Showing 1–50 of 115 results for author: Charnley, S B

A JWST Study of Stardust. I. Infrared Spectroscopy of Comet 81P/Wild 2 and Overall Composition

Authors: Nathan X. Roth, Stefanie N. Milam, Diane H. Wooden, Charles E. Woodward, Dominique Bockelee-Morvan, Michael S. P. Kelley, Steven B. Charnley, Simon J. Clemett, Martin A. Cordiner, Perry A. Gerakines, David E. Harker, Els Peeters, Ella Sciamma-O'Brien, Geronimo L. Villanueva

Abstract: We report observations of comet 81P/Wild 2, target of the Stardust sample return mission, on UT 2023 March 20 and 24 at a heliocentric distance ($r_H$) of 1.85 au using the NIRSpec and MIRI integral field unit spectrographs on board the James Webb Space Telescope (JWST). This study is the first compositional comparison between JWST remote-sensing spectroscopy of a solar system object against terre… ▽ More We report observations of comet 81P/Wild 2, target of the Stardust sample return mission, on UT 2023 March 20 and 24 at a heliocentric distance ($r_H$) of 1.85 au using the NIRSpec and MIRI integral field unit spectrographs on board the James Webb Space Telescope (JWST). This study is the first compositional comparison between JWST remote-sensing spectroscopy of a solar system object against terrestrial analysis of its returned samples. We securely detected molecular emission from H$_2$O, CH$_4$, C$_2$H$_6$, CH$_3$OH, CO, CO$_2$, $^{13}$CO$_2$, OCS, HCN, and CN and find molecular abundances consistent within $2σ$ with those reported during previous perihelion passages. The water ortho-to-para ratio was $2.76\pm0.05$, and the $^{12}$CO$_2$/$^{13}$CO$_2$ ratio was $85\pm4$. Thermal emission from the nucleus and dust was detected and modeled, providing an effective nucleus radius of $1.77\pm0.04$ km and a dust composition (relative mass fraction of the submicron grains) of $\sim36\%$ amorphous carbon, $\sim25\%$ amorphous Mg:Fe olivine, $\sim23\%$ Mg-rich crystalline olivine, and $\sim15\%$ amorphous Mg:Fe pyroxene. The crystalline mass fraction of the sub-micron grains in the coma was $0.362\pm0.003$. Comparison of the JWST-derived thermal model against the fine-grained materials in Stardust returned samples demonstrates complementarity between the missions, with each most sensitive to a different population of the coma dust grains. △ Less

Submitted 3 August, 2026; originally announced August 2026.

Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. III. SiO and SiS abundances

Authors: R. Unnikrishnan, E. De Beck, L. -Å. Nyman, H. Olofsson, W. H. T. Vlemmings, M. Maercker, M. Van de Sande, T. J. Millar, T. Danilovich, M. Andriantsaralaza, S. B. Charnley, M. G. Rawlings

Abstract: The present understanding of C-rich AGB chemistry largely relies on observations of the archetypal carbon star IRC+10216. Current molecular abundance estimates for carbon stars are based either on single-dish spectra sampling a range of excitation conditions, or on interferometric mapping of a few lines. We aim to estimate the circumstellar abundances of SiO, SiS, and their most abundant isotopolo… ▽ More The present understanding of C-rich AGB chemistry largely relies on observations of the archetypal carbon star IRC+10216. Current molecular abundance estimates for carbon stars are based either on single-dish spectra sampling a range of excitation conditions, or on interferometric mapping of a few lines. We aim to estimate the circumstellar abundances of SiO, SiS, and their most abundant isotopologues ($^{29}$SiO, $^{30}$SiO, $^{29}$SiS, $^{30}$SiS, and Si$^{34}$S) for a sample of five carbon stars. We derived molecular abundances using detailed 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer (RT) modelling, constrained by both morphological and excitation information obtained from spatially resolved ALMA maps and single-dish observations. We further compared the derived abundances to chemical modelling results. We obtain good fits to the SiO and SiS line profiles, and derived well-constrained abundance profiles and reliable isotopic ratios for all sources except AFGL 3068. While the SiS peak abundances are very similar across the sample (2.0$\times$10$^{-6}-4.7\times$10$^{-6}$), we find that the SiO peak abundances of the rest of the stars are a factor of $\sim$5 larger than that of IRC+10216. The $e$-folding radii ($R_\mathrm{e}$) are in the range 1.3$\times$10$^{16}-7.0\times$10$^{16}$ cm for SiO and 6.0$\times$10$^{15}-1.0\times$10$^{17}$ cm for SiS. The $R_\mathrm{e}$ increases with gas density for both SiO and SiS. Chemical models reproduce the derived SiO abundance profiles well, while over-predicting the SiS $R_\mathrm{e}$ values. Our models highlight the necessity of having spatially resolved observations across a broad range of excitation conditions, while also making evident the limitations inherent in 1D RT modelling using simplified (circum)stellar models. We find that the currently assumed SiS photodissociation rate in chemical models is underestimated. △ Less

Submitted 14 May, 2026; originally announced May 2026.

Comments: 14 pages, with 10 figures and 3 tables in the main text. Two appendices contain additional figures and tables. Total 22 pages. Accepted for publication in Astronomy & Astrophysics

Monitoring Volatile Evolution in Disrupting Comet D/2021 A1 (Leonard) with NOEMA and APEX

Authors: Timothy N. Proudkii, Nathan X. Roth, Jérémie Boissier, Dominique Bockelée-Morvan, Nicolas Biver, Steven B. Charnley, Stefanie N. Milam, Martin A. Cordiner, Michael A. DiSanti, Boncho P. Bonev, Neil Dello Russo

Abstract: We report a pre-perihelion survey of volatile emissions from comet D/2021 A1 (Leonard) with the Northern Extended Millimeter Array (NOEMA; UT 2021 Nov. 5, 21, and Dec. 1) and the Atacama Pathfinder Experiment (APEX; UT 2021 Dec. 9-10), spanning heliocentric distances ($r_H$) from 1.3 to 0.80 au. We securely detected HCN and CS and place 3$σ$ upper limits on CH$_3$OH, H$_2$CO, and CO abundances. Li… ▽ More We report a pre-perihelion survey of volatile emissions from comet D/2021 A1 (Leonard) with the Northern Extended Millimeter Array (NOEMA; UT 2021 Nov. 5, 21, and Dec. 1) and the Atacama Pathfinder Experiment (APEX; UT 2021 Dec. 9-10), spanning heliocentric distances ($r_H$) from 1.3 to 0.80 au. We securely detected HCN and CS and place 3$σ$ upper limits on CH$_3$OH, H$_2$CO, and CO abundances. Line kinematics and NOEMA spatial constraints indicate that HCN was released at or near the nucleus (parent scale length $<300$ km), while CS showed higher gas expansion velocities and mixing ratios that increased with decreasing $r_H$ $-$ consistent with production from a distributed source. Across our campaign, CS mixing ratios relative to H$_2$O increased by a factor of $\sim$5, from $0.02 \pm 0.01\%$ at $r_H$ = 1.3 au to $0.10\pm0.02\%$ by $r_H$ = 0.80 au. HCN mixing ratios in our data rose modestly, from $0.04 \pm 0.02\%$ at $r_H$ = 1.3 au to $0.07 \pm 0.02\%$ by $r_H$ = 0.81 au. However, contemporaneous measurements from other facilities placed HCN consistently at a higher absolute level ($\sim\!0.08\%$) with additional variability. Once cross-facility measurements were included, the HCN abundance showed no statistically robust monotonic dependence on $r_H$. Variability in both species during the mid-December outbursts and fragmentation suggests that D/2021 A1's volatile evolution reflected not only solar insolation but also disruption processes, underscoring the value of multi-epoch, multi-instrument monitoring to capture rapid, species-dependent changes. △ Less

Submitted 13 April, 2026; v1 submitted 2 April, 2026; originally announced April 2026.

Comments: 21 pages, 7 figures, accepted to PSJ

Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS

Authors: Nathan X. Roth, Martin A. Cordiner, Stefanie N. Milam, Geronimo L. Villanueva, Steven B. Charnley, Nicolas Biver, Dominique Bockelee-Morvan, Dennis Bodewits, Steven J. Bromley, Jacques Crovisier, Maria N. Drozdovskaya, Sara Faggi, Davide Farnocchia, Kenji Furuya, Michael S. P. Kelley, Marco Micheli, John W. Noonan, Cyrielle Opitom, Megan E. Schwamb, Cristina A. Thomas

Abstract: We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance ($r_H$) of $2.37-2.41$ au. These measurements of CO, CO$_2$, H$_2$O, CH$_3$OH, and CH$_4$ sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entere… ▽ More We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance ($r_H$) of $2.37-2.41$ au. These measurements of CO, CO$_2$, H$_2$O, CH$_3$OH, and CH$_4$ sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H$_2$O ice line -- the region between $r_H$ = $2-3$ au where the temperature becomes too low for H$_2$O to vigorously sublime and CO and CO$_2$ begin to control the overall activity. CO was the most abundant molecule, followed by H$_2$O and CO$_2$, whose molecular abundances with respect to CO were $(40.5\pm3.1)\%$ and ($41.6\pm0.3)\%$, respectively. This work presents spatial-spectral maps of column density and rotational temperature as a function of distance from the nucleus for all detected species. The spatial distributions of both quantities were highly anisotropic for the apolar species in the coma of 3I/ATLAS, yet were more nearly symmetric for the polar molecules. These results demonstrate how volatiles were segregated in the nucleus ices of 3I/ATLAS and reveal heating and cooling mechanisms in its coma. Derived maps of the ortho-to-para ratio (OPR) for H$_2$O were flat with increasing distance from the nucleus and consistent with a coma-averaged value $\mathrm{OPR}=2.7\pm0.2$, slightly less than the expected equilibrium value of three. △ Less

Submitted 27 May, 2026; v1 submitted 20 March, 2026; originally announced March 2026.

Water D/H in 3I/ATLAS as a Probe of Formation Conditions in Another Planetary System

Authors: Luis E. Salazar Manzano, Teresa Paneque-Carreño, Martin A. Cordiner, Edwin A. Bergin, Hsing Wen Lin, Dariusz C. Lis, David W. Gerdes, Jennifer B. Bergner, Nicolas Biver, Dominique Bockelée-Morvan, Dennis Bodewits, Steven B. Charnley, Jacques Crovisier, Davide Farnocchia, Viviana V. Guzmán, Stefanie N. Milam, John W. Noonan, Anthony J. Remijan, Nathan X. Roth, John J. Tobin

Abstract: Water reservoirs in the Solar System exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth. Gas-phase and ice-grain deuterium enrichments occur through chemical processes that operate at low temperatures ($<$~30~K) pointing towards an origin in the prestellar molecular cloud or in the outer parts of the protoplanetary disk. However, not all stars a… ▽ More Water reservoirs in the Solar System exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth. Gas-phase and ice-grain deuterium enrichments occur through chemical processes that operate at low temperatures ($<$~30~K) pointing towards an origin in the prestellar molecular cloud or in the outer parts of the protoplanetary disk. However, not all stars are born in environments similar to our Sun, nor do their subsequent evolutionary histories follow the same path. These environmental differences can be traced by the water deuterium-to-hydrogen (D/H) ratio. Here we use ALMA observations of the interstellar comet 3I/ATLAS to constrain the water D/H ratio in extrasolar cometary material. With a water D/H value of [D/H]$_{\mathrm{H_2O}} > 6.6\times10^{-3}$, 3I/ATLAS shows a deuterium enrichment exceeding Earth's ocean value by more than a factor of $\gtrsim40$ and typical Solar System cometary values by more than a factor of $\gtrsim30$. The elevated deuterium enrichment points to water that formed under colder, less irradiated conditions and from less thermally processed material, consistent with an origin in a planetary system that formed under different physical and chemical conditions than our own. △ Less

Submitted 14 April, 2026; v1 submitted 6 March, 2026; originally announced March 2026.

Comments: Accepted for publication in Nature Astronomy. 3 Figures, 2 Extended Data Tables, 3 Extended Data Figures, 1 Supplementary Table, and 4 Supplementary Figures

CH$_3$OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH$_3$OH/HCN Production Rate Ratio

Authors: Nathan X. Roth, Martin A. Cordiner, Dominique Bockelée-Morvan, Nicolas Biver, Jacques Crovisier, Stefanie N. Milam, Emmanuel Lellouch, Pablo Santos-Sanz, Dariusz C. Lis, Chunhua Qi, K. D. Foster, Jérémie Boissier, Kenji Furuya, Raphael Moreno, Steven B. Charnley, Anthony J. Remijan, Yi-Jehng Kuan, Lillian X. Hart

Abstract: We report the detection of methanol (CH$_3$OH) toward interstellar comet 3I/ATLAS using the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances ($r_H$) of 2.6 -- 1.7 au. The molecules showed out… ▽ More We report the detection of methanol (CH$_3$OH) toward interstellar comet 3I/ATLAS using the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances ($r_H$) of 2.6 -- 1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH$_3$OH was enhanced in that direction. Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH$_3$OH included production from coma sources at $L_p>258$ km at 99% confidence. However low signal-to-noise on long baselines, which sample emission on small spatial scales closest to the nucleus, prevented definitively ruling out CH$_3$OH as purely a parent species. In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH$_3$OH production rate increased sharply from August through October, including an uptick near the inner edge of the H$_2$O sublimation zone at $r_H$ = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH$_3$OH/HCN ratios in 3I/ATLAS of $124^{+30}_{-34}$ and $79^{+11}_{-14}$ on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS). △ Less

Submitted 2 March, 2026; v1 submitted 25 November, 2025; originally announced November 2025.

The Evolution in Coma Molecular Composition of Comet C/2017 K2 (PanSTARRS) Across the H$_2$O Sublimation Zone: ALMA Imaging of an H$_2$O-Dominated Coma

Authors: Nathan X. Roth, Stefanie N. Milam, Martin A. Cordiner, Anthony J. Remijan, Dominique Bockelee-Morvan, Nicolas Biver, Jeremie Boissier, Steven B. Charnley, Charles E. Woodward, Lillian X. Hart, Timothy N. Proudkii

Abstract: We report a survey of molecular emission from cometary volatiles using the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2017 K2 (PanSTARRS) carried out on UT 2022 September 21, 22, and 23 at a heliocentric distance ($r_H$) of 2.1 au. These measurements of HCN, CS, CO, CH$_3$OH, and H$_2$CO (along with continuum emission from dust) sampled molecular chemistry in C/2017 K2 at t… ▽ More We report a survey of molecular emission from cometary volatiles using the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2017 K2 (PanSTARRS) carried out on UT 2022 September 21, 22, and 23 at a heliocentric distance ($r_H$) of 2.1 au. These measurements of HCN, CS, CO, CH$_3$OH, and H$_2$CO (along with continuum emission from dust) sampled molecular chemistry in C/2017 K2 at the inner edge of the H$_2$O sublimation zone, the region from $r_H$ = 2-3 au where H$_2$O begins vigorously subliming and increasingly dominating comet activity, discerning parent from daughter or extended source species. This work presents spectrally integrated flux maps, production rates, and parent scale lengths for each molecule. CH$_3$OH, CO, and HCN were produced within $\sim$250 km of the nucleus, potentially including contributions from sublimation of icy grains. CS was consistent with production from CS$_2$ photolysis, and H$_2$CO required production from extended sources in the coma. An ortho-to-para ratio OPR=$2.9\pm0.4$ for H$_2$CO was derived from simultaneously measured transitions of each spin species. The continuum was extended and spatially resolved, consistent with thermal emission from dust in the coma. Analysis of the continuum visibilities provided an upper limit on the nucleus diameter $d<6.6$ km and coma dust masses of $1.2-2.4\times10^{11}$ kg. △ Less

Submitted 2 March, 2026; v1 submitted 7 November, 2025; originally announced November 2025.

Protostars at Subsolar Metallicity: First Detection of Large Solid-state Complex Organic Molecules in the Large Magellanic Cloud

Authors: Marta Sewiło, Will R. M. Rocha, Martijn van Gelder, Maria Gabriela Navarro, Steven B. Charnley, Miwha Jin, Joana M. Oliveira, Jacco Th. van Loon, Logan Francis, Jennifer Wiseman, Remy Indebetouw, C. -H. Rosie Chen, Roya Hamedani Golshan, Danna Qasim

Abstract: We present the results of James Webb Space Telescope observations of the protostar ST6 in the Large Magellanic Cloud (LMC) with the Medium Resolution Spectrograph of the Mid-Infrared Instrument (4.9-27.9 $μ$m). Characterized by one-third to half-solar metallicity and strong UV radiation fields, the environment of the LMC allows us to study the physics and chemistry of star-forming regions under th… ▽ More We present the results of James Webb Space Telescope observations of the protostar ST6 in the Large Magellanic Cloud (LMC) with the Medium Resolution Spectrograph of the Mid-Infrared Instrument (4.9-27.9 $μ$m). Characterized by one-third to half-solar metallicity and strong UV radiation fields, the environment of the LMC allows us to study the physics and chemistry of star-forming regions under the conditions similar to those at earlier cosmological epochs. We detected five icy complex organic molecules (COMs): methanol (CH$_3$OH), acetaldehyde (CH$_3$CHO), ethanol (CH$_3$CH$_2$OH), methyl formate (HCOOCH$_3$), and acetic acid (CH$_3$COOH). This is the first conclusive detection of CH$_3$COOH ice in an astrophysical context, and CH$_3$CHO, CH$_3$CH$_2$OH, and HCOOCH$_3$ ices are the first secure detections outside the Galaxy and in a low-metallicity environment. We address the presence of glycolaldehyde (HOCH$_2$CHO, a precursor of biomolecules), an isomer of HCOOCH$_3$ and CH$_3$COOH, but its detection is inconclusive. ST6's spectrum is also rich in simple ices: H$_2$O, CO$_2$, CH$_4$, SO$_2$, H$_2$CO, HCOOH, OCN$^{-}$, HCOO$^{-}$, NH$_3$, and NH$_4^{+}$. We obtain the composition and molecular abundances in the icy dust mantles by fitting the spectrum in the 6.8-8.4 $μ$m range with a large sample of laboratory ice spectra using the ENIIGMA fitting tool or the local continuum method. We found differences in the simple and COM ice abundances with respect to H$_2$O ice between ST6 and Galactic protostars that likely reflect differences in metallicity and UV flux. More laboratory ice spectra of COMs are needed to better reconstruct the observed infrared spectra of protostars. △ Less

Submitted 20 October, 2025; originally announced October 2025.

Comments: 28 pages, 16 figures, 2 tables (including appendices); Published in The Astrophysical Journal Letters

Journal ref: The Astrophysical Journal Letters, Volume 992, Issue 2, article id. L30 (2025)

JCMT detection of HCN emission from 3I/ATLAS at 2.1 AU

Authors: Iain M. Coulson, Yi-Jehng Kuan, Steven B. Charnley, Martin A. Cordiner, Yo-Ling Chuang, Yueh-Ning Lee, Min-Kai Lin, Stefanie N. Milam, Bannawit Pimpanuwat, Nathan X. Roth, Michał Żółtowski

Abstract: We report the detection of HCN ($J=3-2$) rotational emission from comet 3I/ATLAS at a heliocentric distance of 2.13 AU with the James Clerk Maxwell Telescope (JCMT). Observations were conducted from 07 August 2025 and 14 September 2025 (UT) using the $^{\prime}\overline U^{\prime}\overline u$ heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at $>6σ$ on 14 Sep 2025 (UT… ▽ More We report the detection of HCN ($J=3-2$) rotational emission from comet 3I/ATLAS at a heliocentric distance of 2.13 AU with the James Clerk Maxwell Telescope (JCMT). Observations were conducted from 07 August 2025 and 14 September 2025 (UT) using the $^{\prime}\overline U^{\prime}\overline u$ heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at $>6σ$ on 14 Sep 2025 (UT) and a production rate of $Q({\rm HCN})=(4.0\pm1.7)\times10^{25}\ {\rm s}^{-1}$ was derived by non-LTE radiative transfer modelling. Preliminary estimates of the HCN/H$_2$O and CN/HCN abundance ratios suggest values similar to Solar System comets. △ Less

Submitted 13 January, 2026; v1 submitted 3 October, 2025; originally announced October 2025.

Journal ref: Mon Not R Astron Soc (2026)

The Molecular Inventory of TMC-1 with GOTHAM Observations

Authors: Ci Xue, Alex N. Byrne, Larry Morgan, Gabi Wenzel, P. Bryan Changala, Zachary T. P. Fried, Ryan A. Loomis, Anthony Remijan, Edwin A. Bergin, Ilsa R. Cooke, David Frayer, Andrew M. Burkhardt, Steven B. Charnley, Martin A. Cordiner, Andrew Lipnicky, Michael C. McCarthy, Brett A. McGuire

Abstract: Spectral line surveys of the Taurus Molecular Cloud-1 (TMC-1) have led to the detection of more than 100 new molecular species, making it the most prolific source of interstellar molecular discoveries. These wide-band, high-sensitivity line surveys have been enabled by advances in telescope and receiver technology, particularly at centimeter and millimeter wavelengths. In this work, we present a s… ▽ More Spectral line surveys of the Taurus Molecular Cloud-1 (TMC-1) have led to the detection of more than 100 new molecular species, making it the most prolific source of interstellar molecular discoveries. These wide-band, high-sensitivity line surveys have been enabled by advances in telescope and receiver technology, particularly at centimeter and millimeter wavelengths. In this work, we present a statistical analysis of the molecular inventory of TMC-1 as probed by the GOTHAM large program survey from 3.9 to 36.4 GHz. To fully unlock the potential of the $\sim$29 GHz spectral bandwidth, we developed an automated pipeline for data reduction and calibration. We applied a Bayesian approach with Markov-Chain Monte Carlo fitting to the calibrated spectra and constrained column densities for 102 molecular species detected in TMC-1, including 75 main isotopic species, 20 carbon-13 substituted species, and seven deuterium-substituted species. This list of the detected gas-phase molecules is populated by unsaturated hydrocarbons, in stark contrast to the oxygen-rich organics found in sublimated ices around protostars. Of note, ten individual aromatic molecules were identified in the GOTHAM observations, contributing 0.011% of the gas-phase carbon budget probed by detected molecules when including CO and 6% when excluding CO. This work provides a reference set of observed gas-phase molecular abundances for interstellar clouds, offering a new benchmark for astrochemical theoretical models. △ Less

Submitted 7 September, 2025; originally announced September 2025.

Comments: Accepted for publication in The Astrophysical Journal Supplement Series. 12 tables, 9 figures, 1 figure set

JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS

Authors: Martin A. Cordiner, Nathaniel X. Roth, Michael S. P. Kelley, Dennis Bodewits, Steven B. Charnley, Maria N. Drozdovskaya, Davide Farnocchia, Marco Micheli, Stefanie N. Milam, Cyrielle Opitom, Megan E. Schwamb, Cristina A. Thomas, Stefano Bagnulo

Abstract: 3I/ATLAS is the third confirmed interstellar object to visit our Solar System, and only the second to display a clear coma. Infrared spectroscopy with the James Webb Space Telescope (JWST) provides the opportunity to measure its coma composition and determine the primary activity drivers. We report the first results from our JWST NIRSpec campaign for 3I/ATLAS, at an inbound heliocentric distance o… ▽ More 3I/ATLAS is the third confirmed interstellar object to visit our Solar System, and only the second to display a clear coma. Infrared spectroscopy with the James Webb Space Telescope (JWST) provides the opportunity to measure its coma composition and determine the primary activity drivers. We report the first results from our JWST NIRSpec campaign for 3I/ATLAS, at an inbound heliocentric distance of $r_H=3.32$ au. The spectral images (spanning 0.6-5.3 $μ$m) reveal a CO2 dominated coma, with enhanced outgassing in the sunward direction, and the presence of H2O, CO, OCS, water ice and dust. The coma CO2/H2O mixing ratio of $7.6\pm0.3$ is among the highest ever observed in a comet, and is 4.5-sigma above the trend as a function of heliocentric distance for long-period and Jupiter-family comets (excluding the outlier C/2016 R2). Our observations are compatible with an intrinsically CO2-rich nucleus, which may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System comets, or that it formed close to the CO2 ice line in its parent protoplanetary disk. A low coma H2O gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, which could suppress the H2O sublimation rate relative to CO2 and CO. △ Less

Submitted 10 September, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: Accepted at ApJ Letters 2025-09-10

A D/H Ratio Consistent with Earth's Water in Halley-type Comet 12P from ALMA HDO Mapping

Authors: M. A. Cordiner, E. L. Gibb, Z. Kisiel, N. X. Roth, N. Biver, D. Bockelée-Morvan, J. Boissier, B. P. Bonev, S. B. Charnley, I. M. Coulson, J. Crovisier, M. N. Drozdovskaya, K. Furuya, M. Jin, Y. -J. Kuan, M. Lippi, D. C. Lis, S. N. Milam, C. Opitom, C. Qi, A. J. Remijan

Abstract: Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. The D/H ratio in particular, helps reveal the relationship between (and heritage of) different H$_2$O reservoirs within the Solar System. Here we present interferometric maps of water (H$_2$O) and semiheavy water (HDO) in the gas-phase coma of a comet… ▽ More Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. The D/H ratio in particular, helps reveal the relationship between (and heritage of) different H$_2$O reservoirs within the Solar System. Here we present interferometric maps of water (H$_2$O) and semiheavy water (HDO) in the gas-phase coma of a comet (Halley-type comet 12P/Pons-Brooks), obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). The maps are consistent with outgassing of both H$_2$O and HDO directly from the nucleus, and imply a coma D/H ratio (for water) of $(1.71 \pm 0.44)\times10^{-4}$. This is at the lower end of the range of previously-observed values in comets, and is consistent with D/H in Earth's ocean water. Our results suggest a possible common heritage between a component of the Oort cloud's water ice reservoir, and the water that was delivered to the young Earth during the early history of the Solar System. △ Less

Submitted 7 August, 2025; originally announced August 2025.

Comments: Published in Nature Astronomy, 8 August 2025

ALMA Observations of Molecular Complexity in the Large Magellanic Cloud: Probing the Star-forming Region N160

Authors: Amanda Broadmeadow, Marta Sewiło, Lee Mundy, Roya Hamedani Golshan, Kazuki Tokuda, Thomas Möller, Remy Indebetouw, Joana M. Oliveira, Steven B. Charnley, Jennifer Wiseman, Naoto Harada, Peter Schilke

Abstract: Hot cores are small ($\lesssim$0.1 pc), dense ($\geq$10$^6$ cm$^{-3}$), and hot ($>$100 K) regions around massive protostars and are one of the main production sites of complex organic molecules (COMs, $\geq6$ atoms, including carbon). The Large Magellanic Cloud (LMC) is an ideal place to study hot core and COM formation in an environment that is different from our Galaxy, though prior to this stu… ▽ More Hot cores are small ($\lesssim$0.1 pc), dense ($\geq$10$^6$ cm$^{-3}$), and hot ($>$100 K) regions around massive protostars and are one of the main production sites of complex organic molecules (COMs, $\geq6$ atoms, including carbon). The Large Magellanic Cloud (LMC) is an ideal place to study hot core and COM formation in an environment that is different from our Galaxy, though prior to this study there have only been nine detections of extragalactic hot cores (seven in the LMC and two in the Small Magellanic Cloud, SMC). Here, we report 1.2 mm continuum and molecular line observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in the star-forming region N160 that we named N160A-mm. We identify six 1.2 mm continuum sources, four of which are associated with methanol (CH$_3$OH) emission. Another COM, methyl cyanide (CH$_3$CN) is associated with the brightest source, N160A-mmA, the most chemically rich source in the field. Using the XCLASS software, we perform spectral modeling to estimate rotational temperatures and total column densities of detected molecular species for four sources. Based on the temperature exceeding 100 K, small size, and high H$_2$ number density, we identify N160A-mmA as a hot core. We compare the molecular abundances of this newly detected hot core with those previously detected in the LMC and SMC, as well as with a sample of Galactic hot cores, and discuss the complex nature of N160A-mmA. △ Less

Submitted 25 June, 2025; originally announced June 2025.

Comments: Comments: 49 pages, 34 figures, 5 tables (including appendices); Accepted for publication in the Astrophysical Journal

Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS

Authors: R. Unnikrishnan, M. Andriantsaralaza, E. De Beck, L. -Å. Nyman, H. Olofsson, W. H. T. Vlemmings, M. Maercker, M. Van de Sande, T. Danilovich, T. J. Millar, S. B. Charnley, M. G. Rawlings

Abstract: The circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars harbour a rich variety of molecules and are sites of complex chemistry. Our current understanding of the circumstellar chemical processes of carbon-rich AGB stars is predominantly based on observations of a single star, IRC+10216, often regarded as an archetypical carbon star. We aim to estimate stellar and circumstellar pro… ▽ More The circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars harbour a rich variety of molecules and are sites of complex chemistry. Our current understanding of the circumstellar chemical processes of carbon-rich AGB stars is predominantly based on observations of a single star, IRC+10216, often regarded as an archetypical carbon star. We aim to estimate stellar and circumstellar properties for five carbon stars, and constrain their circumstellar CS abundances. This study compares the CS abundances among the sources, informs circumstellar chemical models, and helps to assess if IRC+10216 is a good representative of the physics and chemistry of carbon star CSEs. We modelled the spectral energy distributions (SEDs) and CO line emission to derive the stellar and outflow properties. Using these, we then retrieved CS abundance profiles with detailed radiative transfer modelling, imposing spatial and excitation constraints from ALMA and single-dish observations. We obtain good fits to the SEDs and CO lines for all sources and reproduce the CS line emission across various transitions and apertures, yielding robust estimates of the CS abundance profiles. Peak CS fractional abundances range from 1$\times$10$^{-6}$ - 4$\times$10$^{-6}$, with e-folding radii of 1.8$\times$10$^{16}$ - 6.8$\times$10$^{16}$ cm. We also derive reliable $^{12}$C/$^{13}$C and $^{32}$S/$^{34}$S ratios from CS isotopologue modelling. Our results refine previous single-dish CS abundance estimates and improve the relative uncertainty on the CS e-folding radius for IRAS 07454$-$7112 by a factor of $\sim$2.5. Chemical models reproduce our estimates of the CS radial extent, corroborating the CS photodissociation framework used therein. We find no significant differences between the derived CS abundance profiles for IRC+10216 and the rest of the sample, apart from the expected density-driven variations. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 18 pages, with 7 figures and 4 tables in the main text. 4 appendices contain additional figures and tables. Total 31 pages. Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A, 699, A48 (2025)

First Detection of Molecular Activity in the Largest Known Oort Cloud Comet: ALMA Imaging of C/2014 UN271 (Bernardinelli-Bernstein) at 16.6 au from the Sun

Authors: Nathan X. Roth, Stefanie N. Milam, Martin A. Cordiner, Dominique Bockelee-Morvan, Nicolas Biver, Michael S. P. Kelley, Anthony J. Remijan, Steven B. Charnley, Carrie E. Holt, Kiernan D. Foster, Joseph Chatelain, Edward Gomez, Sarah Greenstreet, Tim Lister, Helen Usher

Abstract: We report observations of comet C/2014 UN271 (Bernardinelli-Bernstein) carried out on UT 2024 March 8 and 17 at a heliocentric distance ($r_{\mathrm{H}}$) of 16.6 au using the Atacama Large Millimeter/Submillimeter Array (ALMA). The CO ($J$=2-1) line at 230 GHz was detected along with continuum emission from its dust coma and large ($\sim$140 km) nucleus, revealing the nature of the activity drive… ▽ More We report observations of comet C/2014 UN271 (Bernardinelli-Bernstein) carried out on UT 2024 March 8 and 17 at a heliocentric distance ($r_{\mathrm{H}}$) of 16.6 au using the Atacama Large Millimeter/Submillimeter Array (ALMA). The CO ($J$=2-1) line at 230 GHz was detected along with continuum emission from its dust coma and large ($\sim$140 km) nucleus, revealing the nature of the activity drivers and outgassing kinematics of the largest Oort cloud comet discovered to date. This work presents spectrally integrated flux maps, autocorrelation spectra, production rates, and parent scale lengths for CO and a stringent upper limit for the H$_2$CO production rate. CO outgassing displayed multiple active jets which evolved from one epoch to the next. The continuum emission was compact and spatially unresolved, and is consistent with thermal emission from the large nucleus and a tentative detection of a dust coma. Complementary optical observations provided activity context for the ALMA epochs, indicating that UN271 underwent an outburst in late February before returning to a quiescent brightness in mid--late March. These results represent the first secure detection of molecular activity reported in the literature for C/2014 UN271 and highlight the dynamic nature of this distantly active small world. △ Less

Submitted 16 July, 2025; v1 submitted 7 May, 2025; originally announced May 2025.

Comments: Published in The Astrophysical Journal Letters

Journal ref: The Astrophysical Journal Letters, 986, L22 (2025)

Discovery of the 7-ring PAH Cyanocoronene (C$_{24}$H$_{11}$CN) in GOTHAM Observations of TMC-1

Authors: Gabi Wenzel, Siyuan Gong, Ci Xue, P. Bryan Changala, Martin S. Holdren, Thomas H. Speak, D. Archie Stewart, Zachary T. P. Fried, Reace H. J. Willis, Edwin A. Bergin, Andrew M. Burkhardt, Alex N. Byrne, Steven B. Charnley, Andrew Lipnicky, Ryan A. Loomis, Christopher N. Shingledecker, Ilsa R. Cooke, Anthony J. Remijan, Michael C. McCarthy, Alison E. Wendlandt, Brett A. McGuire

Abstract: We present the synthesis and laboratory rotational spectroscopy of the 7-ring polycyclic aromatic hydrocarbon (PAH) cyanocoronene (C$_{24}$H$_{11}$CN) using a laser-ablation assisted cavity-enhanced Fourier transform microwave spectrometer. A total of 71 transitions were measured and assigned between 6.8--10.6\,GHz. Using these assignments, we searched for emission from cyanocoronene in the GBT Ob… ▽ More We present the synthesis and laboratory rotational spectroscopy of the 7-ring polycyclic aromatic hydrocarbon (PAH) cyanocoronene (C$_{24}$H$_{11}$CN) using a laser-ablation assisted cavity-enhanced Fourier transform microwave spectrometer. A total of 71 transitions were measured and assigned between 6.8--10.6\,GHz. Using these assignments, we searched for emission from cyanocoronene in the GBT Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM) project observations of the cold dark molecular cloud TMC-1 using the 100\,m Green Bank Telescope (GBT). We detect a number of individually resolved transitions in ultrasensitive X-band observations and perform a Markov Chain Monte Carlo analysis to derive best-fit parameters, including a total column density of $N(\mathrm{C}_{24}\mathrm{H}_{11}\mathrm{CN}) = 2.69^{+0.26}_{-0.23} \times 10^{12}\,\mathrm{cm}^{-2}$ at a temperature of $6.05^{+0.38}_{-0.37}\,$K. A spectral stacking and matched filtering analysis provides a robust 17.3$\,σ$ significance to the overall detection. The derived column density is comparable to that of cyano-substituted naphthalene, acenaphthylene, and pyrene, defying the trend of decreasing abundance with increasing molecular size and complexity found for carbon chains. We discuss the implications of the detection for our understanding of interstellar PAH chemistry and highlight major open questions and next steps. △ Less

Submitted 7 April, 2025; originally announced April 2025.

Improved Carbon and Nitrogen Isotopic Ratios for CH$_3$CN in Titan's Atmosphere Using ALMA

Authors: J. Nosowitz, M. A. Cordiner, C. A. Nixon, A. E. Thelen, Z. Kisiel, N. A. Teanby, P. G. J. Irwin, S. B. Charnley, V. Vuitton

Abstract: Titan, Saturn's largest satellite, maintains an atmosphere composed primarily of nitrogen (N$_2$) and methane (CH$_4$) that leads to a complex organic chemistry. Some of the nitriles (CN-bearing organics) on Titan are known to have substantially enhanced $^{15}$N abundances compared to Earth and to Titan's dominant nitrogen (N$_2$) reservoir. The $^{14}$N/$^{15}$N isotopic ratio in Titan's nitrile… ▽ More Titan, Saturn's largest satellite, maintains an atmosphere composed primarily of nitrogen (N$_2$) and methane (CH$_4$) that leads to a complex organic chemistry. Some of the nitriles (CN-bearing organics) on Titan are known to have substantially enhanced $^{15}$N abundances compared to Earth and to Titan's dominant nitrogen (N$_2$) reservoir. The $^{14}$N/$^{15}$N isotopic ratio in Titan's nitriles can provide better constraints on the synthesis of nitrogen-bearing organics in planetary atmospheres as well as insights into the origin of Titan's large nitrogen abundance. Using high signal-to-noise ratio ($>13$), disk-integrated observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 receiver (211-275 GHz), we measure the $^{14}$N/$^{15}$N and $^{12}$C/$^{13}$C isotopic ratios of acetonitrile (CH$_3$CN) in Titan's stratosphere. Using the Nonlinear optimal Estimator for MultivariatE spectral analySIS (NEMESIS), we derived the CH$_3$CN/$^{13}$CH$_3$CN ratio to be 89.2 $\pm$ 7.0 and the CH$_3$CN/CH$_3$$^{13}$CN ratio to be 91.2 $\pm$ 6.0, in agreement with the $^{12}$C/$^{13}$C ratio in Titan's methane, and other Solar System species. We found the $^{14}$N/$^{15}$N isotopic ratio to be 68.9 $\pm$ 4.2, consistent with previously derived values for HCN and HC$_3$N, confirming an enhanced $^{15}$N abundance in Titan's nitriles compared with the bulk atmospheric N$_2$ value of $^{14}$N/$^{15}$N = 168, in agreement with chemical models incorporating isotope-selective photodissociation of N$_2$ at high altitudes. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Comments: Accepted for publication in PSJ March 2025

FAUST XX. The chemical structure and temperature profile of the IRAS 4A2 hot corino at 20-50 au

Authors: J. Frediani, M. De Simone, L. Testi, L. Podio, C. Codella, C. J. Chandler, C. Ceccarelli, L. Loinard, A. López-Sepulcre, B. Svoboda, N. Sakai, L. Chahine, Y. Aikawa, E. Bianchi, M. Bouvier, L. Cacciapuoti, P. Caselli, S. B. Charnley, I. Jimenez-Serra, D. Johnstone, G. Sabatini, Y. Shirley, S. Yamamoto

Abstract: Young low-mass protostars often possess hot corinos, compact, hot and dense regions bright in interstellar Complex Organic Molecules (iCOMs). Besides of their prebiotic role, iCOMs can be used as a powerful tool to characterize the chemical and physical properties of hot corinos. Using ALMA/FAUST data we aim to explore the iCOMs emission at < 50 au scale around the Class 0 prototypical hot corino… ▽ More Young low-mass protostars often possess hot corinos, compact, hot and dense regions bright in interstellar Complex Organic Molecules (iCOMs). Besides of their prebiotic role, iCOMs can be used as a powerful tool to characterize the chemical and physical properties of hot corinos. Using ALMA/FAUST data we aim to explore the iCOMs emission at < 50 au scale around the Class 0 prototypical hot corino IRAS 4A2. We imaged IRAS 4A2 in six abundant, common iCOMs (CH$_3$OH, HCOOCH$_3$, CH$_3$CHO, CH$_3$CH$_2$OH, CH$_2$OHCHO, and NH$_2$CHO), and derived their emitting size. The column density and gas temperature for each species were derived at 1$σ$ from a multi-line analysis by applying a non-LTE approach for CH$_3$OH, and LTE population or rotational diagram analysis for the other iCOMs. Thanks to the unique estimates of the absorption from foreground millimeter dust toward IRAS 4A2, we derived for the first time unbiased gas temperatures and column densities. We resolved the IRAS 4A2 hot corino finding evidence for a chemical spatial distribution in the inner 50 au, with the outer emitting radius increasing from ~ 22-23 au for NH$_2$CHO and CH$_2$OHCHO, followed by CH$_3$CH$_2$OH (~ 27 au), CH$_3$CHO (~ 28 au), HCOOCH$_3$ (~ 36 au), and out to ~ 40 au for CH$_3$OH. Combining our estimate of the gas temperature probed by each iCOM with their beam-deconvolved emission sizes, we inferred the gas temperature profile of the hot corino on scales of 20-50 au in radius, finding a power-law index $q$ of approximately -1. We observed, for the first time, a chemical segregation in iCOMs of the IRAS 4A2 hot corino, and derived the gas temperature profile of its inner envelope. The derived profile is steeper than when considering a simple spherical collapsing and optically-thin envelope, hinting at a partially optically-thick envelope or a gravitationally unstable disk-like structure. △ Less

Submitted 31 January, 2025; originally announced January 2025.

Comments: 21 pages, 16 figures, 4 tables. Accepted in A&A

Journal ref: A&A 695, A78 (2025)

Ice inventory towards the protostar Ced 110 IRS4 observed with the James Webb Space Telescope. Results from the ERS Ice Age program

Authors: W. R. M. Rocha, M. K. McClure, J. A. Sturm, T. L. Beck, Z. L. Smith, H. Dickinson, F. Sun, E. Egami, A. C. A. Boogert, H. J. Fraser, E. Dartois, I. Jimenez-Serra, J. A. Noble, J. Bergner, P. Caselli, S. B. Charnley, J. Chiar, L. Chu, I. Cooke, N. Crouzet, E. F. van Dishoeck, M. N. Drozdovskaya, R. Garrod, D. Harsono, S. Ioppolo , et al. (15 additional authors not shown)

Abstract: This work focuses on the ice features toward the binary protostellar system Ced 110 IRS 4A and 4B, and observed with JWST as part of the Early Release Science Ice Age collaboration. We aim to explore the JWST observations of the binary protostellar system Ced~110~IRS4A and IRS4B to unveil and quantify the ice inventories toward these sources. We compare the ice abundances with those found for the… ▽ More This work focuses on the ice features toward the binary protostellar system Ced 110 IRS 4A and 4B, and observed with JWST as part of the Early Release Science Ice Age collaboration. We aim to explore the JWST observations of the binary protostellar system Ced~110~IRS4A and IRS4B to unveil and quantify the ice inventories toward these sources. We compare the ice abundances with those found for the same molecular cloud. The analysis is performed by fitting or comparing laboratory infrared spectra of ices to the observations. Spectral fits are carried out with the ENIIGMA fitting tool that searches for the best fit. For Ced~110~IRS4B, we detected the major ice species H$_2$O, CO, CO$_2$ and NH$_3$. All species are found in a mixture except for CO and CO$_2$, which have both mixed and pure ice components. In the case of Ced~110~IRS4A, we detected the same major species as in Ced~110~IRS4B, as well as the following minor species CH$_4$, SO$_2$, CH$_3$OH, OCN$^-$, NH$_4^+$ and HCOOH. Tentative detection of N$_2$O ice (7.75~$μ$m), forsterite dust (11.2~$μ$m) and CH$_3^+$ gas emission (7.18~$μ$m) in the primary source are also presented. Compared with the two lines of sight toward background stars in the Chameleon I molecular cloud, the protostar has similar ice abundances, except in the case of the ions that are higher in IRS4A. The clearest differences are the absence of the 7.2 and 7.4~$μ$m absorption features due to HCOO$^-$ and icy complex organic molecules in IRS4A and evidence of thermal processing in both IRS4A and IRS4B as probed by the CO$_2$ ice features. We conclude that the binary protostellar system Ced~110~IRS4A and IRS4B has a large inventory of icy species. The similar ice abundances in comparison to the starless regions in the same molecular cloud suggest that the chemical conditions of the protostar were set at earlier stages in the molecular cloud. △ Less

Submitted 29 November, 2024; originally announced November 2024.

Comments: 33 pages, 19 Figures. Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 693, A288 (2025)

Detections of interstellar 2-cyanopyrene and 4-cyanopyrene in TMC-1

Authors: Gabi Wenzel, Thomas H. Speak, P. Bryan Changala, Reace H. J. Willis, Andrew M. Burkhardt, Shuo Zhang, Edwin A. Bergin, Alex N. Byrne, Steven B. Charnley, Zachary T. P. Fried, Harshal Gupta, Eric Herbst, Martin S. Holdren, Andrew Lipnicky, Ryan A. Loomis, Christopher N. Shingledecker, Ci Xue, Anthony J. Remijan, Alison E. Wendlandt, Michael C. McCarthy, Ilsa R. Cooke, Brett A. McGuire

Abstract: Polycyclic aromatic hydrocarbons (PAHs) are among the most ubiquitous compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. We report the detection of 2- a… ▽ More Polycyclic aromatic hydrocarbons (PAHs) are among the most ubiquitous compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. We report the detection of 2- and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in TMC-1. △ Less

Submitted 4 October, 2024; v1 submitted 1 October, 2024; originally announced October 2024.

Comments: Submitted version to comply with licensing agreements

Discovery of interstellar 1-cyanopyrene: a four-ring polycyclic aromatic hydrocarbon in TMC-1

Authors: Gabi Wenzel, Ilsa R. Cooke, P. Bryan Changala, Edwin A. Bergin, Shuo Zhang, Andrew M. Burkhardt, Alex N. Byrne, Steven B. Charnley, Martin A. Cordiner, Miya Duffy, Zachary T. P. Fried, Harshal Gupta, Martin S. Holdren, Andrew Lipnicky, Ryan A. Loomis, Hannah Toru Shay, Christopher N. Shingledecker, Mark A. Siebert, D. Archie Stewart, Reace H. J. Willis, Ci Xue, Anthony J. Remijan, Alison E. Wendlandt, Michael C. McCarthy, Brett A. McGuire

Abstract: Polycyclic aromatic hydrocarbons (PAHs) are expected to be the most abundant class of organic molecules in space. Their interstellar lifecycle is not well understood, and progress is hampered by difficulties detecting individual PAH molecules. Here, we present the discovery of CN-functionalized pyrene, a 4-ring PAH, in the dense cloud TMC-1 using the 100-m Green Bank Telescope. We derive an abunda… ▽ More Polycyclic aromatic hydrocarbons (PAHs) are expected to be the most abundant class of organic molecules in space. Their interstellar lifecycle is not well understood, and progress is hampered by difficulties detecting individual PAH molecules. Here, we present the discovery of CN-functionalized pyrene, a 4-ring PAH, in the dense cloud TMC-1 using the 100-m Green Bank Telescope. We derive an abundance of 1-cyanopyrene of ~1.52 x $10^{12}$ cm$^{-2}$, and from this estimate that the un-substituted pyrene accounts for up to ~0.03-0.3% of the carbon budget in the dense interstellar medium which trace the birth sites of stars and planets. The presence of pyrene in this cold (~10 K) molecular cloud agrees with its recent measurement in asteroid Ryugu where isotopic clumping suggest a cold, interstellar origin. The direct link to the birth site of our solar system is strengthened when we consider the solid state pyrene content in the pre-stellar materials compared to comets, which represent the most pristine material in the solar system. We estimate that solid state pyrene can account for 1% of the carbon within comets carried by this one single organic molecule. The abundance indicates pyrene is an "island of stability" in interstellar PAH chemistry and suggests a potential cold molecular cloud origin of the carbon carried by PAHs that is supplied to forming planetary systems, including habitable worlds such as our own. △ Less

Submitted 4 October, 2024; v1 submitted 1 October, 2024; originally announced October 2024.

Comments: Version of manuscript revised to comply with licensing requirements

Evidence for Surprising Heavy Nitrogen Isotopic Enrichment in Comet 46P/Wirtanen's Hydrogen Cyanide

Authors: M. A. Cordiner, K. Darnell, D. Bockeleé-Morvan, N. X. Roth, N. Biver, S. N. Milam, S. B. Charnley, J. Boissier, B. P. Bonev, C. Qi, J. Crovisier, A. J. Remijan

Abstract: 46P/Wirtanen is a Jupiter-family comet, probably originating from the Solar System's Kuiper belt, that now resides on a 5.4 year elliptical orbit. During its 2018 apparition, comet 46P passed unusually close to the Earth (within 0.08 au), presenting an outstanding opportunity for close-up observations of its inner coma. Here we present observations of HCN, H$^{13}$CN and HC$^{15}$N emission from 4… ▽ More 46P/Wirtanen is a Jupiter-family comet, probably originating from the Solar System's Kuiper belt, that now resides on a 5.4 year elliptical orbit. During its 2018 apparition, comet 46P passed unusually close to the Earth (within 0.08 au), presenting an outstanding opportunity for close-up observations of its inner coma. Here we present observations of HCN, H$^{13}$CN and HC$^{15}$N emission from 46P using the Atacama Compact Array (ACA). The data were analyzed using the SUBLIME non-LTE radiative transfer code to derive $^{12}$C/$^{13}$C and $^{14}$N/$^{15}$N ratios. The HCN/H$^{13}$CN ratio is found to be consistent with a lack of significant $^{13}$C fractionation, whereas the HCN/HC$^{15}$N ratio of $68\pm27$ (using our most conservative $1σ$ uncertainties), indicates a strong enhancement in $^{15}$N compared with the solar and terrestrial values. The observed $^{14}$N/$^{15}$N ratio is also significantly lower than the values of $\sim140$ found in previous comets, implying a strong $^{15}$N enrichment in 46P's HCN. This indicates that the nitrogen in Jupiter-family comets could reach larger isotopic enrichments than previously thought, with implications for the diversity of $^{14}$N/$^{15}$N ratios imprinted into icy bodies at the birth of the Solar System. △ Less

Submitted 9 September, 2024; originally announced September 2024.

Comments: Accepted for publication in PSJ, September 2024

FAUST XVII: Super deuteration in the planet forming system IRS 63 where the streamer strikes the disk

Authors: L. Podio, C. Ceccarelli, C. Codella, G. Sabatini, D. Segura-Cox, N. Balucani, A. Rimola, P. Ugliengo, C. J. Chandler, N. Sakai, B. Svoboda, J. Pineda, M. De Simone, E. Bianchi, P. Caselli, A. Isella, Y. Aikawa, M. Bouvier, E. Caux, L. Chahine, S. B. Charnley, N. Cuello, F. Dulieu, L. Evans, D. Fedele , et al. (33 additional authors not shown)

Abstract: Recent observations suggest that planets formation starts early, in protostellar disks of $\le10^5$ yrs, which are characterized by strong interactions with the environment, e.g., through accretion streamers and molecular outflows. To investigate the impact of such phenomena on disk physical and chemical properties it is key to understand what chemistry planets inherit from their natal environment… ▽ More Recent observations suggest that planets formation starts early, in protostellar disks of $\le10^5$ yrs, which are characterized by strong interactions with the environment, e.g., through accretion streamers and molecular outflows. To investigate the impact of such phenomena on disk physical and chemical properties it is key to understand what chemistry planets inherit from their natal environment. In the context of the ALMA Large Program Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars (FAUST), we present observations on scales from ~1500 au to ~60 au of H$_2$CO, HDCO, and D$_2$CO towards the young planet-forming disk IRS~63. H$_2$CO probes the gas in the disk as well as in a large scale streamer (~1500 au) impacting onto the South-East (SE) disk side. We detect for the first time deuterated formaldehyde, HDCO and D$_2$CO, in a planet-forming disk, and HDCO in the streamer that is feeding it. This allows us to estimate the deuterium fractionation of H$_2$CO in the disk: [HDCO]/[H$_2$CO]$\sim0.1-0.3$ and [D$_2$CO]/[H$_2$CO]$\sim0.1$. Interestingly, while HDCO follows the H$_2$CO distribution in the disk and in the streamer, the distribution of D$_2$CO is highly asymmetric, with a peak of the emission (and [D]/[H] ratio) in the SE disk side, where the streamer crashes onto the disk. In addition, D$_2$CO is detected in two spots along the blue- and red-shifted outflow. This suggests that: (i) in the disk, HDCO formation is dominated by gas-phase reactions similarly to H$_2$CO, while (ii) D$_2$CO was mainly formed on the grain mantles during the prestellar phase and/or in the disk itself, and is at present released in the gas-phase in the shocks driven by the streamer and the outflow. These findings testify on the key role of streamers in the build-up of the disk both concerning the final mass available for planet formation and its chemical composition. △ Less

Submitted 5 July, 2024; originally announced July 2024.

Comments: 12 pages, 10 figures, accepted for publication on A&A

Journal ref: A&A 688, L22 (2024)

Observations of Titan's Stratosphere During Northern Summer: Temperatures, CH3CN and CH3D Abundances

Authors: Alexander E. Thelen, Conor A. Nixon, Martin A. Cordiner, Emmanuel Lellouch, Sandrine Vinatier, Nicholas A. Teanby, Bryan Butler, Steven B. Charnley, Richard G. Cosentino, Katherine de Kleer, Patrick G. J. Irwin, Mark A. Gurwell, Zbigniew Kisiel, Raphael Moreno

Abstract: Titan's atmospheric composition and dynamical state have previously been studied over numerous epochs by both ground- and space-based facilities. However, stratospheric measurements remain sparse during Titan's northern summer and fall. The lack of seasonal symmetry in observations of Titan's temperature field and chemical abundances raises questions about the nature of the middle atmosphere's mer… ▽ More Titan's atmospheric composition and dynamical state have previously been studied over numerous epochs by both ground- and space-based facilities. However, stratospheric measurements remain sparse during Titan's northern summer and fall. The lack of seasonal symmetry in observations of Titan's temperature field and chemical abundances raises questions about the nature of the middle atmosphere's meridional circulation and evolution over Titan's 29-yr seasonal cycle that can only be answered through long-term monitoring campaigns. Here, we present maps of Titan's stratospheric temperature, acetonitrile (or methyl cyanide; CH$_3$CN), and monodeuterated methane (CH$_3$D) abundances following Titan's northern summer solstice obtained with Band 9 ($\sim0.43$ mm) ALMA observations. We find that increasing temperatures towards high-southern latitudes, currently in winter, resemble those observed during Titan's northern winter by the Cassini mission. Acetonitrile abundances have changed significantly since previous (sub)millimeter observations, and we find that the species is now highly concentrated at high-southern latitudes. The stratospheric CH$_3$D content is found to range between 4-8 ppm in these observations, and we infer the CH$_4$ abundance to vary between $\sim0.9-1.6\%$ through conversion with previously measured D/H values. A global value of CH$_4=1.15\%$ was retrieved, lending further evidence to the temporal and spatial variability of Titan's stratospheric methane when compared with previous measurements. Additional observations are required to determine the cause and magnitude of stratospheric enhancements in methane during these poorly understood seasons on Titan. △ Less

Submitted 3 May, 2024; originally announced May 2024.

Comments: Accepted for publication in the Planetary Science Journal. 9 Figures, 1 table

ALMA Spectroscopy of Europa: A Search for Active Plumes

Authors: M. A. Cordiner, A. E. Thelen, I. -L. Lai, W. -L. Tseng, C. A. Nixon, Y. -J. Kuan, G. L. Villanueva, L. Paganini, S. B. Charnley, K. D. Retherford

Abstract: The subsurface ocean of Europa is a high priority target in the search for extraterrestrial life, but direct investigations are hindered by the presence of a thick, exterior ice shell. Here we present spectral line and continuum maps of Europa obtained over four epochs in May-June 2021 using the Atacama Large Millimeter/submillimeter Array (ALMA), to search for molecular emission from atmospheric… ▽ More The subsurface ocean of Europa is a high priority target in the search for extraterrestrial life, but direct investigations are hindered by the presence of a thick, exterior ice shell. Here we present spectral line and continuum maps of Europa obtained over four epochs in May-June 2021 using the Atacama Large Millimeter/submillimeter Array (ALMA), to search for molecular emission from atmospheric plumes, with the aim of investigating subsurface processes. Using a 3D physical model, we obtained upper limits for the plume abundances of HCN, H$_2$CO, SO$_2$ and CH$_3$OH. If active plume(s) were present, they contained very low abundances of these molecules. Assuming a total gas production rate of $10^{29}$ s$^{-1}$, our H$_2$CO abundance upper limit of $<0.016$\% is more than an order of magnitude less than measured in the Enceladus plume by the Cassini spacecraft, implying a possible chemical difference between the plume source materials for these two icy moons. △ Less

Submitted 8 April, 2024; originally announced April 2024.

Comments: Submitted to IAU Symposium 383 conference proceedings --- Astrochemistry VIII: From the First Galaxies to the Formation of Habitable Worlds

Variability in Titan's Mesospheric HCN and Temperature Structure as Observed by ALMA

Authors: A. E. Thelen, C. A. Nixon, R. Cosentino, M. A. Cordiner, N. A. Teanby, C. E. Newman, P. G. J. Irwin, S. B. Charnley

Abstract: The temperature structure of Titan's upper atmosphere exhibits large variability resulting from numerous spatially and temporally irregular external energy sources, seasonal changes, and the influence of molecular species produced via photochemistry. In particular, Titan's relatively abundant HCN is thought to provide substantial cooling to the upper atmosphere through rotational emission, balanci… ▽ More The temperature structure of Titan's upper atmosphere exhibits large variability resulting from numerous spatially and temporally irregular external energy sources, seasonal changes, and the influence of molecular species produced via photochemistry. In particular, Titan's relatively abundant HCN is thought to provide substantial cooling to the upper atmosphere through rotational emission, balancing UV/EUV heating and thermal conduction. Here, we present the analysis of ALMA observations of Titan from 2012, 2014, 2015, and 2017, corresponding to planetocentric solar longitudes of ~34-89$^{\circ}$, including vertical HCN and temperature profiles retrieved from the lower mesosphere through the thermosphere (~350-1200 km; $3\times10^{-2}$-$2\times10^{-8}$ mbar). Throughout the atmosphere, temperature profiles differ by 10 to 30 K between observations approximately one Earth year apart, particularly from 600-900 km. We find evidence for a large imbalance in Titan's upper atmospheric energy budget between 2014 and 2015, where the mesospheric thermal structure changes significantly and marks the transition between a mesopause located at ~600 km ($2\times10^{-4}$ mbar) and at ~800 km ($3\times10^{-6}$ mbar). The retrieved HCN abundances vary dramatically during the 2012 to 2017 time period as well, showing close to 2 orders of magnitude difference in abundance at 1000 km. However, the change in HCN abundance does not appear to fully account for the variation in mesospheric temperatures over the $L_S\sim$34-89$^{\circ}$ period. These measurements provide additional insight into the variability of Titan's mesospheric composition and thermal structure following its 2009 vernal equinox, and motivate continued investigation of the origins of such rapid changes in Titan's atmosphere throughout its seasonal cycle. △ Less

Submitted 2 February, 2024; originally announced February 2024.

Comments: Published in the Planetary Science Journal. 25 pages, 11 figures, 1 table

Charting Circumstellar Chemistry of Carbon-rich AGB Stars: I. ALMA 3 mm spectral surveys

Authors: R. Unnikrishnan, E. De Beck, L. A. Nyman, H. Olofsson, W. H. T. Vlemmings, D. Tafoya, M. Maercker, S. B. Charnley, M. A. Cordiner, I. de Gregorio, E. Humphreys, T. J. Millar, M. G. Rawlings

Abstract: AGB stars are major contributors to the chemical enrichment of the ISM through nucleosynthesis and extensive mass loss. Most of our current knowledge of AGB atmospheric and circumstellar chemistry, in particular in a C-rich environment, is based on observations of the carbon star IRC+10216. We aim to obtain a more generalised understanding of the chemistry in C-rich AGB CSEs by studying a sample o… ▽ More AGB stars are major contributors to the chemical enrichment of the ISM through nucleosynthesis and extensive mass loss. Most of our current knowledge of AGB atmospheric and circumstellar chemistry, in particular in a C-rich environment, is based on observations of the carbon star IRC+10216. We aim to obtain a more generalised understanding of the chemistry in C-rich AGB CSEs by studying a sample of three carbon stars, IRAS15194-5115, IRAS15082-4808, and IRAS07454-7112, and test the archetypal status often attributed to IRC+10216. We performed spatially resolved, unbiased spectral surveys in ALMA Band 3. We identify a total of 132 rotational transitions from 49 molecular species. There are two main morphologies of the brightness distributions: centrally-peaked (e.g. HCN) and shell-like (e.g. C$_2$H). We estimated the sizes of the molecular emitting regions using azimuthally-averaged radial profiles of the line brightness distributions, and derived abundance estimates. Of the shell distributions, the cyanopolyynes peak at slightly smaller radii than the hydrocarbons, and CN and HNC show the most extended emission. The emitting regions for each species are the smallest for IRAS07454-7112. We find that, within the uncertainties of the analysis, the three stars present similar abundances for most species, also compared to IRC+10216. We find that SiO is more abundant in our three stars compared to IRC+10216. Our estimated isotopic ratios match well the literature values for the sources. The observed circumstellar chemistry appears very similar across our sample and compared to that of IRC+10216, both in terms of the relative location of the emitting regions and molecular abundances. This implies that, to a first approximation, the chemical models tailored to IRC+10216 are able to reproduce the observed chemistry in C-rich envelopes across roughly an order of magnitude in wind density. △ Less

Submitted 15 December, 2023; originally announced December 2023.

Comments: 21 pages. 13 figures and 8 tables in the main text. 5 appendices contain additional figures and tables. Appendix tables are available in electronic form at the CDS, along with the reprocessed ALMA cubes and spectra, at http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ or via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5). Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A, 684, A4 (2024)

The Detection of Higher-Order Millimeter Hydrogen Recombination Lines in the Large Magellanic Cloud

Authors: Marta Sewiło, Kazuki Tokuda, Stan E. Kurtz, Steven B. Charnley, Thomas Möller, Jennifer Wiseman, C. -H. Rosie Chen, Remy Indebetouw, Álvaro Sánchez-Monge, Kei E. I. Tanaka, Peter Schilke, Toshikazu Onishi, Naoto Harada

Abstract: We report the first extragalactic detection of the higher-order millimeter hydrogen recombination lines ($Δn>2$). The $γ$-, $ε$-, and $η$-transitions have been detected toward the millimeter continuum source N105-1A in the star-forming region N105 in the Large Magellanic Cloud (LMC) with the Atacama Large Millimeter/submillimeter Array (ALMA). We use the H40$α$ line, the brightest of the detected… ▽ More We report the first extragalactic detection of the higher-order millimeter hydrogen recombination lines ($Δn>2$). The $γ$-, $ε$-, and $η$-transitions have been detected toward the millimeter continuum source N105-1A in the star-forming region N105 in the Large Magellanic Cloud (LMC) with the Atacama Large Millimeter/submillimeter Array (ALMA). We use the H40$α$ line, the brightest of the detected recombination lines (H40$α$, H36$β$, H50$β$, H41$γ$, H57$γ$, H49$ε$, H53$η$, and H54$η$), and/or the 3 mm free-free continuum emission to determine the physical parameters of N105-1A (the electron temperature, emission measure, electron density, and size) and study ionized gas kinematics. We compare the physical properties of N105-1A to a large sample of Galactic compact and ultracompact (UC) H II regions and conclude that N105-1A is similar to the most luminous ($L>10^5$ $L_{\odot}$) UC H II regions in the Galaxy. N105-1A is ionized by an O5.5 V star, it is deeply embedded in its natal molecular clump, and likely associated with a (proto)cluster. We incorporate high-resolution molecular line data including CS, SO, SO$_2$, and CH$_3$OH ($\sim$0.12 pc), and HCO$^{+}$ and CO ($\sim$0.087 pc) to explore the molecular environment of N105-1A. Based on the CO data, we find evidence for a cloud-cloud collision that likely triggered star formation in the region. We find no clear outflow signatures, but the presence of filaments and streamers indicates on-going accretion onto the clump hosting the UC H II region. Sulfur chemistry in N105-1A is consistent with the accretion shock model predictions. △ Less

Submitted 5 September, 2023; originally announced September 2023.

Comments: 51 pages, 30 figures, 2 tables (including appendices); accepted for publication in The Astrophysical Journal (ApJ)

Linking ice and gas in the Coronet cluster in Corona Australis

Authors: G. Perotti, J. K. Jørgensen, W. R. M. Rocha, A. Plunkett, E. Artur de la Villarmois, L. E. Kristensen, M. Sewiło, P. Bjerkeli, H. J. Fraser, S. B. Charnley

Abstract: During the journey from the cloud to the disc, the chemical composition of the protostellar envelope material can be either preserved or processed to varying degrees depending on the surrounding physical environment. This works aims to constrain the interplay of solid (ice) and gaseous methanol (CH$_3$OH) in the outer regions of protostellar envelopes located in the Coronet cluster in Corona Austr… ▽ More During the journey from the cloud to the disc, the chemical composition of the protostellar envelope material can be either preserved or processed to varying degrees depending on the surrounding physical environment. This works aims to constrain the interplay of solid (ice) and gaseous methanol (CH$_3$OH) in the outer regions of protostellar envelopes located in the Coronet cluster in Corona Australis (CrA), and assess the importance of irradiation by the Herbig Ae/Be star R CrA. CH$_3$OH is a prime test-case as it predominantly forms as a consequence of the solid-gas interplay (hydrogenation of condensed CO molecules onto the grain surfaces) and it plays an important role in future complex molecular processing. We present 1.3 mm Submillimeter Array (SMA) and Atacama Pathfinder Experiment (APEX) observations towards the envelopes of four low-mass protostars in the Coronet. Eighteen molecular transitions of seven species are identified. We calculate CH$_3$OH gas-to-ice ratios in this strongly irradiated cluster and compare them with ratios determined towards protostars located in less irradiated regions such as the Serpens SVS 4 cluster in Serpens Main and the Barnard 35A cloud in the $λ$ Orionis region. The CH$_3$OH gas-to-ice ratios in the Coronet vary by one order of magnitude (from 1.2$\times$10$^{-4}$ to 3.1$\times$10$^{-3}$) which is similar to less irradiated regions as found in previous studies. We find that the CH$_3$OH gas-to-ice ratios estimated in these three regions are remarkably similar despite the different UV radiation field intensities and formation histories. This result suggests that the overall CH$_3$OH chemistry in the outer regions of low-mass envelopes is relatively independent of variations in the physical conditions and hence that it is set during the prestellar stage. △ Less

Submitted 25 August, 2023; originally announced August 2023.

Comments: 20 pages, 17 figures, accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 678, A78 (2023)

Low NH$_{3}$/H$_{2}$O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun

Authors: Maria N. Drozdovskaya, Dominique Bockelée-Morvan, Jacques Crovisier, Brett A. McGuire, Nicolas Biver, Steven B. Charnley, Martin A. Cordiner, Stefanie N. Milam, Cyrielle Opitom, Anthony J. Remijan

Abstract: A lower-than-solar elemental nitrogen content has been demonstrated for several comets, including 1P/Halley and 67P/C-G with independent in situ measurements of volatile and refractory budgets. The recently discovered semi-refractory ammonium salts in 67P/C-G are thought to be the missing nitrogen reservoir in comets. The thermal desorption of ammonium salts from cometary dust particles leads to t… ▽ More A lower-than-solar elemental nitrogen content has been demonstrated for several comets, including 1P/Halley and 67P/C-G with independent in situ measurements of volatile and refractory budgets. The recently discovered semi-refractory ammonium salts in 67P/C-G are thought to be the missing nitrogen reservoir in comets. The thermal desorption of ammonium salts from cometary dust particles leads to their decomposition into ammonia and a corresponding acid. The NH$_{3}$/H$_{2}$O ratio is expected to increase with decreasing heliocentric distance with evidence for this in near-infrared observations. NH$_{3}$ has been claimed to be more extended than expected for a nuclear source. Here, the aim is to constrain the NH$_{3}$/H$_{2}$O ratio in comet C/2020 F3 (NEOWISE) during its July 2020 passage. OH emission from comet C/2020 F3 (NEOWISE) was monitored for 2 months with NRT and observed from GBT on 24 July and 11 August 2020. Contemporaneously with the 24 July 2020 OH observations, the NH$_{3}$ hyperfine lines were targeted with GBT. The concurrent GBT and NRT observations allowed the OH quenching radius to be determined at $\left(5.96\pm0.10\right)\times10^{4}$ km on 24 July 2020, which is important for accurately deriving $Q(\text{OH})$. C/2020 F3 (NEOWISE) was a highly active comet with $Q(\text{H}_{2}\text{O}) \approx 2\times10^{30}$ molec s$^{-1}$ one day before perihelion. The $3σ$ upper limit for $Q_{\text{NH}_{3}}/Q_{\text{H}_{2}\text{O}}$ is $<0.29\%$ at $0.7$ au from the Sun. The obtained NH$_{3}$/H$_{2}$O ratio is a factor of a few lower than measurements for other comets at such heliocentric distances. The abundance of NH$_{3}$ may vary strongly with time depending on the amount of water-poor dust in the coma. Lifted dust can be heated, fragmented, and super-heated; whereby, ammonium salts, if present, can rapidly thermally disintegrate and modify the NH$_{3}$/H$_{2}$O ratio. △ Less

Submitted 21 July, 2023; originally announced July 2023.

Comments: Accepted for publication in A&A; 18 pages, 8 figures, 6 tables

Journal ref: A&A 677, A157 (2023)

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

Authors: M. A. Cordiner, N. X. Roth, S. N. Milam, G. Villanueva, D. Bockelee-Morvan, A. J. Remijan, S. B. Charnley, N. Biver, D. C. Lis, C. Qi, B. Bonev, J. Crovisier, J. Boissier

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… ▽ More 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 towards comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) during the comet's unusually close (~0.1 au) approach to Earth in December 2018. 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 near 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-length $L_p=36\pm7$ km). On the other hand, H2CO, 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, 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. △ Less

Submitted 19 June, 2023; v1 submitted 8 May, 2023; originally announced May 2023.

Comments: Accepted for publication in the Astrophysical Journal on 2023-06-17

New Bistable Solutions in Molecular Cloud Chemistry: Nitrogen and Carbon Autocatalysis

Authors: Dufour Gwenaëlle, Steven B. Charnley

Abstract: We have investigated the chemistry of dense interstellar clouds and found new bistable solutions in the nitrogen and carbon chemistries. We identify the autocatalytic processes that are present in the pure, reduced, chemical networks and, as previously found for oxygen chemistry, that He$^+$ plays an important role. The applicability of these results to astronomical environments is briefly discu… ▽ More We have investigated the chemistry of dense interstellar clouds and found new bistable solutions in the nitrogen and carbon chemistries. We identify the autocatalytic processes that are present in the pure, reduced, chemical networks and, as previously found for oxygen chemistry, that He$^+$ plays an important role. The applicability of these results to astronomical environments is briefly discussed. The bistable solutions found for carbon chemistry occur for low densities and high ionization fractions that are not compatible with that found cold, dense clouds. Bistability in the pure nitrogen chemistry occurs for conditions that are relevant for prestellar cores in which significant CO depletion has taken place. We conclude that several autocatalyses are embedded in gas-phase interstellar chemistry and that many more are potentially present. △ Less

Submitted 27 April, 2023; originally announced April 2023.

Comments: 12 pages, 5 figures, 1 table

Oscillations in Gas-grain Astrochemical Kinetics

Authors: Gwenaëlle Dufour, Steven B. Charnley, Johan E. Lindberg

Abstract: We have studied gas-grain chemical models of interstellar clouds to search for nonlinear dynamical evolution. A prescription is given for producing oscillatory solutions when a bistable solution exists in the gas-phase chemistry and we demonstrate the existence of limit cycle and relaxation oscillation solutions. As the autocatalytic chemical processes underlying these solutions are common to all… ▽ More We have studied gas-grain chemical models of interstellar clouds to search for nonlinear dynamical evolution. A prescription is given for producing oscillatory solutions when a bistable solution exists in the gas-phase chemistry and we demonstrate the existence of limit cycle and relaxation oscillation solutions. As the autocatalytic chemical processes underlying these solutions are common to all models of interstellar chemistry, the occurrence of these solutions should be widespread. We briefly discuss the implications for interpreting molecular cloud composition with time-dependent models and some future directions for this approach. △ Less

Submitted 27 April, 2023; originally announced April 2023.

Comments: 9 pages, 3 figures, 1 table

Molecular Outgassing in Centaur 29P/Schwassmann-Wachmann 1 During Its Exceptional 2021 Outburst: Coordinated Multi-Wavelength Observations Using nFLASH at APEX and iSHELL at the NASA-IRTF

Authors: Nathan X. Roth, Stefanie N. Milam, Michael A. DiSanti, Geronimo L. Villanueva, Sara Faggi, Boncho P. Bonev, Martin A. Cordiner, Anthony J. Remijan, Dominique Bockelée-Morvan, Nicolas Biver, Jacques Crovisier, Dariusz C. Lis, Steven B. Charnley, Emmanuel Jehin, Eva. S. Wirström, Adam J. McKay

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 multi-wavelength spectroscopic observations of 29P/Schwassmann-Wachmann 1 using iSHELL at the NASA Infrared Telescope Facility and nFLASH at the Atacama Pathfinder EXperiment (… ▽ More 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 multi-wavelength spectroscopic observations of 29P/Schwassmann-Wachmann 1 using iSHELL at the NASA Infrared Telescope Facility 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 with 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 (3-sigma) 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. △ Less

Submitted 27 April, 2023; originally announced April 2023.

An Ice Age JWST inventory of dense molecular cloud ices

Authors: M. K. McClure, W. R. M. Rocha, K. M. Pontoppidan, N. Crouzet, L. E. U. Chu, E. Dartois, T. Lamberts, J. A. Noble, Y. J. Pendleton, G. Perotti, D. Qasim, M. G. Rachid, Z. L. Smith, Fengwu Sun, Tracy L Beck, A. C. A. Boogert, W. A. Brown, P. Caselli, S. B. Charnley, Herma M. Cuppen, H. Dickinson, M. N. Drozdovskaya, E. Egami, J. Erkal, H. Fraser , et al. (17 additional authors not shown)

Abstract: Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and composition of their atmospheres. The initial ice composition is set in the cold, dense parts of molecular clouds, prior to the onset of star formation. With the exquisite sensitivity of JWST, this critical stage of ice evolution is now acces… ▽ More Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and composition of their atmospheres. The initial ice composition is set in the cold, dense parts of molecular clouds, prior to the onset of star formation. With the exquisite sensitivity of JWST, this critical stage of ice evolution is now accessible for detailed study. Here we show the first results of the Early Release Science program "Ice Age" that reveal the rich composition of these dense cloud ices. Weak ices, including, $^{13}$CO$_2$, OCN$^-$, $^{13}$CO, OCS, and COMs functional groups are now detected along two pre-stellar lines of sight. The $^{12}$CO$_2$ ice profile indicates modest growth of the icy grains. Column densities of the major and minor ice species indicate that ices contribute between 2 and 19% of the bulk budgets of the key C, O, N, and S elements. Our results suggest that the formation of simple and complex molecules could begin early in a water-ice rich environment. △ Less

Submitted 22 January, 2023; originally announced January 2023.

Comments: To appear in Nature Astronomy on January 23rd, 2023. 33 pages, 16 figures, 3 tables; includes extended and supplemental data sections. Part of the JWST Ice Age Early Release Science program's science enabling products. Enhanced spectra downloadable on Zenodo at the following DOI: 10.5281/zenodo.7501239

Astronomical Detection of the Interstellar Anion C10H- towards TMC-1 from the GOTHAM Large Program on the GBT

Authors: Anthony Remijan, Haley N. Scolati, Andrew M. Burkhardt, P. Bryan Changala, Steven B. Charnley, Ilsa R. Cooke, Martin A. Cordiner, Harshal Gupta, Eric Herbst, Kin Long Kelvin Lee, Ryan Loomis, Christopher N. Shingledecker, Mark A. Siebert, Ci Xue, Michael C. McCarthy, Brett A. McGuire

Abstract: Using data from the GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) survey, we report the first astronomical detection of the C10H- anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C10H-. From the velocity stacked data and the matched filter response, C10H- is detected at >9σ confidence level at a column density of 4… ▽ More Using data from the GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) survey, we report the first astronomical detection of the C10H- anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C10H-. From the velocity stacked data and the matched filter response, C10H- is detected at >9σ confidence level at a column density of 4.04e11 cm-2. A dedicated search for the C10H radical was also conducted towards TMC-1. In this case, the stacked molecular emission of C10H was detected at a ~3.2σ confidence interval at a column density of 2.02e11 cm-2. However, since the determined confidence level is currently <5σ, we consider the identification of C10H as tentative. The full GOTHAM dataset was also used to better characterize the physical parameters including column density, excitation temperature, linewidth, and source size for the C4H, C6H and C8H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C10H-/C10H column density ratio is ~2.0 - the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain. △ Less

Submitted 18 January, 2023; originally announced January 2023.

Comments: 38 Pages, 24 Figures, 12 Tables, 8 Appendices

MSC Class: 85-11 ACM Class: A.1

Phosphine in the Venusian Atmosphere: A Strict Upper Limit from SOFIA GREAT Observations

Authors: M. A. Cordiner, G. L. Villanueva, H. Wiesemeyer, S. N. Milam, I. de Pater, A. Moullet, R. Aladro, C. A. Nixon, A. E. Thelen, S. B. Charnley, J. Stutzki, V. Kofman, S. Faggi, G. Liuzzi, R. Cosentino, B. A. McGuire

Abstract: The presence of phosphine (PH$_3$) in the atmosphere of Venus was reported by Greaves et al. (2021a), based on observations of the J=1-0 transition at 267 GHz using ground-based, millimeter-wave spectroscopy. This unexpected discovery presents a challenge for our understanding of Venus's atmosphere, and has led to a reappraisal of the possible sources and sinks of atmospheric phosphorous-bearing g… ▽ More The presence of phosphine (PH$_3$) in the atmosphere of Venus was reported by Greaves et al. (2021a), based on observations of the J=1-0 transition at 267 GHz using ground-based, millimeter-wave spectroscopy. This unexpected discovery presents a challenge for our understanding of Venus's atmosphere, and has led to a reappraisal of the possible sources and sinks of atmospheric phosphorous-bearing gases. Here we present results from a search for PH$_3$ on Venus using the GREAT instrument aboard the SOFIA aircraft, over three flights conducted in November 2021. Multiple PH$_3$ transitions were targeted at frequencies centered on 533 GHz and 1067 GHz, but no evidence for atmospheric PH$_3$ was detected. Through radiative transfer modeling, we derived a disk-averaged upper limit on the PH$_3$ abundance of 0.8 ppb in the altitude range 75-110 km, which is more stringent than previous ground-based studies. △ Less

Submitted 24 October, 2022; originally announced October 2022.

Comments: Accepted for publication in Geophysical Research Letters

The Detection of Deuterated Water in the Large Magellanic Cloud with ALMA

Authors: Marta Sewiło, Agata Karska, Lars E. Kristensen, Steven B. Charnley, C. -H. Rosie Chen, Joana M. Oliveira, Martin Cordiner, Jennifer Wiseman, Álvaro Sánchez-Monge, Jacco Th. van Loon, Remy Indebetouw, Peter Schilke, Emmanuel Garcia-Berrios

Abstract: We report the first detection of deuterated water (HDO) toward an extragalactic hot core. The HDO 2$_{11}$-2$_{12}$ line has been detected toward hot cores N105-2A and 2B in the N105 star-forming region in the low-metallicity Large Magellanic Cloud (LMC) dwarf galaxy with the Atacama Large Millimeter/submillimeter Array (ALMA). We have compared the HDO line luminosity ($L_{\rm HDO}$) measured towa… ▽ More We report the first detection of deuterated water (HDO) toward an extragalactic hot core. The HDO 2$_{11}$-2$_{12}$ line has been detected toward hot cores N105-2A and 2B in the N105 star-forming region in the low-metallicity Large Magellanic Cloud (LMC) dwarf galaxy with the Atacama Large Millimeter/submillimeter Array (ALMA). We have compared the HDO line luminosity ($L_{\rm HDO}$) measured toward the LMC hot cores to those observed toward a sample of seventeen Galactic hot cores covering three orders of magnitude in $L_{\rm HDO}$, four orders of magnitude in bolometric luminosity ($L_{\rm bol}$), and a wide range of Galactocentric distances (thus metallicities). The observed values of $L_{\rm HDO}$ for the LMC hot cores fit very well into the $L_{\rm HDO}$ trends with $L_{\rm bol}$ and metallicity observed toward the Galactic hot cores. We have found that $L_{\rm HDO}$ seems to be largely dependent on the source luminosity, but metallicity also plays a role. We provide a rough estimate of the H$_2$O column density and abundance ranges toward the LMC hot cores by assuming that HDO/H$_2$O toward the LMC hot cores is the same as that observed in the Milky Way; the estimated ranges are systematically lower than Galactic values. The spatial distribution and velocity structure of the HDO emission in N105-2A is consistent with HDO being the product of the low-temperature dust grain chemistry. Our results are in agreement with the astrochemical model predictions that HDO is abundant regardless of the extragalactic environment and should be detectable with ALMA in external galaxies. △ Less

Submitted 9 May, 2022; originally announced May 2022.

Comments: 21 pages, 2 tables, 9 figures (including appendices); Accepted for publication in the Astrophysical Journal

A Search for Heterocycles in GOTHAM Observations of TMC-1

Authors: Timothy J. Barnum, Mark A. Siebert, Kin Long Kelvin Lee, Ryan A. Loomis, P. Bryan Changala, Steven B. Charnley, Madelyn L. Sita, Ci Xue, Anthony J. Remijan, Andrew M. Burkhardt, Brett A. McGuire, Ilsa R. Cooke

Abstract: We have conducted an extensive search for nitrogen-, oxygen- and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon… ▽ More We have conducted an extensive search for nitrogen-, oxygen- and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection. △ Less

Submitted 10 April, 2022; originally announced April 2022.

Comments: Accepted in the Journal of Physical Chemistry A

The Isotopic Links from Planet Forming Regions to the Solar System

Authors: H. Nomura, K. Furuya, M. A. Cordiner, S. B. Charnley, C. M. O'D. Alexander, C. A. Nixon, V. V. Guzman, H. Yurimoto, T. Tsukagoshi, T. Iino

Abstract: Isotopic ratios provide a powerful tool for understanding the origins of materials, including the volatile and refractory matter within solar system bodies. Recent high sensitivity observations of molecular isotopologues, in particular with ALMA, have brought us new information on isotopic ratios of hydrogen, carbon, nitrogen and oxygen in star and planet forming regions as well as the solar syste… ▽ More Isotopic ratios provide a powerful tool for understanding the origins of materials, including the volatile and refractory matter within solar system bodies. Recent high sensitivity observations of molecular isotopologues, in particular with ALMA, have brought us new information on isotopic ratios of hydrogen, carbon, nitrogen and oxygen in star and planet forming regions as well as the solar system objects. Solar system exploration missions, such as Rosetta and Cassini, have given us further new insights. Meanwhile, the recent development of sophisticated models for isotope chemistry including detailed gas-phase and grain surface reaction network has made it possible to discuss how isotope fractionation in star and planet forming regions is imprinted into the icy mantles of dust grains, preserving a record of the initial isotopic state of solar system materials. This chapter reviews recent progress in observations of molecular isotopologues in extra-solar planet forming regions, prestellar/protostellar cores and protoplanetary disks, as well as objects in our solar system -- comets, meteorites, and planetary/satellite atmospheres -- and discusses their connection by means of isotope chemical models. △ Less

Submitted 21 March, 2022; originally announced March 2022.

Comments: To appear in Protostars and Planets VII, University of Arizona Press, eds. Shu-ichiro Inutsuka, Yuri Aikawa, Takayuki Muto, Kengo Tomida, and Motohide Tamura

A SUBLIME 3D Model for Cometary Coma Emission: the Hypervolatile-Rich Comet C/2016 R2 (PanSTARRS)

Authors: M. A. Cordiner, I. M. Coulson, E. Garcia-Berrios, C. Qi, F. Lique, M. Zoltowski, M. de Val-Borro, Y. -J. Kuan, W. -H. Ip, S. Mairs, N. X. Roth, S. B. Charnley, S. N. Milam, W. -L Tseng, Y. -L Chuang

Abstract: 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 N2+/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 resoluti… ▽ More 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 N2+/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 (JCMT) and Submillimeter Array (SMA) to elucidate the outgassing behaviour of C/2016 R2. Results are analyzed using a new, time-dependent, three dimensional radiative transfer code (SUBLIME), 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 $(3.8-7.6)\times10^{28}$ s$^{-1}$ between 2018-01-13 and 2018-02-01, with a mean value of $(5.3\pm0.6)\times10^{28}$ s$^{-1}$ at r_H = 2.8-2.9 au. The emission is concentrated in a near-sunward jet, with an outflow velocity $0.51\pm0.01$ km/s, compared to $0.25\pm0.01$ km/s in the ambient (and night-side) coma. Evidence was also found for an extended source of CO emission, possibly due to icy grain sublimation around $1.2\times10^5$ km 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. △ Less

Submitted 23 February, 2022; originally announced February 2022.

Comments: Accepted for publication in ApJ

ALMA Observations of Molecular Complexity in the Large Magellanic Cloud: The N105 Star-Forming Region

Authors: Marta Sewiło, Martin Cordiner, Steven B. Charnley, Joana M. Oliveira, Emmanuel Garcia Berrios, Peter Schilke, Jacob L. Ward, Jennifer Wiseman, Remy Indebetouw, Kazuki Tokuda, Jacco Th. van Loon, Álvaro Sánchez-Monge, Veronica Allen, C. -H. Rosie Chen, Roya Hamedani Golshan, Agata Karska, Lars E. Kristensen, Stan E. Kurtz, Toshikazu Onishi, Sarolta Zahorecz

Abstract: The Large Magellanic Cloud (LMC) is the nearest laboratory for detailed studies on the formation and survival of complex organic molecules (COMs), including biologically important ones, in low-metallicity environments--typical for earlier cosmological epochs. We report the results of 1.2 mm continuum and molecular line observations of three fields in the star-forming region N105 with the Atacama L… ▽ More The Large Magellanic Cloud (LMC) is the nearest laboratory for detailed studies on the formation and survival of complex organic molecules (COMs), including biologically important ones, in low-metallicity environments--typical for earlier cosmological epochs. We report the results of 1.2 mm continuum and molecular line observations of three fields in the star-forming region N105 with the Atacama Large Millimeter/submillimeter Array (ALMA). N105 lies at the western edge of the LMC bar with on-going star formation traced by H$_2$O, OH, and CH$_3$OH masers, ultracompact H II regions, and young stellar objects. Based on the spectral line modeling, we estimated rotational temperatures, column densities, and fractional molecular abundances for twelve 1.2 mm continuum sources. We identified sources with a range of chemical make-ups, including two bona fide hot cores and four hot core candidates. The CH$_3$OH emission is widespread and associated with all the continuum sources. COMs CH$_3$CN and CH$_3$OCH$_3$ are detected toward two hot cores in N105 together with smaller molecules typically found in Galactic hot cores (e.g., SO$_2$, SO, and HNCO) with the molecular abundances roughly scaling with metallicity. We report a tentative detection of the astrobiologically relevant formamide molecule (NH$_2$CHO) toward one of the hot cores; if confirmed, this would be the first detection of NH$_2$CHO in an extragalactic sub-solar metallicity environment. We suggest that metallicity inhomogeneities resulting from the tidal interactions between the LMC and the Small Magellanic Cloud (SMC) might have led to the observed large variations in COM abundances in LMC hot cores. △ Less

Submitted 24 January, 2022; originally announced January 2022.

Comments: 75 pages, 48 figures, 7 tables (including appendices); Accepted for publication in the Astrophysical Journal

Thermal desorption of interstellar ices. A review on the controlling parameters and their implications fromsnowlines to chemical complexity

Authors: Marco Minissale, Yuri Aikawa, Edwin Bergin, M. Bertin, Wendy A. Brown, Stephanie Cazaux, Steven B. Charnley, Audrey Coutens, Herma M. Cuppen, Victoria Guzman, Harold Linnartz, Martin R. S. McCoustra, Albert Rimola, Johanna G. M. Schrauwen, Celine Toubin, Piero Ugliengo, Naoki Watanabe, Valentine Wakelam, Francois Dulieu

Abstract: The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, that, in turn, is determined by the physico-chemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particles adsorption and desorption). Gas-grain and grain-gas transitions as well as formation and sublimation o… ▽ More The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, that, in turn, is determined by the physico-chemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particles adsorption and desorption). Gas-grain and grain-gas transitions as well as formation and sublimation of interstellar ices are thus essential elements of understanding astrophysical observations of cold environments (e.g., pre-stellar cores) where unexpected amounts of a large variety of chemical species have been observed in the gas phase. Adsorbed atoms and molecules also undergo chemical reactions which are not efficient in the gas phase. Therefore, the parameterization of the physical properties of atoms and molecules interacting with dust grain particles is clearly a key aspect to interpret astronomical observations and to build realistic and predictive astrochemical models. In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways towards molecular complexity and define the location of snowlines, which ultimately influence the planet formation process. We review different crucial aspects of desorption parameters both from a theoretical and experimental point of view. We critically assess the desorption parameters commonly used in the astrochemical community for astrophysical relevant species and provide tables with recommended values. In addition, we show that a non-trivial determination of the pre-exponential factor nu using the Transition State Theory can affect the binding energy value. Finally, we conclude this work by discussing the limitations of theoretical and experimental approaches currently used to determine the desorption properties with suggestions for future improvements. △ Less

Submitted 19 January, 2022; originally announced January 2022.

Comments: 115 pages, 17 figures

Journal ref: ACS Earth and Space Science, 2022

Detection of Dynamical Instability in Titan's Thermospheric Jet

Authors: M. A. Cordiner, E. Garcia-Berrios, R. G. Cosentino, N. A. Teanby, C. E. Newman, C. A. Nixon, A. E. Thelen, S. B. Charnley

Abstract: Similar to Earth, Saturn's largest moon, Titan, possesses a system of high-altitude zonal winds (or jets) that encircle the globe. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in August 2016, Lellouch et al. (2019) discovered an equatorial jet at much higher altitudes than previously known, with a surprisingly fast speed of up to ~340 m/s, but the origin of such high velocities is… ▽ More Similar to Earth, Saturn's largest moon, Titan, possesses a system of high-altitude zonal winds (or jets) that encircle the globe. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in August 2016, Lellouch et al. (2019) discovered an equatorial jet at much higher altitudes than previously known, with a surprisingly fast speed of up to ~340 m/s, but the origin of such high velocities is not yet understood. We obtained spectrally and spatially resolved ALMA observations in May 2017 to map Titan's 3D global wind field and compare our results with a reanalysis of the August 2016 data. Doppler wind velocity maps were derived in the altitude range ~300-1000 km (from the upper stratosphere to the thermosphere). At the highest, thermospheric altitudes, a 47% reduction in the equatorial zonal wind speed was measured over the 9-month period (corresponding to L_s = 82-90 degrees on Titan). This is interpreted as due to a dramatic slowing and loss of confinement (broadening) of the recently-discovered thermospheric equatorial jet, as a result of dynamical instability. These unexpectedly-rapid changes in the upper-atmospheric dynamics are consistent with strong variability of the jet's primary driving mechanism. △ Less

Submitted 3 June, 2021; originally announced June 2021.

Journal ref: Astrophysical Journal Letters, 2020, Volume 904, id.L12

Detection of Interstellar H$_2$CCCHC$_3$N

Authors: C. N. Shingledecker, K. L. K. Lee, J. T. Wandishin, N. Balucani, A. M. Burkhardt, S. B. Charnley, R. Loomis, M. Schreffler, M. Siebert, M. C. McCarthy, B. A. McGuire

Abstract: The chemical pathways linking the small organic molecules commonly observed in molecular clouds to the large, complex, polycyclic species long-suspected to be carriers of the ubiquitous unidentified infrared emission bands remain unclear. To investigate whether the formation of mono- and poly-cyclic molecules observed in cold cores could form via the bottom-up reaction of ubiquitous carbon-chain s… ▽ More The chemical pathways linking the small organic molecules commonly observed in molecular clouds to the large, complex, polycyclic species long-suspected to be carriers of the ubiquitous unidentified infrared emission bands remain unclear. To investigate whether the formation of mono- and poly-cyclic molecules observed in cold cores could form via the bottom-up reaction of ubiquitous carbon-chain species with, e.g. atomic hydrogen, a search is made for possible intermediates in data taken as part of the GOTHAM (GBT Observations of TMC-1 Hunting for Aromatic Molecules) project. Markov-Chain Monte Carlo (MCMC) Source Models were run to obtain column densities and excitation temperatures. Astrochemical models were run to examine possible formation routes, including a novel grain-surface pathway involving the hydrogenation of C$_6$N and HC$_6$N, as well as purely gas-phase reactions between C$_3$N and both propyne (CH$_3$CCH) and allene (CH$_2$CCH$_2$), as well as via the reaction CN + H$_2$CCCHCCH. We report the first detection of cyanoacetyleneallene (H$_2$CCCHC$_3$N) in space toward the TMC-1 cold cloud using the Robert C. Byrd 100 m Green Bank Telescope (GBT). Cyanoacetyleneallene may represent an intermediate between less-saturated carbon-chains, such as the cyanopolyynes, that are characteristic of cold cores and the more recently-discovered cyclic species like cyanocyclopentadiene. Results from our models show that the gas-phase allene-based formation route in particular produces abundances of H$_2$CCCHC$_3$N that match the column density of $2\times10^{11}$ cm$^{-2}$ obtained from the MCMC Source Model, and that the grain-surface route yields large abundances on ices that could potentially be important as precursors for cyclic molecules. △ Less

Submitted 7 May, 2021; originally announced May 2021.

Comments: Accepted in A&A Letters

Discovery of the Pure Polycyclic Aromatic Hydrocarbon Indene ($c$-C$_9$H$_8$) with GOTHAM Observations of TMC-1

Authors: Andrew M. Burkhardt, Kin Long Kelvin Lee, P. Bryan Changala, Christopher N. Shingledecker, Ilsa R. Cooke, Ryan A. Loomis, Hongji Wei, Steven B. Charnley, Eric Herbst, Michael C. McCarthy, Brett A. McGuire

Abstract: Polycyclic Aromatic Hydrocarbons (PAHs) have long been invoked in the study of interstellar and protostellar sources, but the unambiguous identification of any individual PAH has proven elusive until very recently. As a result, the formation mechanisms for this important class of molecules remain poorly constrained. Here we report the first interstellar detection of a pure hydrocarbon PAH, indene… ▽ More Polycyclic Aromatic Hydrocarbons (PAHs) have long been invoked in the study of interstellar and protostellar sources, but the unambiguous identification of any individual PAH has proven elusive until very recently. As a result, the formation mechanisms for this important class of molecules remain poorly constrained. Here we report the first interstellar detection of a pure hydrocarbon PAH, indene (C$_9$H$_8$), as part of the GBT Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey. This detection provides a new avenue for chemical inquiry, complementing the existing detections of CN-functionalized aromatic molecules. From fitting the GOTHAM observations, indene is found to be the most abundant organic ring detected in TMC-1 to date. And from astrochemical modeling with NAUTILUS, the observed abundance is greater than the model's prediction by several orders of magnitude suggesting that current formation pathways in astrochemical models are incomplete. The detection of indene in relatively high abundance implies related species such as cyanoindene, cyclopentadiene, toluene, and styrene may be detectable in dark clouds. △ Less

Submitted 27 May, 2021; v1 submitted 30 April, 2021; originally announced April 2021.

Comments: 18 pages, 2 figures and 3 tables in the main text. 1 table and 1 figure in the Appendix. Published in The Astrophysical Journal Letters. Supplementary data available in the DataVerse entry provided in text

Journal ref: ApJL 2021, 913 L18

Neutral-Neutral Synthesis of Organic Molecules in Cometary Comae

Authors: M. A Cordiner, S. B. Charnley

Abstract: Remote and in-situ observations of cometary gases have revealed the presence of a wealth of complex organic molecules, including carbon chains, alcohols, imines and the amino acid glycine. Such chemical complexity in cometary material implies that impacts by comets could have supplied reagents for prebiotic chemistry to young planetary surfaces. However, the assumption that some of the molecules o… ▽ More Remote and in-situ observations of cometary gases have revealed the presence of a wealth of complex organic molecules, including carbon chains, alcohols, imines and the amino acid glycine. Such chemical complexity in cometary material implies that impacts by comets could have supplied reagents for prebiotic chemistry to young planetary surfaces. However, the assumption that some of the molecules observed in cometary comae at millimetre wavelengths originate from ices stored inside the nucleus has not yet been proven. In fact, the comae of moderately-active comets reach sufficient densities within a few thousand kilometers of the nucleus for an active (solar radiation-driven) photochemistry to ensue. Here we present results from our latest chemical-hydrodynamic models incorporating an updated reaction network, and show that the commonly-observed HC3N (cyanoacetylene) and NH2CHO (formamide) molecules can be efficiently produced in cometary comae as a result of two-body, neutral-neutral, gas-phase reactions involving well-known coma gases. In the presence of a near-nucleus distributed source of CN (similar to that observed by the Rosetta spacecraft at comet 67P), we find that sufficient HC$_3$N and NH2CHO can be synthesized to match the abundances of these molecules in previous observations of Oort Cloud comets. The precise origin of these (and other) complex organic molecules in cometary comae can be verified through interferometric mapping observations, for example, using the Atacama Large Millimeter/submillimeter Array (ALMA). △ Less

Submitted 15 April, 2021; originally announced April 2021.

Comments: Accepted for publication in MNRAS, April 15th 2021

Leveraging the ALMA Atacama Compact Array for Cometary Science: An Interferometric Survey of Comet C/2015 ER61 (PanSTARRS) and Evidence for a Distributed Source of Carbon Monosulfide

Authors: Nathan X. Roth, Stefanie N. Milam, Martin A. Cordiner, Dominique Bockelée-Morvan, Nicolas Biver, Jérémie Boissier, Dariusz C. Lis, Anthony J. Remijan, Steven B. Charnley

Abstract: We report the first survey of molecular emission from cometary volatiles using standalone Atacama Compact Array (ACA) observations of the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2015 ER61 (PanSTARRS) carried out on UT 2017 April 11 and 15, shortly after its April 4 outburst. These measurements of HCN, CS, CH$_3$OH, H$_2$CO, and HNC (along with continuum emission from dus… ▽ More We report the first survey of molecular emission from cometary volatiles using standalone Atacama Compact Array (ACA) observations of the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2015 ER61 (PanSTARRS) carried out on UT 2017 April 11 and 15, shortly after its April 4 outburst. These measurements of HCN, CS, CH$_3$OH, H$_2$CO, and HNC (along with continuum emission from dust) probed the inner coma of C/2015 ER61, revealing asymmetric outgassing and discerning parent from daughter/distributed source species. This work presents spectrally integrated flux maps, autocorrelation spectra, production rates, and parent scale lengths for each molecule, and a stringent upper limit for CO. HCN is consistent with direct nucleus release in C/2015 ER61, whereas CS, H$_2$CO, HNC, and potentially CH$_3$OH are associated with distributed sources in the coma. Adopting a Haser model, parent scale lengths determined for H$_2$CO (L$_p$ $\sim$ 2200 km) and HNC (L$_p$ $\sim$ 3300 km) are consistent with previous work in comets, whereas significant extended source production (L$_p$ $\sim$ 2000 km) is indicated for CS, suggesting production from an unknown parent in the coma. The continuum presents a point-source distribution, with a flux density implying an excessively large nucleus, inconsistent with other estimates of the nucleus size. It is best explained by the thermal emission of slowly-moving outburst ejectas, with total mass 5--8 $\times$ 10$^{10}$ kg. These results demonstrate the power of the ACA for revealing the abundances, spatial distributions, and locations of molecular production for volatiles in moderately bright comets such as C/2015 ER61. △ Less

Submitted 25 October, 2021; v1 submitted 7 April, 2021; originally announced April 2021.

Rapidly Varying Anisotropic Methanol (CH$_3$OH) Production in the Inner Coma of Comet 46P/Wirtanen as Revealed by the ALMA Atacama Compact Array

Authors: Nathan X. Roth, Stefanie N. Milam, Martin A. Cordiner, Dominique Bockelée-Morvan, Michael A. DiSanti, Jérémie Boissier, Nicolas Biver, Jacques Crovisier, Neil Dello Russo, Boncho P. Bonev, Chunhua Qi, Anthony J. Remijan, Steven B. Charnley, Erika L. Gibb, Miguel de Val-Borro, Emmanuël Jehin

Abstract: We report the detection of CH$_3$OH emission in comet 46P/Wirtanen on UT 2018 December 8 and 9 using the Atacama Compact Array (ACA), part of the Atacama Large Millimeter/Submillimeter Array (ALMA). These interferometric measurements of CH$_3$OH along with continuum emission from dust probed the inner coma ($<$2000 km from the nucleus) of 46P/Wirtanen approximately one week before its closest appr… ▽ More We report the detection of CH$_3$OH emission in comet 46P/Wirtanen on UT 2018 December 8 and 9 using the Atacama Compact Array (ACA), part of the Atacama Large Millimeter/Submillimeter Array (ALMA). These interferometric measurements of CH$_3$OH along with continuum emission from dust probed the inner coma ($<$2000 km from the nucleus) of 46P/Wirtanen approximately one week before its closest approach to Earth ($Δ$ = 0.089 -- 0.092 au), revealing rapidly varying and anisotropic CH$_3$OH outgassing during five separate ACA executions between UT 23:57 December 7 and UT 04:55 December 9, with a clear progression in the spectral line profiles over a timescale of minutes. We present spectrally integrated flux maps, production rates, rotational temperatures, and spectral line profiles of CH$_3$OH during each ACA execution. The variations in CH$_3$OH outgassing are consistent with Wirtanen's 9 hr nucleus rotational period derived from optical and millimeter wavelength measurements and thus are likely coupled to the changing illumination of active sites on the nucleus. The consistent blue offset of the line center indicates enhanced CH$_3$OH sublimation from the sunward hemisphere of the comet, perhaps from icy grains. These results demonstrate the exceptional capabilities of the ACA for time-resolved measurements of comets such as 46P/Wirtanen. △ Less

Submitted 11 March, 2021; originally announced March 2021.

Interstellar Detection of 2-Cyanocyclopentadiene, C$_5$H$_5$CN, a Second Five-Membered Ring Toward TMC-1

Authors: Kin Long Kelvin Lee, P. Bryan Changala, Ryan A. Loomis, Andrew M. Burkhardt, Ci Xue, Martin A. Cordiner, Steven B. Charnley, Michael C. McCarthy, Brett A. McGuire

Abstract: Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in the dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional observations of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used a combination of spectral stacking, Markov chain Monte Carlo (MC… ▽ More Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in the dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional observations of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used a combination of spectral stacking, Markov chain Monte Carlo (MCMC), and matched filtering techniques to detect 2-cyanocyclopentadiene, a low-lying isomer of 1-cyanocyclopentadiene, which was recently discovered there by the same methods. The new observational data also yields a considerably improved detection significance for the more stable isomer and evidence for several individual transitions between 23 - 32 GHz. Through our MCMC analysis, we derive total column densities of $8.3\times10^{11}$ and $1.9\times10^{11}$ cm$^{-2}$ for 1- and 2-cyanocyclopentadiene respectively, corresponding to a ratio of 4.4(6) favoring the former. The derived abundance ratios point towards a common formation pathway - most likely being cyanation of cyclopentadiene by analogy to benzonitrile. △ Less

Submitted 18 February, 2021; originally announced February 2021.

Comments: 6 pages, 3 figures and 1 table in the main text. 2 tables and 2 figures in the Appendix. Accepted for publication in The Astrophysical Journal Letters. Supplementary data available in the DataVerse entry provided in text