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1. Volatile Abundances, Extended Coma Sources, and Nucleus Ice Associations in Comet C/2014 Q2 (Lovejoy)

   https://doi.org/10.3847/PSJ/ac323c  

   Dello Russo, Neil; Vervack, Ronald J.; Kawakita, Hideyo; Bonev, Boncho P.; DiSanti, Michael A.; Gibb, Erika L.; McKay, Adam J.; Cochran, Anita L.; Weaver, Harold A.; Biver, Nicolas; et al (January 2022, The Planetary Science Journal)

   Abstract High-resolution infrared spectra of comet C/2014 Q2 Lovejoy were acquired with NIRSPEC at the W. M. Keck Observatory on two post-perihelion dates (UT 2015 February 2 and 3). H 2 O was measured simultaneously with CO, CH 3 OH, H 2 CO, CH 4 , C 2 H 6 , C 2 H 4 , C 2 H 2 , HCN, and NH 3 on both dates, and rotational temperatures, production rates, relative abundances, H 2 O ortho-to-para ratios, and spatial distributions in the coma were determined. The first detection of C 2 H 4 in a comet from ground-based observations is reported. Abundances relative to H 2 O for all species were found to be in the typical range compared with values for other comets in the overall population to date. There is evidence of variability in rotational temperatures and production rates on timescales that are small compared with the rotational period of the comet. Spatial distributions of volatiles in the coma suggest complex outgassing behavior. CH 3 OH, HCN, C 2 H 6 , and CH 4 spatial distributions in the coma are consistent with direct release from associated ices in the nucleus and are peaked in a more sunward direction compared with co-measured dust. H 2 O spatial profiles are clearly distinct from these other four species, likely due to a sizable coma contribution from icy grain sublimation. Spatial distributions for C 2 H 2 , H 2 CO, and NH 3 suggest substantial contributions from extended coma sources, providing further evidence for distinct origins and associations for these species in comets. CO shows a different spatial distribution compared with other volatiles, consistent with jet activity from discrete nucleus ice sources. 

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2. 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)

   Abstract 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, CH 3 OH, CH 3 CN, H 2 CO, 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, CH 3 OH, and CH 3 CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH 3 OH originating from sublimation of icy grains in the near-nucleus coma (at a scale length L p = 36 ± 7 km). On the other hand, H 2 CO, CS, and HNC originate primarily from distributed coma sources (with L p values in the range 550–16,000 km), the identities of which remain to be established. The HCN, CH 3 OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H 2 CO, CH 3 CN, and CS abundances are relatively low. 

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3. Molecular Outgassing in Centaur 29P/Schwassmann–Wachmann 1 during Its Exceptional 2021 Outburst: Coordinated Multiwavelength Observations Using nFLASH at APEX and iSHELL at the NASA-IRTF

   https://doi.org/10.3847/PSJ/ace1e9  

   Roth, Nathan X; Milam, Stefanie N; DiSanti, Michael A; Villanueva, Geronimo L; Faggi, Sara; Bonev, Boncho P; Cordiner, Martin A; Remijan, Anthony J; Bockelée-Morvan, Dominique; Biver, Nicolas; et al (September 2023, The Planetary Science Journal)

   Abstract The extraordinary 2021 September–October outburst of Centaur 29P/Schwassmann–Wachmann 1 afforded an opportunity to test the composition of primitive Kuiper disk material at high sensitivity. We conducted nearly simultaneous multiwavelength spectroscopic observations of 29P/Schwassmann–Wachmann 1 using iSHELL at the NASA Infrared Telescope Facility (IRTF) and nFLASH at the Atacama Pathfinder EXperiment (APEX) on 2021 October 6, with follow-up APEX/nFLASH observations on 2021 October 7 and 2022 April 3. This coordinated campaign between near-infrared and radio wavelengths enabled us to sample molecular emission from a wealth of coma molecules and to perform measurements that cannot be accomplished at either wavelength alone. We securely detected CO emission on all dates with both facilities, including velocity-resolved spectra of the CO (J= 2–1) transition with APEX/nFLASH and multiple CO (v= 1–0) rovibrational transitions with IRTF/iSHELL. We report rotational temperatures, coma kinematics, and production rates for CO and stringent (3σ) upper limits on abundance ratios relative to CO for CH₄, C₂H₆, CH₃OH, H₂CO, CS, and OCS. Our upper limits for CS/CO and OCS/CO represent their first values in the literature for this Centaur. Upper limits for CH₄, C₂H₆, CH₃OH, and H₂CO are the most stringent reported to date, and are most similar to values found in ultra CO-rich Oort cloud comet C/2016 R2 (PanSTARRS), which may have implications for how ices are preserved in cometary nuclei. We demonstrate the superb synergy of coordinated radio and near-infrared measurements, and advocate for future small-body studies that jointly leverage the capabilities of each wavelength. 

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4. Observations of comet C/2020 F3 (NEOWISE) with IRAM telescopes

   https://doi.org/10.1051/0004-6361/202244970  

   Biver, N.; Boissier, J.; Bockelée-Morvan, D.; Crovisier, J.; Cottin, H.; Cordiner, M. A.; Roth, N. X.; Moreno, R. (December 2022, Astronomy & Astrophysics)

   We present the results of millimetre-wave spectroscopic and continuum observations of the comet C/2020 F3 (NEOWISE) undertaken with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m and the NOrthern Extended Millimeter Array (NOEMA) telescopes on 22, 25–27 July, and 7 August 2020. Production rates of HCN, HNC, CH 3 OH CS, H 2 CO, CH 3 CN, H 2 S, and CO were determined with upper limits on six other species. The comet shows abundances within the range observed for other comets. The CO abundance is low (3.2% relative to water), while H 2 S is relatively abundant (1.1% relative to water). The H 2 CO abundance shows a steep variation with heliocentric distance, possibly related to a distributed production from the dust or macro-molecular source. The CH 3 OH and H 2 S production rates show a slower decrease post-perihelion than water. There was no detection of the nucleus point source contribution based on the interferometric map of the continuum (implying a size of r < 4.7 km), but this yielded an estimate of the dust production rate, leading to a relatively low dust-to-gas ratio of 0.7 ± 0.3 on 22.4 July 2020. 

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5. 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)

   Abstract 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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