Adam Ginsburg

We have detected salts (NaCl and KCl) and hot water in Orion Source I and a few other disks around high-mass young stellar objects. Source I is the closest high-mass (15 Msun) YSO to the sun by a factor of at least two, so we might hope that it provides a generalizable archetype for its class (but it might be unique). The salts around Src I are highly excited, exhibiting vibrational emission with upper state energies in the thousands of K. The rotational and vibrational excitation temperatures do not agree: the vibrational temperatures are much higher than the rotational. I'll give some speculative hypotheses about why and why the salt is in the gas phase at all. I'll also show some first results on demographics: while salts have been detected in a few other sources (G17 by Maud+ and I16547 by Tanaka+), there are many high-mass disks that do not have brine vapor atmospherese.

Yichen Zhang

Detailed observations of massive protostellar disks are still very limited, partially due to the lack of knowledge of lines which can effectively trace the innermost region. In this talk, I will present high-resolution ALMA observations toward the O-type proto-binary system IRAS 16547–4247. We detect salt, silicon compounds, and hot water lines as probes of the individual protostellar disks at a scale of 100 au, which are complementary to volatile hot-core molecules tracing the circumbinary structures on a 1000-au scale. The vibrationally excited water line tracing inner-disks has an upper-state energy of Eu/k > 3000 K, indicating a high temperature of the disks. On the other hand, despite the detected transitions of NaCl, SiO, and SiS not necessarily having high upper-state energies, they are enhanced only in the vicinity of the protostars. We interpret that these molecules are the products of dust destruction, which only happens in the inner disks. The excitation conditions of the detected species are explored with Non-LTE modeling to constrain the physical properties (such as temperature, density, and radiation) of these disks. The kinematics of these lines further show hints that the twin disks are counter-rotating, which might indicate the origin of this massive proto-binary system. These results suggest this set of lines has great potential for future research of disks in massive star formation. Extending from the hot-core chemistry induced by the ice sublimation at ~100K, such a characteristic chemical pattern of "hot disks" will open a new avenue of astrochemistry with a unique insight into astromineralogy. The lower-energy transitions of refractory molecules are excellent targets for future radio observations by ngVLA.

Melisse Bonfand

The complex interplay between the physical and chemical processes involved in the formation of high-mass stars have been long-standing issues in astrophysics. In this context, the new capabilities of state-of-the art radio-interferometers have opened a new window on the study of high-mass star-forming sites. In particular, the high-sensitivity wide-bandwidth observations delivered by ALMA give access to a large population of dense clumps, of which we are able to probe the structure and the chemical content, over scales from clouds to individual proto-stellar envelopes.

I will present a detailed comparative study of the physical structure and chemical composition of four young high-mass proto-stellar objects embedded in Sgr B2(N), one of the most prominent regions forming high-mass stars in our Galaxy. Given its exposure to the extreme environment of the Galactic center region, Sgr B2(N) provides us with an excellent case study to investigate the impact of extreme environmental conditions on the high-mass star-formation process. The chemistry associated to this process plays a key role in diagnosing the sources' physical properties and their evolution. I will show how the coupled analysis of observational data and time-dependent astrochemical kinetics models allows us to constrain the environmental factors (cosmic-ray ionization rate, external radiation field, and minimum dust temperature) that best reproduce the observed chemical complexity, and thus best characterize the Galactic center region.

Danielle Simkus

Amino acids – the monomers of protein in terrestrial biology – are detected in a wide range of meteorite sample types. Assessing their abiotic origins is important for understanding their abundance and distribution beyond Earth. Through laboratory analyses of meteorites and other extraterrestrial samples in the Astrobiology Analytical Laboratory at NASA Goddard Space Flight Center, we investigate the synthesis and preservation of extraterrestrial amino acids and their chemical precursors. Though much of the prebiotic chemistry involved in amino acid synthesis is believed to take place within the meteorite parent bodies themselves, questions remain regarding the pre-accretion chemistry and the origins of amino acids precursors. In this talk, I highlight some of our laboratory analyses and recent results that aim to offer insights into these important science questions.

Cornelia Meinert

‘How did life choose its handedness?’ Just like our hands mirror each other, but cannot be superimposed on each other, amino acids and sugars exist in left- and right-handed forms. Even if there appears to be no biochemical reason to favor one enantiomer over the other, life on Earth uses almost exclusively left-handed amino acids and right-handed sugars. This is called biological homochirality and it is inevitable for building functional proteins and RNA/DNA.

Several asymmetric processes have been experimentally tested to induce chirality in molecular systems, but those focusing on stellar circularly polarized light (CPL) appear to us to be most encouraging, especially given results reported on CPL-induced chirality in amino acids. The astrophysical origin of homochirality is strengthened by i) the detection of L-enriched amino and D-enriched sugar acids in meteoritic samples, ii) the detection of CPL in several star-forming regions as well as iii) experiments studying the interaction of UV CPL with prebiotically relevant chiral species. In this talk, I will highlight significant results on our on-going cometary ice simulation experiments as well as on circular dichroism and anisotropy spectroscopy as a key tool to decipher the response of chiral molecules to UV CPL. Moreover, I will present our major findings on recent asymmetric photosynthesis/photolysis experiments to discuss whether stellar UV CPL could have induced a common chiral bias across molecular families?

Brendan Moore

Our understanding of amino acid chemistry in interstellar environments is always evolving. In condensed phases such as interstellar ices amino acids are typically found in their zwitterionic form, whereas in the gas phase amino acids exist in their neutral form. Amino acids readily decompose when exposed to the types of UV radiation found in interstellar environments, and the form of the amino acid affects its photochemistry. Due to the challenges of working with amino acids in the gas phase, the photochemistry of amino acids in their neutral form is still not well understood.

Here we report the photochemistry of glycine, α-alanine, and leucine isolated in a solid parahydrogen matrix. Solid parahydrogen matrix isolation spectroscopy utilizes the advantages of matrix isolation to accumulate large samples of amino acids in their neutral form, and combines this with the “cage-free” environment of solid parahydrogen to study the photodissociation pathways of these amino acids. It was found that excitation by 213 nm light resulted in cleavage of the Cα-C carbonyl bond to produce trans-HOCO radicals from all three amino acids as the primary dissociation process. The direct production of trans-HOCO radicals α-alanine was further supported by deuterated experiments where DOCO was the primary product. The major co-products of this dissociation were methanimine, ethanimine, and 3-methylbutane-1-imine from glycine, α-alanine, and leucine, respectively. This imine formation was attributed to rapid hydrogen loss from the corresponding excited state amine radicals immediately following photodissociation. Our latest results for 193 nm photochemistry will also be presented. These photodissociation pathways have not been observed in previous photochemical studies of neutral amino acids, and we hope that further work is able to quantify their importance for amino acid astrochemistry.

Kedron Silsbee

Cosmic rays (CRs) play a very important role in determining the chemistry and dynamics of prestellar cores, and protoplanetary disks. At visual extinctions greater than a few, CRs are the dominant source of ionization, thus driving the chemistry, and altering the gas dynamics (as the ionization determines the coupling to the magnetic field). Interestingly, the CR abundance in these regions is rather poorly constrained. I will describe the work we’ve done in calculating the propagation of cosmic rays in dense molecular gas. We have considered several different transport regimes and make specific predictions for the decrease of the ionization rate towards more shielded environments in each case. I will then discuss the comparison of these predictions with observations, and what uncertainties (many!) remain.

Ameek Sidhu

The mid-infrared (IR) spectrum of almost all objects in the Universe is dominated by a set of strong emission features characteristic of a class of large organic molecules made of carbon and hydrogen known as polycyclic aromatic hydrocarbons (PAHs). These molecules account for ~ 15% of the cosmic carbon and ~20% of the total IR power of the Milky Way and star-forming galaxies. They are strong absorbers of ultraviolet (UV) photons and release the absorbed energy through vibrational transitions that result in strong IR emission features. PAHs play a critical role in the evolution of the interstellar medium (ISM) as they drive much of the ISM’s heating and ionization balance. As a result, detailed knowledge of the molecular astrophysics of PAHs, including a thorough understanding of their molecular properties and their interactions with the environment in which they reside, is crucial to understand the evolution of the ISM. Although decades of experimental, theoretical, and observational work have helped gain important insights into the behaviour of PAHs in the ISM, our understanding is far from complete. In this thesis, we investigate the astrophysical behaviour of PAHs from both an observational and theoretical standpoint.

Our observational study focuses on identifying the key parameters that drive the PAH behaviour in two well-known Galactic reflection nebulae, NGC 2023 and NGC 7023, using a Principal Component Analysis. We find that the amount of PAH emission, which represents the PAH abundance and excitation, and the PAH charge state are the only two parameters that drive their behaviour in both environments. In our theoretical study, we develop a model that determines the charge distribution of PAHs and uses it to compute the PAH emission spectrum in astrophysical environments. The relative strengths of the PAH emission features predicted by our model in the Orion Bar, NGC 2023, NGC 7023, the Horsehead nebula, and the diffuse ISM compare well to those obtained from observations. Furthermore, the results of our model highlight the necessity of experimentally determined electron-recombination rates of PAHs and the molecular characteristics of PAH anions, both of which are crucial in understanding PAH behaviour but for which the data is scarce to date.

Milan Sil

It is of prime interest to explore interstellar chemistry by implementing astrochemical models. However, due to the unavailability of adequate parameters, it often produces misleading results. Dust particles (carbonaceous, silicate) play an essential role in shaping this complex species in the star-forming regions. In the super-cold part (~10-20 K prevalent in the dense cloud, proto-planetary disk, etc.), submicron-sized grains are covered by several layers of ices. Water molecules mostly dominate these ices. The molecules formed during the cold phase can revert to the gas phase by various thermal and nonthermal evaporation or sublimation processes. During the later evolutionary stage, these species are again frozen out around the outer part of the proto-planetary disk and form so-called snow-lines depending on their condensation front. Thus, the binding energies (BEs) of these species with the prevailing dust particles or over their icy layers play a crucial role in determining the structural information of this disk and consequently the star and planet formation processes. This presentation will discuss a reliable computational methodology to evaluate the BEs of a large set of species over a wide range of substrates of astrochemical interest.

Planets form and obtain their compositions in dust- and gas-rich disks around young stars. Dust substructure at the 1-10 au scale is commonplace in these disks, but far fewer observations have probed gas substructure at similar scales. The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program was designed to remedy this and provide a comprehensive view of the chemistry of planet formation. MAPS reveals ubiquitous chemical substructures and a striking diversity in their radial morphologies. In the inner planet-forming zones, most line emission substructures are spatially coincident with those in the dust, while at larger radii, most cannot be directly linked to dust substructure. In this talk, I will discuss these findings in the context of gas and dust interactions and the use of line emission substructures as powerful probes of disk physical characteristics.

The temperature structure of protoplanetary disks is key to interpreting observations, predicting the physical and chemical evolution of the disk, and modeling planet formation processes. To constrain the temperature profile in disks, I use a thermo-chemical code (RAC2D) to create a model that reproduces spatially resolved ALMA observations of CO, the upper limit of HD J=1-0 and J=2-1 observations, as well as the SED and continuum morphology of the disk. I applied these methods to determine the 2D temperature profile of the disk around HD 163296 using RAC2D and the high resolution (up to 0.12'’) MAPS observations. My final model finds it necessary to incorporate radial depletion of CO, with moderate depletion inside of 50 au and a larger depletion outside of 50 au (a factor of 0.5 and 0.1, respectively). Additionally, to reproduce the 12CO radial profile, the final model indicates additional heating above a z/r=0.21 within 100 au. We speculate this to be from either PAH heating or mechanical heating not accounted for in our model setup. This HD 163296 model agrees with empirically derived temperatures and observed emitting surfaces from the J=2-1 12CO, 13CO, and C18O MAPS observations. We find an upper limit of 0.35 solar masses using the upper limit HD observations.

With our final thermal structure, we explore the impact that gaps have on the temperature structure constrained by observations of the resolved gaps. Our model suggests that the temperature increases inside the gap. Adding a corresponding gap in the gas and small dust at the location of the large-dust gap additionally increases gas temperature in the gap by only 5-10%.

Sulfur-bearing molecules play an important role in prebiotic chemistry and planet habitability. They are also proposed probes of chemical ages, elemental C/O ratio, and grain chemistry processing. Commonly detected in diverse astrophysical objects, including the Solar System, their distribution and chemistry remain, however, largely unknown in planet-forming disks. I will present new CS (2 − 1) observations at ~0.3'' resolution performed within the ALMA-MAPS Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. CS is detected in all five disks, displaying a variety of radial intensity profiles and spatial distributions across the sample, including intriguing apparent azimuthal asymmetries. Transitions of SO were also serendipitously covered but only upper limits are found. Using astrochemical disk modeling, we demonstrate that N(CS)/N(SO) is a promising probe for the elemental C/O ratio and that the comparison with the observations tends toward a super-solar C/O.

For the MWC 480 case, I will also present complementary ALMA observations at 0.5'' of H2CS which we used to derive a column density ratio of N(H2CS)/N(CS)~2/3. This surprisingly high ratio is an important result for two reasons. Firstly, it suggests that a substantial portion of the volatile sulphur in disks may be in organic form (i.e., CxHySz), questioning our current understanding of chemical reservoirs in such disks. Secondly, it reveals some tensions between astrochemical models and observations indicative of an incomplete theoretical understanding of the sulphur chemistry in disks. Pursuing studies of sulphur chemistry is therefore crucial to better constrain current astrochemical models, which in turns constitute also important tools to indirectly inform us on the potential compositions of unseen reservoirs such as the ice reservoir hosted on dust grains.

Nienke van der Marel

The chemistry of planet-forming disks sets the exoplanet atmosphere composition and the prebiotic molecular content. Dust traps are of particular importance as pebble growth and transport are crucial for setting the chemistry where giant planets are forming. The Oph IRS 48 dust trap is a unique environment for this type of study, as its asymmetric nature provides a direct way to trace the icy pebble chemistry. I will present the results of a recent ALMA Band 7 study, which revealed multiple transitions of complex molecules H2CO and CH3OH, and the sulphur-bearing molecules SO2 and SO, which are all found to be cospatial with the dust trap. The analysis indicates that these molecules are the result of desorption of icy dust pebbles at the edge of the dust cavity, that were trapped at this location. Furthermore, the C/O ratio appears to be lower than other protoplanetary disks. These results show the importance of including dust evolution and transport in chemical disk models for our interpretation of the disk composition.

Ilsa R. Cooke

Measurements of kinetic parameters down to low temperatures (<10 K) are critical to understanding the presence and abundance of molecules in interstellar space and star-forming regions. I will present the latest measurements of neutral-neutral reaction rate constants in Rennes, including reactions between the CN radical and small aromatic molecules, using the well-established CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) combined with Pulsed Laser Photolysis-Laser-Induced Fluorescence (PLP-LIF).

While CRESU PLP-LIF measurements can be used to measure rate coefficients for the overall reactions, the product-channel-specific reaction rate coefficients cannot be measured using this technique. Virtually no product branching ratios have been measured experimentally at the low temperatures relevant to dense interstellar clouds. I will discuss our recent experimental developments in Rennes to enable measurements of low-temperature product branching ratios.

Co-authors: Divita Gupta, Joseph P. Messinger, Brian Hays, Théo Guillaume, Omar Abdelkader Khedaoui, Thomas S. Hearne, Myriam Drissi, Mitchio Okumura and Ian Sims

Brett A. McGuire

It's been nearly three years since my 2018 Census was published - and boy howdy have things exploded in the world of molecular astrochemistry! Most of this talk will be dedicated to sharing with everyone what the landscape of our known molecular inventories looks like right now: trends, patterns, statistics, and newcomer molecules to the stage.

I'll also be talking about what we're considering as an SOC for the future of Astrochemistry Discussions. We'll take a look back at what we've done - what's worked and what hasn't - and then quickly outline some of the ideas taking shape in our heads for how Astrochemistry Discussions can evolve as our world situation evolves. There's a survey for your input!

Julianna Palotás

With the detection of C60, C70 and C60+, the fullerenes constitute by far the largest molecular species individually identified in the interstellar medium (ISM). Proton affinities of C60 and C70 are significant which support the hypothesis that protonated fullerenes may also be abundant in the ISM.

We present the experimental vibrational spectra of gaseous C60H+ and C70H+. The protonated fullerenes were investigated with IR multiple photon dissociation (IRMPD) spectroscopy using the wide wavelength tunability and high pulse energies of the FELIX free-electron laser.

The protonation of the highly symmetrical molecules causes a drastic symmetry lowering resulting in a rich vibrational spectrum. As compared to C60, where all C-atoms are equivalent due to the icosahedral symmetry, C70 belongs to the D5h point group and has five non-equivalent C-atoms, which are available as protonation sites. Combined analysis of the experimental spectrum and spectra computed at the density functional theory level enables us to evaluate the protonation isomers being formed. We compare the IR spectra of C60H+ and C70H+ to IR emission spectra from planetary nebulae, which suggests that a mixture of these fullerene analogues could contribute to their IR emission.

Co-Authors: Jonathan Martens, Giel Berden, Jos Oomens

Thomas G. Bisbas

To determine the physical properties and evolution of the ISM, we need to model its chemical conditions as these set its heating and cooling rates and ionization state, which mediates coupling to magnetic fields. Calculating the intensity of various emission lines is important not only for estimating cooling rates, but also for predicting the diagnostic information they carry, so we can assess how well ISM conditions can be inferred from a given set of observables. To this end, many groups worldwide focus on algorithms constructing synthetic observations of various distributions under different ISM conditions. The general goal is to understand how these conditions affect the trends of emissivities of the different coolants. In this talk, I will present results from recent three-dimensional PDR simulations and synthetic observations of molecular clouds in various ISM environments and explore the behaviour of the most commonly used diagnostics. In the era of ALMA, SOFIA, and the forthcoming CCAT-prime telescope, it is crucial to understand the emission of lines in a wide range of galactic and extragalactic environments to study their ISM.

Charlie Markus

Molecular ions play a critical role in the chemistry of the interstellar medium. They are able to rapidly react at extremely low temperatures, allowing for gas phase reactions to occur in molecular clouds. Laboratory spectroscopy provides transition frequencies necessary for astronomical searches and can provide insight into their reactivity. However, measuring the weak signals from ions in laboratory discharges is challenging. Here I discuss investigations using highly sensitive spectroscopic methods, including sub-Doppler spectroscopy in laboratory discharges and action spectroscopy utilizing a 22-pole ion trap. These resulted in new highly accurate transition frequencies for molecular ions such as H3+, D2H+, OH+, and CH2NH2+. In addition, action spectroscopy was used to investigate an exothermic ion-neutral reaction with non-Langevin behavior, c-C3H2+ + H2, which was found to be strongly inhibited by excitation of its asymmetric C-H stretching mode.

Jennifer Bergner

Phosphorus is a critical ingredient in terrestrial biochemistry, yet is rarely observed during the low-mass star formation sequence, and so its chemistry in protosolar analogs is poorly understood. We recently detected the phosphorus carriers PO and PN towards the low-mass protostar B1-a using the IRAM 30m telescope, representing the second detection of P-bearing molecules in a solar-type star forming region. I will discuss the inferred abundances and emission origin of PO and PN in this source, and implications for phosphorus partitioning between the volatile and refractory phases. Comparing B1-a to other dense ISM regions with PO and PN detections, the PO/PN ratio is surprisingly uniform in different environments, which I will discuss in the context of current astrochemical models. I will also introduce follow-up efforts to further characterize the phosphorus molecule emission in B1-a and additional low-mass protostars. Together these observations are advancing our understanding of how this key prebiotic element is inherited in planetary system progenitors.

Jacob Bernal

The PO molecule (X2r) has been identified for the first time in Orion-KL via its J = 2.5→1.5 transition, as part of a 3mm survey using the Arizona Radio Observatory 12m telescope. The J = 2.5→1.5 transition consists of two lambda doublets, both of which were detected. The PN (X1Σ) J = 2 →1 transition was also detected. Both PO and PN exhibited line profiles with ΔV1/2 ~ 21 - 25 km s-1, and VLSR ~ 10 km s-1, characteristic of the Orion Plateau region. The presence of both species in the Plateau region indicates that shocks are primarily responsible for their formation. Abundances of both species were calculated, with a PO/PN value of ~3, similar to other sources where both species have been observed. The implications of the PO/PN detections for chemical formation pathways will be discussed.

Co-Authors: Lucy M. Ziurys and Lilia Koelemay

Víctor Rivilla

Phosphorus is a key chemical biogenic element for the development of life. But how it arrived on the early Earth is still a mystery. To track the interstellar journey of phosphorus from molecular clouds to planets, we have studied the phosphorus chemical reservoir in a large sample of star-forming regions in our Galaxy and in the comet 67P/Churyumov–Gerasimenko. In this talk I will present the main results of several observational campaigns carried out with single dish telescopes and ALMA, and measurements of the ESA Rosetta mission. Our findings, combined with newly developed chemical models, have allowed us to understand how phosphorus-bearing molecules are formed in star-forming regions, and how they could have been transferred to our own Solar System.

Zachary Buchanan

Detection of new molecules in the interstellar medium has been ongoing for almost 100 years, with over 200 detected so far. To facilitate these searches, in depth characterization of molecules with potential to be detected is required. Choosing which species to study is based on what has been detected in the past guided by computational methods, or intuition on what molecules could be built from those pieces. With recent detection of benzonitrile in TMC-1, the characterization of other nitrogen containing aromatic rings is an intuitive path of investigation. In this talk, I'll present the work our team has performed on phenylpropiolonitrile (C6H5C3N) with microwave, mm-wave and infrared spectroscopies. Over 6200 transitions in the 8-200 GHz range have been assigned and used to derive rotational spectroscopic constants (rotational, centrifugal distortion, and nuclear hyperfine) for C6H5C3N. In addition, 14 fundamental vibrational bands were measured and identified with help from computational work. With the measurement of these parameters, an interstellar search for C6H5C3N can be performed with confidence as many of the lines have either been assigned, or can be calculated with high accuracy below 300 GHz.

Maria Zdanovskaia

Nitriles are a high interest target for extraterrestrial detection due to their high prevalence and their typically large dipole moments, which aid in their detection by radioastronomy. Additional interest exists in finding pyridine (C₅H₅N), a molecule that may provide insight into formation of nitrogen-containing polycyclic aromatic hydrocarbons and prebiotic chemistry occurring in space, or molecules potentially related to its formation. Our group has synthesized four cyanobutadiene (C₅H₅N) isomers: 2-cyano-1,3-butadiene, E-1-cyano-1,3-butadiene, Z-1-cyano-1,3-butadiene, and 4-cyano-1,2-butadiene. We have analyzed their ground vibrational state rotational spectra in the 130 – 370 GHz frequency range. While some isomers require a Coriolis-coupled multi-state least-squares fit to correctly model their spectra, we have acquired spectroscopic constants for all of the isomers to predict the most intense transitions that would be used to seek these molecules in the interstellar medium, and these constants have already proven useful in detecting some of the cyanobutadienes in reaction spectra.

Co-Authors: P. Matisha Dorman, Vanessa L. Orr, Andrew N. Owen, Samuel M. Kougias, Aatmik R. Patel, Brian J. Esselman, R. Claude Woods, and Robert J. McMahon

Kelvin Lee

A large portion of molecular discoveries in the interstellar medium have been motivated and guided by exhaustive laboratory studies. These experiments provide, for instance, precise rest frequencies and reactive branching ratios in typically targeted studies focusing on specific molecules of interest. With the advent of broadband chirped-pulse microwave spectroscopy, we are now able to collect rotational spectra over many gigahertz—closer to the capabilities of many radio telescopes—covering multiple transitions for potentially many molecules at high signal-to-noise within only a few hours of integration. With these capabilities, we have access to rapid experimental assays covering different precursors and other experimental parameters; the potential discovery space is enormous.

In this talk, I will present exhaustive studies of unbiased assays of benzene in electrical discharges, conducted at the Center for Astrophysics | Harvard & Smithsonian, which highlight just how much new data we can obtain in single experiments. Moreover, these experiments demonstrate how automation of spectroscopic analysis is highly necessary where high quality data can be obtained at a higher cadence than can be regularly analyzed; I will discuss the open-source tools (PySpecTools, https://github.com/laserkelvin/pyspectools) we have recently developed that simultaneously decreases the amount of human oversight required, thereby improving the overall reproducibility and collaborative nature of spectral analysis. Overall, these improvements allow the focus of analysis to be shifted towards unknown unknowns—potentially new molecules and states that are previously unknown and unsought that are of interest to molecular astrophysics.

Alexander Thelen

Saturn’s largest moon, Titan, harbors a dense, organic-rich atmosphere primarily composed of N₂ and CH₄. A wealth of additional hydrocarbon, O- and N-bearing species make up the remaining atmospheric composition, formed through the photodissociation of nitrogen and methane, interactions with the Saturnian magnetosphere and galactic cosmic rays, or from exogenic sources such as Enceladus. While in orbit around Saturn, the Cassini mission detected a vast array of previously unidentified positive and negative ion species in Titan’s upper atmosphere, revealing the complexity of the moon’s naturally occurring atmospheric chemistry. The extent of Titan’s neutral atmospheric inventory, the incorporation of these molecular species into its stratospheric haze layer, and their participation in Titan’s methane-based meteorological cycle are all important processes that enhance our understanding of Titan’s global climate and the connections between the atmosphere, organic regolith and hydrocarbon lakes.

Though many early detections of molecular species in Titan’s atmosphere were made by the Voyager 1 and Cassini missions and ground-based telescopes in the infrared, we now harness the power of cutting-edge (sub)millimeter facilities, such as ALMA, to search for additional trace compounds. Here, I detail recent spectral identifications of molecular species found for the first time in Titan’s atmosphere - or, in fact, any planet’s atmosphere thus far. In addition to increasing our understanding of Titan’s atmospheric and surface properties, new molecular discoveries help to further characterize Titan’s atmospheric photochemistry and assess its impact on Titan’s potential for habitability.

Chao He

More than 4000 exoplanets have been discovered in last two decades, including a sample of terrestrial planets in the habitable zone of their host stars. New observing capabilities coming online over the next few years will provide opportunities for characterizing their atmospheres and assessing their potential habitability. Photochemistry could play an important role in their potential atmospheres and lead to the formation of haze that could obscure the detection of major atmospheric constituents. However, the photochemical processes are poorly understood in these exoplanets because their atmospheric parameters are different from that in Solar System bodies. We conducted a series of laboratory experiments that simulate photochemistry in a broad range of temperate (<800 K) exoplanet atmospheres. We investigated three types of atmospheric metallicities (100, 1,000, or 10,000 times solar) at four temperatures (300, 400, 600, and 800 K) using the PHAZER chamber at JHU with one of two energy sources (AC glow plasma and UV photons). We studied the size distributions (He et al. 2018a, 2018b) and the production rates (He et al. 2018a; Hörst et al. 2018a) of solid haze particles that formed, as well as the gas and solid phase chemistry (He et al. 2019, Moran et al. 2020, Vuitton et al. 2020). We find that the particle size is dependent on the experimental conditions, while the production rates are sensitive to atmospheric compositions (He et al. 2018a, b; 2020a, b; Hörst et al. 2018). Even a small amount of H2S can enrich the photochemistry and enhance haze production rate significantly (He et al. 2020a). We observed the photochemical formation of O2, sulfur and organic products in the gas phase, which have been considered as potential biosignatures but are produced abiotically in our experiments. Organic molecules are detected in both gas and solid phases, including potential prebiotic precursors (HCHO and HCN) and compounds with prebiotic molecular formulas (sugars, amino acids, and nucleobases), which could provide a source of organic materials for life to arise.

Eszter Dudás

The SMAUG apparatus (Spectroscopy of Molecules Accelerated in Uniform Gas flows) was developed to produce high-resolution infrared spectra of polyatomic molecules of interest for hot astrophysical atmospheres, like for example those that surround hot Jupiters or cool carbon evolved giant stars, reaching up to 2500 K. In particular, high-temperature infrared spectroscopic data is needed to retrieve temperature and concentration profiles from observed telescope infrared spectra.

SMAUG can operate according to two complementary working regimes: non-LTE (vibrationally hot and rotationally cold) and LTE conditions, to interpret the complex pattern of highly-excited vibrational states. Two different gases, carbon monoxide (CO) and methane (CH₄) were used as test molecules. Using non-LTE conditions a rotational temperature of 30(3) K was measured for CH₄, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with T^I_vib = 894(47) K, while the population distribution within a given polyad was modelled correctly by T^II_vib = 54(4) K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad. Using a "post-shock CRDS" technique CO and CH₄ LTE spectra were recorded at 950 K and 1400 K.

Kamber Schwarz

CO was the first molecule detected in protoplanetary disks. It is widely used as a tracer of the bulk gas and is a major carrier or volatile carbon. The CO snowline has been posited to play an important role in planet formation and in setting the overall chemical composition of the disk. However, basic properties such as the location of the midplane CO snowline and the CO abundance relative to the total gas mass remain difficult to constrain. I will discuss how ALMA observations over the last decade have provided new insights into the distribution of CO gas in protoplanetary disks, as well as raised new questions about volatile carbon chemistry.

Arthur Bosman

The elemental composition of proto-planetary disk gas and ice is a critical parameter for understanding the physics and chemistry in the disk. Furthermore as this gas and ice is incorporated into planets, it is also important for in understanding the composition of planets. I will show how C2H emission is used to constrain the C/O ratio of the gas and apply this to the (possibly) planet containing disks of AS 209, MWC 480 and HD 163296. Observations of these disks suggest that planets forming in the gaps of these disks will accrete gas that has a high C/O but sub-solar C/H. Implying that the atmospheres of pebble- or core-accretion formed planets might have a very perculiar chemical composition.

Alex Cridland

Understanding how planets form is an integral part of understanding the ever growing population of exoplanets as well as our own solar system. Astrochemistry offers a unique probe of the underlying physics of planet formation because the formation history (mass accretion / migration rates) of a giant planet is at least partially encoded in the bulk chemical structure of its atmosphere. In this talk I will outline my ongoing effort to link the planet formation process with protoplanetary disk astrochemistry in order to understand the chemical properties of exoplanetary atmospheres. One particular discovery has been a tight correlation between the fraction of mass brought in by solid accretion and the final bulk carbon-to-oxygen ratio (C/O) in the atmospheres of giant planets. This ‘C/O main sequence’ naturally leads to the empirically derived mass-metallicity relation that has been found for our solar system giant planets, and beyond. Furthermore, this main sequence has shown that understanding the physics of solid accretion into the atmospheres of giant planets will play an important role in understanding the chemical properties of planetary atmospheres as it can dominate the delivery of carbon and oxygen over gas accretion for certain planetary masses.

Dipen Sahu

Studying the physical and chemical properties of low mass star systems is very important as it is directly related to the Solar-like system. Hot-corinos are low mass young stellar objects (YSOs) where the temperature is high enough (>100 K) to release ice-phase complex organic molecules into the gas environment and these COMs can be detected using observation in millimeter wavelength. Some of the COMs are prebiotic and may be related to our chemical heritage. In this talk, I will focus on a protobinary system NGC1333 IRAS 4A and will discuss the presence of hot-corino and an upper limit calculation of a glycine isomer, methyl carbamate. Glycine is a prebiotic molecule, in this connection, I will also discuss how possibly our chemical heritage can be maintained from the early stage of a low mass star formation to planetary objects like our Sun-Earth system.

Originally Broadcast 7 October 2020.

Laura McKemmish

The rapid rise in interest in exoplanetary atmospheres has led to an explosion of interest by astronomers in high accuracy and comprehensive small molecule spectroscopy data. This data is needed to understand and characterise these exoplanetary atmospheres and their host stars (including soon the scientific search for the molecules of life). In this talk, I will discuss in depth a key type of data desired by astronomers - the molecular spectroscopic line list - and how this data is produced by myself and others based on laboratory spectroscopy experiments and high level ab-initio electronic structure calculations, focusing on the key example of diatomic TiO.

The demands for accuracy and breadth of spectral coverage are substantial. New high-resolution techniques now mean that astronomers need these line lists to have strong lines frequencies to an accuracy of < 0.3 cm^-1. Simultaneously, very weak lines must be considered to ensure accurate opacities can be calculated, leading to databases of millions to billions of spectral lines per molecule.

Originally Broadcast 7 October 2020.

Courtney Ennis

Spacecraft exploration of icy outer Solar System bodies such as Titan has alluded to a rich cyanide chemistry. Here, the recombination of radicals, atoms, and ions - generated by photolysis and energetic particle interactions with methane and molecular nitrogen - generate a wide variety complex organic molecules. Such chemistry has been the inspiration for many laboratory investigations over the past decades where cyanides have been a particular focus as they present as carriers of C-N bonds required for species of biological interest such as N-bearing heterocycles.

This short talk will focus on structural and spectroscopic characterisation of cyanide-bearing molecular co-crystals. Perhaps most relevant to the aerosols of Titan’s atmosphere - where fractional condensation of nitriles and hydrocarbons are expected to yield ices of mixed composition - these particles could generate higher order organics via interactions with cosmic rays that penetrate through to the surface. In our preliminary studies on cyanide co-crystals, we have used periodic-DFT to predict the ternary-phase 2C₆H₆-C₂H₂-HCN crystal; a system that shows competitive thermochemistry under Titan conditions. These co-crystal compositions, where polar nitrile species are completely mixed with hydrocarbon material in stable molecular arrangements, provide new environments to explore efficient solid-state chemistry. We discuss our recent endeavours in this pursuit.

Ennis, C.; Cable, M.; Hodyss, R. & Maynard-Casely, H. “Mixed Hydrocarbon and Cyanide Ice Compositions for Titan’s Atmospheric Aerosols: A Ternary-Phase Co-crystal Predicted by Density Functional Theory” ACS Earth Space Chem (2020). DOI: 10.1021/acsearthspacechem.0c00130

Originally Broadcast 7 October 2020.