Highlights from Faraday Discussion: Astrochemistry at high resolution, Baltimore, USA, May 2023

Abstract

The Faraday Discussion ‘Astrochemistry at high resolution’ was held at the Space Telescope Science Institute, Baltimore, United States, and online from May 31–June 2, 2023. The meeting brought together observers, modellers, and experimentalists at different career stages and from different countries to discuss advancements in astrochemistry resulting from improved spatial resolution, spectral resolution, and sensitivity. This conference report provides highlights of the meeting and summaries of the talks presented.

Introduction

Molecules are ubiquitous in the interstellar medium (ISM), and observing them is pivotal for understanding the chemistry of our universe from the formation of stars in cold, dense interstellar clouds to planet formation and evolved planetary systems. Infant star systems bear witness to the incredible chemical richness of the ISM. More than 300 different molecules, including organic and potentially prebiotic species, have thus far been detected in these environments, especially at infrared wavelengths and radio frequencies. Over the last several decades, the advent of new space- and ground-based telescopes, with ever-increasing performance capabilities, has brought us into a new era of astrochemistry. These advances are exemplified by observing facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST), both of which enable molecular observations with unprecedented spatial resolution, spectral resolution, and sensitivity. These new instruments don't just bring more information, but also more questions. Such an influx of questions requires the broader astrochemistry community—including observers, theoreticians, computational modellers, and experimentalists—to work together and shed light on them.

In this context, astrochemists from different research areas, countries, and career stages convened at the Space Telescope Science Institute (STScI, Fig. 1) in Baltimore, Maryland, USA, between May 31 and June 2, 2023, to participate in the Faraday Discussion on ‘Astrochemistry at high resolution’. After many years of planning, and despite postponements due to JWST launch delays and the COVID-19 pandemic, this celebration of the advances in astrochemistry methods was timed perfectly, beginning six weeks prior to the one-year anniversary of the first JWST image release.

Fig. 1 STScI logo welcoming delegates to the Faraday Discussion on the campus of Johns Hopkins University.

The Faraday Discussion provided a unique platform to ask very basic questions across different disciplines and, at the same, very advanced questions within one's discipline. In our experience, such exchanges are not common in the general discussion of other scientific meetings, likely because question-and-answer portions of symposia are typically limited to a mere five minutes. The format of the Faraday Discussion in which a set of talks is followed by a discussion time of approximately 25 minutes per speaker, provided ample time for conversations about a variety of topics in astrochemistry engaging the whole delegation in attendance. Furthermore, the discussion facilitated conversations across different communities within astrochemistry—laboratory scientists, modellers, and observers—enhancing the general discussion with broader perspectives. In this way, possibilities for follow-up research and cross-disciplinary investigations came as a natural part of the meeting. In addition, the meeting format allowed space to have discussions about aspects that might not be typically written about in detail in a manuscript (e.g., the details of control experiments or “failed” investigations).

Finally, interactions between established and early-career astrochemists were central to the Faraday Discussion. The meeting hosted delegates spanning all career stages—from students to academics near retirement—and it seemed participants generally treated other delegates as colleagues and equals. It was also noteworthy that, during the general discussions, many conversations were initiated by more junior members of the community. Beyond the papers presented and the discussions that followed, the meeting comprised a lively poster session, conference dinner, and connections over coffee breaks.

This perspective attempts to consolidate the key themes and what we found to be the most interesting aspects of each session of this Faraday Discussion on Astrochemistry at high resolution. The meeting began with the Introductory Lecture and Session 1: (“Observational astrochemistry in the age of ALMA, NOEMA, JWST and beyond!”) on 31 May. The second day of the meeting encompassed laboratory astrochemistry in Sessions 2 (“Laboratory astrochemistry of the gas phase”) and 3 (“Laboratory astrochemistry of and on dust and ices”). The meeting concluded late in the morning of 2 June with Session 4 (“Computational astrochemistry”) and the Concluding remarks.

Introductory lecture

Following the opening remarks by Martin McCoustra, who chaired the meeting, came the Spiers Memorial Lecture. This honour was bestowed upon Cecilia Ceccarelli of Université Grenoble Alpes (France). In her lecture (Fig. 2), Ceccarelli highlighted the progress toward high-resolution science in the context of both spectral and spatial resolution (DOI: https://doi.org/10.1039/d3fd00106g). She emphasized the contextual importance of an observation's frequency, showing how the distribution of methanol emission in the protostar NGC1333 IRAS4A depends on the telescopes, and thus frequencies, used.

Fig. 2 Delegates listening to Dr Cecilia Ceccarelli's Spiers Memorial Lecture at STScI.

Ceccarelli also gave meeting delegates a crash course in stellar evolution, focusing on interstellar complex organic molecules, which are typically defined as having six or more atoms. She talked about the surprising presence of gas-phase complex organics in prestellar cores despite having cold temperatures around 10 K, the “fountains of interstellar complex organic molecules” observed in protostars, and the refrigeration of these organics in protoplanetary disks. In particular, Ceccarelli described hot corinos, cores of low-mass protostars, as mines of complex organics.

To understand the chemistry in hot corinos (and, in the context of massive star formation, hot cores), a combination of observations and models is needed. Ceccarelli described the advances and challenges in formulating and employing astrochemical models, especially in the context of coupling the gas and dust grain chemistry. Fortunately, progress in improving the spectral and spatial resolution of astrochemical research methods continues to bring us closer to definitively understanding the complex chemical networks that are found in even the earliest stages of star formation.

Session 1: Observational astrochemistry in the age of ALMA, NOEMA, JWST and beyond!

The first session of the meeting began with presentations of three papers about JWST research. Ewine van Dishoeck of Leiden University (The Netherlands) and Inga Kamp of the University of Groningen (The Netherlands) presented the first results of two surveys of young stellar objects using JWST's Mid-InfraRed Instrument (MIRI): JWST Observations of Young protoStars (JOYS) and MIRI INfrared Disk Survey (MINDS). Only a small portion of the surveys' targets have been observed so far, but these programs have already given us some exciting results. For example, the first observed target of the JOYS program, of which van Dishoeck is a principal investigator (PI), yielded new hydrocarbon species, namely diacetylene (C₄H₂) and benzene (C₆H₆), that had not been previously detected in a circumstellar disk (DOI: https://doi.org/10.1039/d3fd00010a). From only a dozen of its ∼50 disk targets, the MINDS survey, of which Kamp is a PI, demonstrated a surprising diversity in disk chemistry, even among stars of the same class (e.g., T Tauri stars, DOI: https://doi.org/10.1039/d3fd00013c). Both programs leverage a combination of JWST's spatial resolution, sensitivity, and—in particular—spectral resolution that are one-to-two orders of magnitude higher than previous state-of-the-art infrared (IR) facilities. With the first results presented at the Faraday Discussion, van Dishoeck and Kamp demonstrated JWST's promise of changing how astronomers understand the innermost parts of protoplanetary disks where most planets are thought to form.

In addition to revealing the chemistry of disks, JWST is expected to impact how we understand the chemistry of already-formed planets. Between the two talks introducing early disk science with JWST, Nikku Madhusudhan of Cambridge University (UK) presented theoretical work exploring the chemistry of Hycean worlds, whether chemistry on such exoplanets could be conducive to life, and the potential for JWST to observe this chemistry, especially given the telescope's superior spectral resolution (DOI: https://doi.org/10.1039/d3fd00075c). The models presented gave delegates an exciting glimpse into some of the exoplanetary chemistry that might be revealed with future JWST observations, but it was a linguistics lesson that evoked a collective gasp of delight from audience members, whose backgrounds were mostly in interstellar and solar system chemistry. Madhusudhan explained that ‘hycean’ is pronounced HI-shin, not hi-SEE-en as most delegates had apparently believed, because it is a portmanteau of hydrogen—describing these planets' atmospheres—and ocean—referring to the liquid water on their surfaces.

The second half of Session 1 focused on papers using ALMA data to look at chemistry in various celestial environments. While spectral resolution is indeed important for astrochemical observations with ALMA, the high spatial resolution is of particular importance for the ALMA observations presented. Eleonora Bianchi of the Excellence Cluster ORIGINS (Germany) presented observations of chemical differentiation—as seen by high-resolution observations of HDO, SO₂, and NH₂CHO—in SVS13-A, a protobinary star system about 300 pc away in Perseus (DOI: https://doi.org/10.1039/d3fd00018d). Going beyond the local interstellar medium, Olivia Wilkins from NASA Goddard Space Flight Center (USA) used the Atacama Compact Array (ACA), a subset of the full ALMA, to conduct a pilot survey of the chemistry in previously unexplored massive star-forming regions in the so-called molecular ring, which spans radii of 4-8 kpc from the galactic center (DOI: https://doi.org/10.1039/d3fd00003f). Ko-Yun Huang of Leiden University (The Netherlands) took delegates even farther away from the solar system into two nearby galaxies to explore large-scale shocks as seen via SiO, HNCO, and CH₃OH emission (DOI: https://doi.org/10.1039/d3fd00007a).

Each of these ALMA papers uses maps of molecular emission to gather clues about the physical environments in the targeted sources, but because they explore different structures across a range of heliocentric distances, the meaning of ‘high resolution’ varies as well. Bianchi described maps with ∼0.2′′ angular resolution, which corresponds to a linear resolution of about 60 au in Perseus, enough to resolve the two components of SVS13-A. For Wilkins, an angular resolution of ∼5′′ is high relative to previous observations, which largely looked at molecular cloud scales; in the molecular-ring sources, this corresponds to linear resolutions of 10⁴–10⁵ au, enough to differentiate between individual dense clumps. In the extragalactic sources presented by Huang, ‘high resolution’ describes spatial resolution on linear scales of ∼28 and ∼56 pc, comparable to the sizes of (giant) molecular clouds. Thus, this set of papers demonstrated how ALMA can be used to probe the chemistry and physical environment across a range of astronomical scales.

Session 2: Laboratory astrochemistry of the gas phase

The second session of the meeting covered recent updates from gas-phase laboratory studies using a wide variety of techniques from researchers across the United States and Europe. Sandra Brünken discussed the unimolecular dissociation chemistry of aromatic molecules. New spectroscopic investigations were presented by Ugo Jacovella, Divita Gupta, Cristina Puzzarini, and Adam Fleisher. Kevin Douglas, Arthur Suits, and Nadia Balucani highlighted the importance of understanding low-temperature reaction kinetics. Polycyclic aromatic hydrocarbons (PAHs), well known to be ubiquitous in the interstellar medium, were covered by Mark Stockett and Alexander Lemmens.

Brünken from Radboud University (The Netherlands) opened the session with results on dissociative ionization of pyridine and benzonitrile (DOI: https://doi.org/10.1039/d3fd00015j). The fragmentation products, identified using infrared action spectroscopy at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory, shed light on fragmentation pathways and the implication of using these products as proxies for their cyclic parent molecules. The next two talks covered low-temperature reaction studies using pulsed Laval nozzle setups as well as theoretical modeling to allow for a complete understanding of the reaction mechanisms. Suits, of the University of Missouri (USA) used near-infrared cavity ringdown spectroscopy to demonstrate that the reaction of the vibrationally excited CN (ν = 1) radical with both isomers of butadiene (C₄H₆) remains fast, on the order of 10⁻¹⁰ cm³ s⁻¹, at temperatures down to 70 K (DOI: https://doi.org/10.1039/d3fd00029j). Douglas, of the University of Leeds (UK) focused on a different radical, NH₂, with acetaldehyde (CH₃CHO) and obtained kinetics for the reaction using pulsed laser photolysis-laser induced fluorescence (DOI: https://doi.org/10.1039/d3fd00046j). Jacovella, of the Université Paris-Saclay (France) then showed recent results on the first microwave spectra of norbornadiene (C₇H₈), a highly photostable bridged molecule, and two of its cyano derivatives (DOI: https://doi.org/10.1039/d3fd00016h). While a search for these molecules in the Taurus Molecular Cloud (TMC-1) was unsuccessful, these gas-phase laboratory measurements bring an opportunity to search for constituents of a new category of bridged species in the ISM.

The second half of the morning session started with Gupta of the Universität zu Köln (Germany) presenting a new leak-out spectroscopic technique to measure high-resolution spectra of ionic species. The rovibrational and rotational spectrum for c—C₃H₂D⁺ provided in this work allows for searches of this cation using both ground-based radio telescopes and JWST (DOI: https://doi.org/10.1039/d3fd00068k). Puzzarini, of the University of Bologna (Italy) and colleagues employed flash vacuum pyrolysis to produce a variety of unstable species like radicals, ions, and unsaturated carbon chains and measure their rotational spectra (DOI: https://doi.org/10.1039/d3fd00052d). By exploiting quantum chemical calculations to guide spectral analysis and assignment, they provide essential information for setting up accurate line catalogs for astronomical searches. Balucani, of the Università degli Studi di Perugia (Italy) continued the discussion on the reaction kinetics of neutral radical species with results on N(²D) with benzene using crossed molecular beams under single-collision conditions (https://doi.org/10.1039/d3fd00057e). The experimental work supported by theoretical calculations provides important updates for the photochemical model of the atmosphere of Titan, the largest moon of Saturn.

The post-lunch session of Day 2 began with Stockett of Stockholm University (Sweden) addressing the challenge of current astrochemical models to reproduce the observed abundances of PAHs in TMC-1. Stockett and colleagues employed a novel experimental method to determine the radiative cooling rate of the cation of 1-cyanonaphthalene (C₁₀H₇CN) at the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University (DOI: https://doi.org/10.1039/d3fd00005b). Fleisher, from NIST (the National Institute of Standards and Technology, USA) presented timely spectral measurements of the 2ν₁ band of H¹³CN in the shortwave infrared region to support observatories like JWST (DOI: https://doi.org/10.1039/d3fd00019b). Lemmens, of Radboud University (The Netherlands) concluded the session with a discussion of PAHs, including a comparison of calculations from the NASA Ames PAH IR spectroscopic database (PAHdb) and state-of-the-art anharmonic calculations with gas-phase direct absorption IR spectra from the NIST chemistry webbook (DOI: https://doi.org/10.1039/d2fd00180b). This work validates the use of the 11.2/3.3 μm intensity ratio as the most reliable tracer for determining the size distribution of PAHs in the ISM.

The gas-phase laboratory session highlighted the significance of accurate spectroscopic investigations, reaction kinetics, and parameters of other astrophysically relevant phenomena. Experimental information on many reactive species, including radicals, ions, and PAHs, proposed to be important in interstellar environments still remains limited. The exciting new developments on continuous-wave cavity ringdown in a pulsed Laval flow presented by Suits, leak-out spectroscopy presented by Gupta, and a cross-dispersed spectrometer with a virtually imaged phased array (VIPA) presented by Fleisher—combined with other established experimental techniques presented in this session—promise to provide crucial information for astrochemical models and understanding the fascinating and exotic chemistry of the ISM.

Session 3: Laboratory astrochemistry of and on dust and ices

The third session of the meeting was dedicated to molecules in the solid phase. At the very low temperatures (∼10 K) of the densest regions of the ISM, physisorbed molecules, which coat dust grains as molecular ices, play a central role in the chemistry of star formation. These ices represent the main reservoir of molecular matter—beside gas-phase H₂—and act as an important chemical reactor to facilitate the increase of molecular complexity via catalytic reactions at their surfaces or in their bulk, thereby strongly influencing gas-phase chemical richness through desorption processes. Each of these steps—from surface reactions and the increase of the chemical complexity in the icy grain mantles to the release of molecules into the gas-phase via desorption phenomena—leads to strong observational impacts, of both the ices themselves and the subsequent gas phase chemical stocks. Laboratory astrophysics, based on either experimental physical–chemical studies or theoretical simulations, is at the forefront for understanding and quantifying these processes to shed light on complex and multiparameter physical–chemical mechanisms.

Session 3 of the Faraday Discussion addressed all these aspects. State-of-the-art computational chemistry simulations now manage to reach intrinsic parameters, such as binding energies of radicals onto water ice surfaces, and establish reaction pathways of these radicals with other adsorbed species. These reaction pathways now include branching ratios for the products formed—a central task for chemical modeling that remains very difficult to address experimentally. W. M. C. Sameera, of Hokkaido University (Japan) presented such calculations between radicals and neutrals adsorbed on an ice surface. Specifically, Sameera summarized a study centered on the reaction between the hydroxyl radical (OH) and methanol (CH₃OH) leading to the formation of adsorbed hydroxymethyl (CH₂ OH) and methoxy (CH₃OH) radicals (DOI: https://doi.org/10.1039/d3fd00033h) and discussed the branching ratio between these two reactive pathways.

Thermal processing and desorption of multi-component ices were also discussed during the session. The ability of the water ice to trap and preserve volatile species, such as CO₂, at higher temperatures (∼140 K) is a well-known phenomenon. This has been further demonstrated in a study of quaternary ice mixtures (DOI: https://doi.org/10.1039/d3fd00048f) presented by Murthy Gudipati of NASA JPL (USA). Thermal processes to eject volatiles off the grains are, however, inoperative in colder regions (≤30 K), and the observed gas-phase abundances of molecules, including organic species, at such cold temperatures imply non-thermal mechanisms to counterbalance their depletion onto dust grains.

Session 3 expanded past thermal processing to discuss the role of photons in non-thermal desorption. Whereas the ability for energetic ultraviolet (UV) photons to efficiently photodesorb simple molecular species has been known for decades now, their effect on the observed abundances of complex gas-phase organics is still debated. Building off previous investigations into methanol-containing ices, which showed that UV photodesorption yields were an order of magnitude lower than what had previously been expected, Mathieu Bertin of Sorbonne Université (France) and colleagues estimated UV photodesorption yields for other complex organics (DOI: https://doi.org/10.1039/d3fd00004d). Although their work demonstrated that formic acid (HCOOH) and methyl formate (HCOOCH₃) ices, like methanol, are negligibly affected by photodesorption, Bertin highlighted that photodesorption efficiencies of organics depend on individual molecules and that the case of methanol cannot be simply extrapolated to other complex species.

Wendy Brown, of the University of Sussex (UK) similarly presented a study focused on the role played by photons, but at much higher wavelengths (DOI: https://doi.org/10.1039/d3fd00024a). Mid-infrared photons, although present in a large amount in some cold regions of ISM, were not previously considered as potential vectors for non-thermal desorption. By studying the IR irradiation of mixed CO:H₂O ices, Brown and colleagues provided evidence that IR photons trigger the loss of condensed molecules via selective excitation of molecular vibrational motions. Although their study was exploratory, it opens the way toward better constraints on IR photodesorption, a promising process that may compete with other non-thermal phenomena and be of high importance in the context of balancing gas and ice abundances in the ISM.

Session 4: Computational astrochemistry

The last session of the meeting focused on astrochemistry using computational methods. Robin Garrod of the University of Virginia (USA) opened the session with astrochemical modeling of glycine, an amino acid relevant to prebiotic chemistry (DOI: https://doi.org/10.1039/d3fd00014a). He and co-author Herbst highlighted some challenging aspects regarding glycine's detection in the ISM, which is still widely awaited. Their models include many new proton-transfer reactions and suggest that molecules, including glycine, with proton affinities greater than that of ammonia will have drastic reductions in abundance and lifetimes.

Serena Viti, of Leiden University (The Netherlands) followed that with an interesting application of machine learning to understand the relationship between binding energies and the abundances of various species (DOI: https://doi.org/10.1039/d3fd00008g). Binding energies continue to be some of the most challenging parameters to be measured, and many discrepancies exist within the literature. Viti and co-authors also determined, using a statistical method, which molecules should be prioritized for future astronomical detections in order to better constrain the values of binding energies.

Whereas much of the Faraday Discussion focused on stellar birth, Marie Van de Sande of the University of Leeds (UK) discussed work toward disentangling the joint chemical and physical complexity of stars approaching death (DOI: https://doi.org/10.1039/d3fd00039g). Van de Sande presented a new chemical model that more thoroughly addresses chemical kinetics in the outflows of asymptotic giant branch (AGB) stars, which contribute about 80% of the gas incorporated into new clouds. The model by Van de Sande and colleagues demonstrated that chemistry can distinguish between stellar and substellar companions, such as red dwarfs and white dwarfs, respectively.

Stefan Bromley, of the University of Barcelona (Spain) then turned the focus of the Faraday Discussion toward models of upcoming JWST observations of interstellar dust. Bromley described quantum chemical calculations to characterize mid-IR optical properties for a library of Mg-containing silicate nanoparticles, which was in turn used to model the IR spectral features associated with Mg-rich olivine and pyroxene dust grains (DOI: https://doi.org/10.1039/d3fd00055a). Using MIRI, the effect of intervening nanosilicate features could potentially be used to detect or place limits on the nanosilicate content in dust grains observed in the diffuse interstellar medium.

Together, these discussions demonstrate the breadth of computational astrochemistry to model chemistry across star formation from birth to death as exemplified by Garrod and Van de Sande, respectively. These discussions also described the necessity of computations for observational work, whether to make decisions about what molecules to target as discussed by Viti or to interpret spectroscopic data as illustrated by Bromley. Together, these papers complemented Sessions 2 and 3 in which the experiments presented were often conducted in conjunction with theory or observations.

Concluding remarks

In closing, Tom Millar of Queen's University Belfast (UK) delivered his concluding remarks. In his talk, Millar listed several themes observed during the meeting, including that “the foundation of astrochemistry is spectroscopy” (Fig. 3). He gave a thoughtful summary of the science presented (DOI: https://doi.org/10.1039/d3fd00131h), but arguably the most thought-provoking comments from Millar's talk centred around the future of the field. Millar discussed the importance of continuing to cultivate the interdisciplinary and international nature of our field. He described the challenges that come with needing to reach across research fields and literal oceans and emphasized the success of existing programs, such as the Dutch Astrochemistry Network, in connecting researchers with different expertise.

Fig. 3 Sketch notes summarizing Dr Tom Millar's concluding remark that “The foundation of astrochemistry is spectroscopy”.

Millar ended his talk by complimenting the involvement of early-career researchers in the meeting, citing both their presentations/posters and their engagement during the discussions. As mentioned in the introduction of this article, we also noticed the strong contributions to the meeting made by early-career researchers, and we are excited that leading astrochemists like Millar found this aspect of the Faraday Discussion rewarding.

Author contributions

All authors contributed to the manuscript's conceptualization, writing the original draft, and reviewing and editing the completed manuscript. In addition, D. G. contributed Fig. 1, and O. W. contributed Fig. 2 and 3.

Acknowledgements

The authors thank the organizers of the Faraday Discussion on ‘Astrochemistry at high resolution’ for inviting them to write this perspective. They especially thank Susanna Widicus Weaver for sharing details about planning the meeting. O. W. is supported by the NASA Postdoctoral Program administered by Oak Ridge Associated Universities. D. G. acknowledges the ERC advanced grant (MissIons: 101020583) for position funding.

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