental settings are dramatically different, which might suggest temporal variability in comet outgassing behavior between the nonsimultaneous measurements. The spatial distributions of simultaneously measured volatile species C₂H₆and CH₄are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H₂O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction.

    

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4. Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

   https://doi.org/10.3847/1538-4357/ace0bc  

   Cordiner, M. A. ; Roth, N. X. ; Milam, S. N. ; Villanueva, G. L. ; Bockelée-Morvan, D. ; Remijan, A. J. ; Charnley, S. B. ; Biver, N. ; Lis, D. C. ; Qi, C. ; et al ( August 2023 , The Astrophysical Journal)

   Gas-phase molecules in cometary atmospheres (comae) originate primarily from (1) outgassing by the nucleus, (2) sublimation of icy grains in the near-nucleus coma, and (3) coma (photo)chemical processes. However, the majority of cometary gases observed at radio wavelengths have yet to be mapped, so their production/release mechanisms remain uncertain. Here we present observations of six molecular species toward comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array during the comet’s unusually close (∼0.1 au) approach to Earth in 2018 December. Interferometric maps of HCN, CH₃OH, CH₃CN, H₂CO, CS, and HNC were obtained at an unprecedented sky-projected spatial resolution of up to 25 km, enabling the nucleus and coma sources of these molecules to be accurately quantified. The HCN, CH₃OH, and CH₃CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH₃OH originating from sublimation of icy grains in the near-nucleus coma (at a scale lengthL_p= 36 ± 7 km). On the other hand, H₂CO, CS, and HNC originate primarily from distributed coma sources (withL_pvalues in the range 550–16,000 km), the identities of which remain to be established. The HCN, CH₃OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H₂CO, CH₃CN, and CS abundances are relatively low.

    

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5. A SUBLIME 3D Model for Cometary Coma Emission: The Hypervolatile-rich Comet C/2016 R2 (PanSTARRS)

   https://doi.org/10.3847/1538-4357/ac5893  

   Cordiner, M. A. ; Coulson, I. M. ; Garcia-Berrios, E. ; Qi, C. ; Lique, F. ; Zołtowski, M. ; de Val-Borro, M. ; Kuan, Y. -J. ; Ip, W. -H. ; Mairs, S. ; et al ( April 2022 , The Astrophysical Journal)

   The coma of comet C/2016 R2 (PanSTARRS) is one of the most chemically peculiar ever observed, in particular due to its extremely high CO/H₂O and${\mathrm{N}}_2^{+}$/H₂O ratios, and unusual trace volatile abundances. However, the complex shape of its CO emission lines, as well as uncertainties in the coma structure and excitation, has lead to ambiguities in the total CO production rate. We performed high-resolution, spatially, spectrally, and temporally resolved CO observations using the James Clerk Maxwell Telescope and Submillimeter Array to elucidate the outgassing behavior of C/2016 R2. Results are analyzed using a new, time-dependent, three-dimensional radiative transfer code (SUBlimating gases in LIME; SUBLIME, based on the open-source version of the LIne Modeling Engine), incorporating for the first time, accurate state-to-state collisional rate coefficients for the CO–CO system. The total CO production rate was found to be in the range of (3.8 − 7.6) × 10²⁸s⁻¹between 2018 January 13 and February 1 (atr_H= 2.8–2.9 au), with a mean value of (5.3 ± 0.6) × 10²⁸s⁻¹. The emission is concentrated in a near-sunward jet, with a half-opening angle of ∼62° and an outflow velocity of 0.51 ± 0.01 km s⁻¹, compared to 0.25 ± 0.01 km s⁻¹in the ambient (and nightside) coma. Evidence was also found for an extended source of CO emission, possibly due to icy grain sublimation around 1.2 × 10⁵km from the nucleus. Based on the coma molecular abundances, we propose that the nucleus ices of C/2016 R2 can be divided into a rapidly sublimating apolar phase, rich in CO, CO₂, N₂, and CH₃OH, and a predominantly frozen (or less abundant), polar phase containing more H₂O, CH₄, H₂CO, and HCN.

    

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