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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)

   Abstract 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. Chemical composition of comets C/2021 A1 (Leonard) and C/2022 E3 (ZTF) from radio spectroscopy and the abundance of HCOOH and HNCO in comets

   https://doi.org/10.1051/0004-6361/202450921  

   Biver, N; Bockelée-Morvan, D; Handzlik, B; Sandqvist, Aa; Boissier, J; Drozdovskaya, M N; Moreno, R; Crovisier, J; Lis, D C; Cordiner, M; et al (October 2024, Astronomy & Astrophysics)

   We present the results of a molecular survey of long period comets C/2021 A1 (Leonard) and C/2022 E3 (ZTF). Comet C/2021 A1 was observed with the Institut de radioastronomie millimétrique (IRAM) 30-m radio telescope in November-December 2021 before perihelion (heliocentric distance 1.22 to 0.76 au) when it was closest to the Earth (≈0.24 au). We observed C/2022 E3 in January-February 2023 with theOdin1-m space telescope and IRAM 30-m, shortly after its perihelion at 1.11 au from the Sun, and when it was closest to the Earth (≈0.30 au). Snapshots were obtained during 12–16 November 2021 period for comet C/2021 A1. Spectral surveys were undertaken over the 8–13 December 2021 period for comet C/2021 A1 (8 GHz bandwidth at 3 mm, 16 GHz at 2 mm, and 61 GHz in the 1 mm window) and over the 3–7 February 2023 period for comet C/2022 E3 (25 GHz at 2 mm and 61 GHz at 1 mm). We report detections of 14 molecular species (HCN, HNC, CH₃CN, HNCO, NH₂CHO, CH₃OH, H₂CO, HCOOH, CH₃CHO, H₂S, CS, OCS, C₂H₅OH and aGg’-(CH₂OH)₂) in both comets. In addition, HC₃N, and CH₂OHCHO were marginally detected in C/2021 A1, and CO and H₂O (withOdin) were detected in C/2022 E3. The spatial distribution of several species (HCN, HNC, CS, H₂CO, HNCO, HCOOH, NH₂CHO, and CH₃CHO) is investigated. Significant upper limits on the abundances of other molecules and isotopic ratios are also presented. The activity of comet C/2021 A1 did not vary significantly between 13 November and 13 December 2021, when observations stopped, just before it started to exhibit major outbursts seen in the visible and from observations of the OH radical. Short-term variability in the outgassing of comet C/2022 E3 of the order of ±20% is present and possibly linked to its 8h rotation period. Both comets exhibit rather low abundances relative to water for volatile species such as CO (<2%) and H₂S (0.15%). Methanol is also rather depleted in comet C/2021 A1 (0.9%). Following their revised photo-destruction rates, HNCO and HCOOH abundances in comets observed at millimetre wavelengths have been reevaluated. Both molecules are relatively enriched in these two comets (~0.2% relative to water). Since the combined abundance of these two acids (0.1–1%) is close to that of ammonia in comets, we cannot exclude that these species could be produced by the dissociation of ammonium formate and ammonium cyanate if present in comets. 

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3. 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, 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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4. Onset of CN Emission in 3I/ATLAS: Evidence for Strong Carbon-chain Depletion

   https://doi.org/10.3847/2041-8213/ae1232  

   Salazar_Manzano, Luis E; Lin_林, Hsing_Wen 省文; Taylor, Aster G; Seligman, Darryl Z; Adams, Fred C; Gerdes, David W; Ruch, Thomas; Frincke, Tessa T; Napier, Kevin J (October 2025, The Astrophysical Journal Letters)

   Abstract Interstellar objects provide a direct window into the environmental conditions around stars other than the Sun. The recent discovery of 3I/ATLAS, a new interstellar comet, offers a unique opportunity to investigate the physical and chemical properties of interstellar objects and to compare them with those of comets in our own solar system. In this Letter we present the results of a 10 night spectroscopic and photometric monitoring campaign with the 2.4 m Hiltner and 1.3 m McGraw–Hill telescopes at the MDM Observatory. The campaign was conducted between August 8 and 17 while 3I/ATLAS was inbound at heliocentric distances of 3.2–2.9 au. Our observations captured the onset of optical gas activity. Nightly spectra reveal a weak CN emission feature in the coma of 3I/ATLAS, absent during the first nights but steadily strengthening thereafter. We measure a CN production rate ofQ(CN) ∼ 6 × 10²⁴s⁻¹, toward the lower end of activity observed in solar system comets. Simultaneous photometry also indicates a small but measurable increase in the coma’s radial profile and increasingr-bandAfρwith values in the order of ∼300 cm. We derived a gas-to-dust production ratio of $\log Q\left(\mathrm{CN}\right)/Af\rho \sim 22.4$. Our upper limit on the C₂-to-CN ratio ( $\log Q\left({\mathrm{C}}_2\right)/Q\left(\mathrm{CN}\right)\lesssim -0.8$) indicates that 3I/ATLAS is a strongly carbon-chain-depleted comet. Further observations of 3I/ATLAS are required to verify the apparent carbon-chain depletion and to explore whether such composition represents a recurring trait of the interstellar comet population. 

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5. Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC ₃ N, CH ₃ CN, and c-C ₃ H ₂

   https://doi.org/10.3847/1538-4365/ac1441  

   Ilee, John D.; Walsh, Catherine; Booth, Alice S.; Aikawa, Yuri; Andrews, Sean M.; Bae, Jaehan; Bergin, Edwin A.; Bergner, Jennifer B.; Bosman, Arthur D.; Cataldi, Gianni; et al (November 2021, The Astrophysical Journal Supplement Series)

   Abstract The precursors to larger, biologically relevant molecules are detected throughout interstellar space, but determining the presence and properties of these molecules during planet formation requires observations of protoplanetary disks at high angular resolution and sensitivity. Here, we present 0.″3 observations of HC 3 N, CH 3 CN, and c -C 3 H 2 in five protoplanetary disks observed as part of the Molecules with ALMA at Planet-forming Scales (MAPS) Large Program. We robustly detect all molecules in four of the disks (GM Aur, AS 209, HD 163296, and MWC 480) with tentative detections of c -C 3 H 2 and CH 3 CN in IM Lup. We observe a range of morphologies—central peaks, single or double rings—with no clear correlation in morphology between molecule or disk. Emission is generally compact and on scales comparable with the millimeter dust continuum. We perform both disk-integrated and radially resolved rotational diagram analysis to derive column densities and rotational temperatures. The latter reveals 5–10 times more column density in the inner 50–100 au of the disks when compared with the disk-integrated analysis. We demonstrate that CH 3 CN originates from lower relative heights in the disks when compared with HC 3 N, in some cases directly tracing the disk midplane. Finally, we find good agreement between the ratio of small to large nitriles in the outer disks and comets. Our results indicate that the protoplanetary disks studied here are host to significant reservoirs of large organic molecules, and that this planet- and comet-building material can be chemically similar to that in our own solar system. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement. 

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