Microwave and Infrared Spectroscopy of Reactive Molecules
Of the nearly 270 molecules identified outside our Solar system, about 60 of them were detected just in the past 2-3 years. Each new detection presents new challenges for our understanding of the overall chemistry in space: how are these molecules formed, what is their fate, and how the answers to these questions vary by environment. Most regions where molecules are observed are kinetically controlled, meaning that the rates of individual chemical reactions determine the composition and chemical evolution rather than thermodynamic considerations. For this reason, reactive molecules such as radicals and ions play a major role in dictating chemical evolution. My research has focused on the spectroscopic detection of these types of molecules as well as studies of their reactivity. In this talk, I will discuss the diverse systems and approaches I have used to perform these studies throughout my career, starting with studies of the fundamental molecular ion H3+ as a graduate student and ending with my group's recent work on nitrogen-containing radicals. Along the way, I will also discuss the various experimental and computational tools that form the foundation of my group's efforts in laboratory astrophysics.
A Philosophical Perspective on Uncertainties and Model Evaluation in Astrochemistry
One of the core questions of philosophy of science is to analyze how different kinds of evidence bear on a theory. Testing whether a scientific theory is adequate amounts to extracting an observational consequence p from it and to confronting p with the available observations. If p is not observed, then the theory must be rejected. If the theory passes this reality check, it can be accepted (temporarily) as valid. This naive version of hypothetico-deductivism, although widely criticized and abandoned nowadays, has greatly shaped the way philosophers and scientists have thought about testing scientific models. Models were seen, and are still seen in many areas, as predictive tools whose adequacy can be validated and verified by checking whether they succeed in matching observations. Yet, this picture does not fit the way scientists have handled model evaluation in the context of high uncertainties, i.e, in a context where it is not clear how observations can be turned into evidence.
The aim of this talk is to analyse how astrochemists have evaluated their models in these cases, based on recent developments in this area. Astrochemical models, based on networks of chemical reactions relying primarily on extrapolated reaction rate constants, sometimes fail to reproduce observations. The spectroscopic observations, performed by ground and space-based telescopes, that permit to identify molecules in space and to derive the physical conditions in astrophysical media, can be both incomplete and uncertain, despite recent significant progress in the resolution of telescopes. Yet, models are indispensable to make the most of observations, to better target future campaign observations, to determine where better experimental data are needed and to motivate further theoretical developments. Thus, astrochemists have no choice but to develop methods to evaluate their models and decide how the disagreement between observations and models should be interpreted, i.e., should guide their future work, despite the uncertainties on both sides. In this talk, I present a philosophical analysis of such methods and how they allow us to address the following questions: How do scientists evaluate their model against observations when uncertainties are so high on both sides? How do scientists improve the accuracy of their models when it is not even clear what accuracy can possibly mean? When is a discrepancy an opportunity to improve a model and when is it a sign that something went wrong with its initial assumptions?
Salt in high-mass "brinary" disks: Location, weird excitation, and what we don't know
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.
Salt, Hot Water, and Silicon Compounds Tracing Massive Twin Disks
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.
Modeling the complex chemistry of young high-mass proto-stellar envelopes
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.
Searching for Amino Acids and their Chemical Precursors through Laboratory Analyses of Meteorites
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.
Life’s Origin Among the Stars: Detection & Symmetry Characterization of Interstellar Chiral Molecules
‘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?
UV Photolysis of Amino Acids in a Solid Parahydrogen Matrix
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.
A Song of Ice and Fire: How Dark Cloud and Extreme PDR Chemistry Reveal Planetary Volatile Budgets
Volatile elements, like C, H, O, N, and S, are critical to the habitability of planets. The similarity between molecular isotopic ratios in Earth's oceans, comets, and protostars suggests that some of the volatile material in these bodies originated as ices in the dense molecular clouds from which stars are born. However, other ices, particularly the most volatile ones like CO, may be destroyed during the initial formation of a protoplanetary disk. Knowing which ices form in clouds, and survive to be incorporated into planetesimals, would be a huge step towards predicting whether exoplanets should have the ingredients to form life.
In the first part of the talk, I will describe how upcoming community-based observations with the successfully launched James Webb Space Telescope (JWST) will combine with laboratory experiments to illuminate the dark chemistry occurring at 10 K in dense molecular cores. The fraction of volatile to less volatile ices (e.g. CO/CH3OH), which depends on the formation routes at this stage, will determine the fraction of carbon-bearing ices that are accessible by forming planetesimals. The question is then which of the available ices are actually incorporated into these planetesimals?
To resolve this question, in the second part of the talk I demonstrate how the retention of ices in disks (either in planetesimals or millimeter sized pebbles) can be measured from infrared gas phase abundances in the hot inner 0.1 AU of protoplanetary disks. This part of the disk is an extremely dense Photon Dominated Region (PDR), which has a distinctly different chemical structure than 'traditional' PDRs in the diffuse Interstellar Medium, with atomic C and H emitting co-spatially. For an example disk, TW Hya, I show how the volatile abundances in the inner disk PDR imply the sequestration of C-rich ices in the disk's planet-formation zone. In combination with ALMA studies of ice traps in protoplanetary disks, this chemistry of cold ices with JWST and hot PDRs will provide a comprehensive record of which ices are (eventually) available to form life on other planets.
Millimeter/submillimeter Spectroscopy of the Potentially Important Interstellar Molecule, 2-chloroethanol
Halogenated hydrocarbons are well known to have problematic effects in the Earth’s troposphere. These molecules are also suggested as potential biomarkers in the search of extraterrestrial life. Chlorine species in particular have been detected in carbonaceous chondrites, on Mars, and in the interstellar medium (ISM). Very little work, however, has focused on the detection and understanding of chlorinated organics, where four of the five neutral species detected in the ISM are triatomic species and one is the organohalogen CH3Cl. The simplest chlorohydrin 2-chloroethanol (ClCH2CH2OH) is predicted to form in the ISM from the reaction of HCl with oxirane or ethylene glycol, all of which are known interstellar constituents. However, attempts to detect 2-chloroethanol towards Sgr B2(N) have been unsuccessful. The spectrum of 2-chloroethanol has been extensively studied in the microwave and infrared, with recent publications of high resolution ro-vibrational spectra. We have extended these spectral measurements into the millimeter/submillimeter region. We measured rotational spectral lines from both chlorine isotopologues in their ground vibrational states as well as multiple vibrationally-excited states. We have analyzed each of these states and refined the molecular constants to the precision needed to guide observational searches with telescopes such as ALMA, increasing the chances of successfully detecting this species in the ISM.
Building the Mid-Infrared Inventory for the Orion Hot Core
Hot molecular cores associated with high mass protostars are a rich source of chemistry in the ISM, connecting star formation to planetary systems such as our own. They represent a key stage in stellar evolution as a young protostar heats its natal, icy mantle to unlock reservoirs of molecules. The mid-infrared (MIR), provides the only access to rovibrational transitions and molecules with no permanent dipole moment, and probes hot core material closest to embedded protostars. With SOFIA/EXES, we conducted a high resolution (R~60,000) spectral survey from 7.2 to 28 um of the hot core Orion IRc2. We have so far established over 350 unique features and have identified nine species with two isotopes. For the first time, we have identified HNC and H13CN in the MIR. Together with HCN, these three species provide key insights into the hot core. We utilize a gas-grain chemical network to model the HCN/HNC evolution, which reaches our derived HCN/HNC=72 after 10^6 years. This is much older than the region’s explosive event 500 years ago, suggesting that the hot core’s origins predate this. Our derived 12C/13C=13 is lower than measurements at longer wavelengths. Several other recent observations towards star-forming regions also show similarly unexpectedly low isotope ratios. This points to the possibility that the isotope chemistry in these regions is not yet fully understood.
Understanding Cosmic Ray Transport in Dense Environments
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.
Revisiting Polycyclic Aromatic Hydrocarbon Emission in Photodissociation Regions
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.
Binding Energy: a Fundamental Parameter to Formulate Interstellar Chemistry
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.
Chemical Substructures at 10 au Scales in Protoplanetary Disks
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.
Determining the 2D Thermal Structure of the HD 163296 disk
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%.
Inferring the C/O and S/H Ratios in Protoplanetary Disks with Sulfur Molecules
Romane Le Gal
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.
From weeds to flowers: Exhaustively investigating the rotational spectra of astrophysical species
Centre National de la Recherche Scientifique (CNRS)
The identification of new molecules in the interstellar medium (ISM) is intrinsically linked to their prior study in the laboratory, especially at centimeter to submillimeter wavelengths where the recorded spectra act as identity cards. Although reactive species represent a significant amount of the known interstellar species (more than 50%), the astronomical detection of relatively large ones remains hindered by the lack of available laboratory data. Indeed, these species are often challenging to produce and characterize in the laboratory compared to their stable, often commercially available, counterparts. It is also interesting to note that even for already known reactive species, laboratory data are often limited to the centimeter and millimeter-wave region (f < 300-400 GHz) while observatories such as ALMA can operate up to about 1 THz. New laboratory data on a wide range of reactive species are thus more than ever needed.
In our group, we are exploiting chirped-pulse millimeter-wave and frequency-multiplication-based (sub)millimeter spectroscopy to record the rotational spectrum of known or postulated astronomical species. Several set-ups (H-abstraction by F atoms, RF and DC discharges) can be coupled to our two spectrometers allowing us to produce radicals and reactive species.
I will present an overview of our experimental capabilities illustrated by recent results on several radicals.
August 2021 Extravaganza - Virtual Lab Tours!
Center for Astrochemical Studies (CAS) Lab Tour - Max-Planck Institute for Extraterrestrial Physics
Puzzarini Lab Tour - Universitá di Bologna
Join Cristina Puzzarini, Luca Bizzochi, and Mattia Melosso for a tour of their microwave spectroscopy laboratory in Bologna!
You can find out more about their work at:
ORIGIN Lab Tour - Universität Bern
Join Niels Ligterink on a tour of the ORIGIN space instrument and testing facilities at the University of Bern, Switzerland.
You can find out more about their work at:
Arumainayagam Lab Tour
Sims Lab Tour
Ice pebble chemistry in a major asymmetric dust trap in a planet-forming disk
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.
Kinetic Measurements of Neutral-Neutral Reactions at Low Temperatures: Rate Constants and Product-Branching Ratios
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
The 2021 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules
-- SLASH --
The Future of Astrochemistry Discussions!
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!
Laboratory IR Spectroscopy of Protonated Fullerenes
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
PDR Diagnostics Across Galactic Environments
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.
Sensitive and High-Precision Spectroscopy of Astronomically Important Molecular Ions
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.
Phosphorus Carriers in the Protosolar Analog B1-a
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.
PO and PN in Orion-KL: Insights to the Phosphorus Chemistry of Molecular Clouds
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
The Interstellar Journey of Phosphorus
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.
Vibrational and rotational characterization of phenylpropiolonitrile to facilitate astrochemical surveys
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.
The 130 – 370 GHz Rotational Spectra of Four Astronomically Relevant Cyanobutadiene Isomers (C5H5N)
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
Finding Unknown Unknowns for Astrochemistry with Broadband Rotational Spectroscopy
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.
The Search for New Molecular Species in the Atmosphere of Titan
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.
Photochemistry in Exoplanet Atmospheres: Insight from Lab Experiments
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.
High-Temperature Non-LTE and LTE Cavity Ringdown Spectroscopy using SMAUG experimental setup
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 TIvib = 894(47) K, while the population distribution within a given polyad was modelled correctly by TIIvib = 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.
Mapping CO Gas in Protoplanetary Disks
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.
The elemental composition of planet feeding gas
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.
Connecting astrochemistry and planet formation: A C/O main sequence for warm and hot giant planets
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.
From Hot-corinos to our chemical heritage; the case of NGC1333 IRAS 4A
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.
Molecular spectroscopic line lists
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.
New solid-state environments for the formation of complex organics
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.
Making a Habitable Planet
Edwin (Ted) Bergin
University of Michigan
Today we stand on the cusp of characterizing potentially living worlds, or habitable planets, planets with orbits where liquid water would exist on a rocky surface. This concept of habitability implicitly assumes water in some form is present on rocky planets - but is it? More broadly are rocky planets generally "chemically habitable" - do they contain the elements needed for life, such as carbon and nitrogen, on their surfaces? In this talk, I will review what we know about the chemical habitability of forming planets. I will follow the most abundant volatile elements needed for life (C, H, O, N) from the vast cold and low pressure environs of the interstellar space to their presence on planetary surfaces of worlds such as our own. I will outline our current understanding of star and planetary birth while discussing the fate of primary carriers of life's elements, both volatile species (e.g. H2O, CO, CO2) and more refractory materials (e.g. silicates, aliphatic/aromatic hydrocarbons). Some of these carriers can be characterized via existing facilities (e.g. the Atacama Large Millimeter Array), planned facilities (the James Webb Space Telescope), while others require future options (such as the Origins Space Telescope). Finally, within a young planet, the ultimate fate of delivered material is not set until the hot young terrestrial world solidifies and core formation ceases - thus the process of planet formation and evolution itself influences whether a mature planet is habitable or uninhabitable. Looking forward, the astrochemical study of life's materials in space, and the astronomical characterization of terrestrial exoplanets, must be intimately linked to grounding knowledge from the planetary sciences. A fascinating interdisciplinary future awaits, where we will seek to ascertain the origin of our own biosphere and provide crucial chemical context for habitability.
Originally Broadcast 23 September 2020.
New, quick talks each week in the month of August from all areas of Astrochemistry and researchers from around the globe! Enjoy them on your own time, and we'll see you all back in September for our regularly scheduled programming.
Physicochemical Modelling: Source-Tailored or Generic?
Beatrice M. Kulterer
Benzene on Ice and in the ISM
University of Hull
Matrix-Isolation FT-IR Spectroscopy of Nitrogen Based Heterocyclic Radicals
Indian Institute of Science Education and Research (IISER) Mohali, India
VUV Spectroscopy for Laboratory Astrophysics and Astrochemistry
National Central University, Taiwan
Sketch Your Science!
Pointing the Green Bank Telescope
My 15 Years of Astrochemistry
Laboratory Measurements of Gas-Phase Reaction Kinetics with CN radical at Low Temperatures
Did Photons or Electrons Create Life?
Photodissociation of CS via Highly Excited Electronic States: Ab Initio and Experimental Study
University of California Davis
Black in Astrochem - A Round-Table Discussion
Ashley Walker, Ayanna Jones, Bryne Hadnott, Kathleen Rink, and Prof. William Jackson
This special session of Astrochemistry Discussions started with a keynote talk given by Dr. William M. Jackson, Distinguished Professor Emeritus at UC Davis, pioneer in the field of astrochemistry, and co-founder of the National Organization for the Professional Advancement of Black Chemists and Chemical Engineers (NOBCChe). A panel followed, moderated by Kathleen Muloma Rink and featuring Ashley Walker, Bryné Hadnott, and Ayanna Jones. This event was conceived of by Ashley Walker (@ThatAstroChic on Twitter), who founded #BlackinAstro week and is well known for highlighting Black astronomers and astrochemists. This is essential watching for all in academia, and most especially for non-Black mentors to Black students.
If you'd like to support the panelists who put their time and emotional labor into this event, their Venmo information is below. Donations to NOBCCHe can be made here and donations to African American Women in Physics can be made here.
Originally Broadcast 15 July 2020. Content Warning: Brief description of sexual assault.
Ashley Walker: Ashley-Walker-210
Ayanna Jones: onlyayanna
Bryné Hadnott: Bryne-Hadnott
Kathleen Muloma Rink: Kathleen-Rink-1
Using reaction rate theory (MESMER) to provide robust kinetic parameters for astrochemical modelling
University of Leeds
Experiments provide kinetic parameters, rate coefficients and product yields, over a limited range of temperatures and pressures. Often this experimental temperature range is not sufficient to cover the temperatures required for astrochemical modelling. Traditionally, simple analytical forms are used to represent rate coefficients, e.g.
k = A Tn exp(-Ea/RT).
However, this approach can result in extrapolated rate coefficients with an order of magnitude uncertainty, especially at very low temperatures.
A more robust approach is to use a theoretical model to fit to the experimental data before extrapolation. For a given reaction, the reaction rate theory code MESMER calculates rate parameters from the information provided by ab initio structure calculations. In addition, MESMER allows the theoretical parameters to be adjusted to best fit the data, and thus provide a robust description of the reaction. The reaction between the hydroxyl radical, OH, and formaldehyde, H2CO, will be used to illustrate how MESMER can provide robust rate coefficients down to 5 K.
Originally Broadcast 8 July 2020.
Bonus Questions & Answers - Mark's Talk
Q: Does MESMER give a range for the rate constant so as to incorporate an ‘error bar’?
A: Yes, you can give the errors for each of the rate constants. It is also possible to propagate the errors to give the MESMER rate constant error for a given T and p.
Q: Does MESMER allow for including and excluding sets of experiments? Some being more accurate than others?
A: It is up to you to assign the error for each rate constant. For measurement with problems you might want to give it a very large error, or not include it at all.
Q: Why is the same species labeled differently (and repeated) multiple times when defining the channels?
A: This was to check which channel the reaction was occurring along. You do not have to do this. By doing this I was able to show that at low T, the reaction was via the pre-reaction complex
Q: How do you think these changes reflect a thin porous ices? Do you expect larger cracks in the ice to influence the structure change?
A: We did study FEL irradiation as a function of ice thickness and found that desorption is more efficient at the surface of thick ices likely due to the bad conductivity of ASW. However changes are seen at all studied thicknesses, i.e. up to a few layers. Of course cracks and nucleation can help the crystallization process.
Q: Do you give any physical significance to any of the parameters in your equation or would you regard them as fitting parameters?
A: Are you referring to the MESMER fitting parameters, if so yes. If you are referring to final expression for k, then I do not.
Q: I think I missed what was "broken" about the 5 K simulation.
A: It was the species profile that was broken. This meant that MESMER did not converge properly, and it is best not to use the rate constant at 5 K.
Q: Which level of theory did you use to optimize minima and TS? how a poor description of van der Waals in the functional influences the rate?
A: The pre-reaction complex does have some very anharmonic vibrations and this can influence the result to some extent. The the fitted parameters can be skewed from the true answer. Need to describe the anharmonic motions as accurately as possible if claiming accuracy.
Q: Just as a matter of curiosity, how much off were the CCSD barrier results from the experimental fit?
A: About 1 kJ mol-1 adjustment of the TS barrier.
Q: Do we ever use the binding energy distributions as parameters in models? In other words, instead of using the average, one pulls from the distribution each time, in a more Monte Carlo approach?
A: It is possible to start the calculation off with different energy distributions. In the OH + acetylene + O2 MESMER example the fraction of O2 affects the products formed.
Q: Can MESMER consider also the 'jump' between two PES (e.g. triplet and singlet) when modelling rate constants?
A: Yes, if you look in the MESMER examples there is such an example.
Simulation of Reactivity on the Surface of Dust Grains: Pitfalls and Opportunities
Gas-phase reaction rates can often be predicted with high accuracy because only a few atoms are involved. Compared to that, the simulation of reactions happening on the surface of a dust grain is much more involved, which often requires compromises concerning accuracy. They are very valuable, however, since the corresponding experiments also face the problem of many parameters that have to be controlled. I will give an overview of techniques to model adsorption and surface reactivity, their estimated accuracy, and their computational costs. It turns out that even an "active" surface, like ASW, often has rather little influence on the reaction barriers. Nevertheless, such surfaces influence the reactivity - but can be treated by a very efficient implicit surface model.
Originally Broadcast 8 July 2020.
Bonus Questions & Answers - Johannes's Talk
Q: Are laboratory experiments like TPD able to provide information about the distribution of binding energies or only a single desorption energy barrier?
A: I am no expert on that, but I talked to a few :-). TPD convolute the (real) binding energy distribution with the thermal desorption probability (which is time dependent). So in my understanding part of the broadening of the desorption time/temperature is due to this broad distribution.
Q: Is there a physical understanding of why those rate constants are so different?
A: In a first instance, because the barriers are quite different. In this particular case, the systems were similar, but not equal: there were different electron-withdrawing groups attached to the aldehyde.
Q: Please comment on the site dependency of the Binding energies.
A: For example different binding sites offer a different number of hydrogen bonds to the adsorbate. They also may lead to more or less buried adsorbates. All that influences the binding energy.
Infrared Resonant Vibrationally Induced Restructuring of Amorphous Solid Water
Queen Mary University of London
Amorphous solid water (ASW) is abundantly present in the interstellar medium, where it forms a mantle on interstellar dust particles and it is the precursor for cometary ices. In space, ASW acts as substrate for interstellar surface chemistry leading to complex molecules and it is postulated to play a critical role in proton-transfer reactions. Although ASW is widely studied and is generally well characterized by different techniques, energetically induced structural changes, such as ion, electron and photon irradiation, in these materials are less well understood. Selective pumping of specific infrared (IR) vibrational modes can aid in understanding the role of vibrations in restructuring of hydrogen bonding networks. In this talk, I will present the first experimental results on hydrogen bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the FELIX Laboratory at the Radboud University in Nijmegen, the Netherlands. Experiments are complimented with Molecular Dynamics simulations to constrain the effect at the molecular level.
Co-Authors: Jennifer A. Noble, Herma M. Cuppen, Stephane Coussan, Britta Redlich
Originally Given 24 June 2020
Bonus Questions & Answers - Sergio's Talk
Answers below from both Sergio and Jennifer Noble.
Q: Do you have a feeling for how things would change not only based on the structure of the ice, but on the roughness or porosity of the surface, since you are investigating surface modes?
A: Yes, we also have a lot of data on compact versus porous water ice. We’re currently working on the analysis, but it does seem to modify the ice response, especially at the surface.
Q: Can you comment on what would happen if you irradiated the H2O combination band? What motivated your choice of band to irradiate?
A: If I recall well, we do see a similar effect when irradiatiating the combination mode. I did not discuss it due to the limited time. We extensively irradiated the MIR range at different frequencies, but here for simplicity I have shown only the main absorption peaks. We are working on a series of papers discussing all these different irradiations. So stay tuned :)
A: One of the biggest motivations to work at FELIX was the wide spectral range, allowing irradiation of vibrational modes from 2.7 microns through the MIR to the THz. In fact, we irradiated all major vibrational bands of water ice (amorphous and crystalline) to determine whether the effects that we observed were mode-dependent or frequency-dependent. The combination mode did give a similar, but less intense, response - maybe due to its low band strength.
Q: Do you think energy is used quickly to change the structure or do you expect there to be local hotspots that live for a short amount of time?
A: The ice should relax quite quickly. MD simulations show that it relaxes somehow between shots, but an overall local heating builds up in time. This is seen in the experiments as well but we are not very sensitive to that because the temperature sensore is far away from the ice, in the substrate. However the effect we see are not global heating.
Q: How do you think these changes reflect a thin porous ices? Do you expect larger cracks in the ice to influence the structure change?
A: We did study FEL irradiation as a function of ice thickness and found that desorption is more efficient at the surface of thick ices likely due to the bad conductivity of ASW. However changes are seen at all studied thicknesses, i.e. up to a few layers. Of course cracks and nucleation can help the crystallization process.
Q: To what extend will VUV be different from IR irradiation in changing the structure of water ice? Could VUV change crystalline back to ASW or do you expect the same behavior as with IR?
A: I forgot to add that VUV/electrons/ions do have enough energy to break molecular bonds disrupting the intermolecular bond network as well. We do not see nor expect that when using IR light. Therefore the final result should be different than selective IR irradiation of ices. However it is important to study the effect of IR photons to isolate and better understand how vibrational energy transfers and dissipate within the ice at the surface vs the bulk.
Q: Hi Sergio, thank you for your talk! Have you experiments led to any new insights on ozone ice chemistry? Have you found new production/destruction pathways?
A: We did not see any chemistry occurring in the pure water ice, but only restructuring. I plan to do experiments with radicals to basically investigate IR photon-induced thermal chemistry. Ozone would be a good candidate. I have not done that yet though.
Q: Is it possible to study ice mixtures on LISA?
A: Yes, absolutely. There is a gas mixing ramp that allows the preparation of gas phase mixtures at a wide range of concentrations. We can quantify the concentrations by a combination of partial pressures in the mixing ramp, and QMS & FTIR calibration experiments in the chamber.
Q: Low energy electrons also have a reasonably high cross-section for absorption of vibrational modes (Mason et al. 2003). Do you think that since a single Galactic Cosmic Ray can generate millions of low energy electrons, this could be a process that is responsible for generating crystallization in the ISM?
A: The answer is probably yes, but electrons will also induce chemistry in the ice, breaking molecular bonds. This effect will compete with crystallization and likely induce amorphyzation. Hence it is important to selectively study both these mechanisms.
Imaging the H2O and CO Snowlines Around Young Stars
Merel van't Hoff
University of Michigan
Planets form in disks of gas and dust around young stars. With the discovery of more than 4000 exoplanets and the ability to study circumstellar disks in great detail we can begin to address the question of how the composition of a planet is linked to its birth environment. Key aspects of circumstellar-disk chemistry are snowlines: radii at which molecular species freeze out from the gas onto dust grains. The temperature at which a molecule freezes out is species dependent. The radial temperature profile in the disk therefore results in radial gradients in the chemical composition, for example, the elemental C/O-ratio of both the gas and ice. The bulk composition of planets may therefore be regulated by their formation location with respect to major snowlines. In this talk I will focus on how we can determine the locations of the two most important snowlines, the H2O and CO snowlines, observationally.
Originally Given 24 June 2020
Bonus Questions & Answers - Merel's Talk
Q: I guess the same question goes for HCO+ and H2O. What kind of uncertainty is there?
A: An uncertainty/degeneracy that is more important for H2O/H13CO+ than for CO/N2H+, especially in disks, is the dust opacity which can create ring-shaped H13CO+ emission if the dust in the inner disk is optically thick. Since the CO snowline is at larger radii this is not so much a problem for N2H+.
Q: Are there other clever tracers like this for the other lines? Like N2? NH3?
A: N2H+ also works for N2, as long as the snow surface is high up in the disk such that there is a substantial drop in the N2H+ column density outside the N2 snowline. Qi et al. (2019) derived N2 snowlines for the three disks I showed. For NH3 or other snowlines, you would need a molecule whose chemistry is dominated by the presence of absence of gas-phase NH3. And then to be really able to use it, this molecule should be abundant and observable in disks. I am not aware of such molecule to trace the NH3 snowline.
Interstellar Comets: A New Window into the Diversity of Protoplanetary Disk Midplane Chemistry
Comets contain a crucial record of the chemistry that occurred in the proto-Solar accretion disk during the epoch of formation of our planets. The recent discovery of interstellar comets provides a unique opportunity to measure the composition of planetary materials originating around other stars. Consequently, we are entering a new era of Galactic astronomy, with the ability to directly investigate the chemistry that occurred in the disk midplanes of extrasolar planetary systems. This talk will summarise our knowledge of the recently-detected interstellar objects 1I/'Oumumua and 2I/Borisov, focusing on our ALMA observations of gas-phase CO and HCN in 2I/Borisov. While the HCN abundance relative to water (~0.11%) was similar to that found in typical Solar System comets, the CO abundance (~68%) was among the highest observed in any comet within 2 au of the Sun, revealing a likely origin in a CO-enriched environment. The importance of this finding in the context our understanding of cometary and protoplanetary disk chemistry - in particular, the ice chemistry occurring close to the CO snowline - will be discussed.
Originally Given 24 June 2020
Bonus Questions & Answers - Martin's Talk
Q: How does the CO/H2O ratio you observed compare to the observations made with the HST? Does that work agree with yours on the interpretation of the origin of the high CO abundance?
A: The CO abundances observed with HST were about as high or higher than we found with ALMA. In fact, there is a clear trend in the HST data of increasing CO/H2O with time.
Q: Staying on theme with the previous talks, how might the structure of the ice alter the abundances that are being shed into the gas phase? Crystalline vs amorphous?
A: There is a lot we don’t know about the structure of cometary ice. Variations in outgassing rates with heliocentric distance could be taken as evidence for trapping of different volatiles in the ice matrix, so I think that’s something that should be looked at in the lab.
Cosmic Rays and Grain Chemistry in Star- and Planet-forming Regions
Max Planck Institute for Extraterrestrial Physics
Interstellar matter is subjected to bombardment by several types of ionizing radiation including cosmic rays, stellar winds, x-rays, and gamma-rays. It is known that such radiation can have a significant physicochemical impact on interstellar environments, and a large body of experimental work has shown that the interaction between such energetic particles and low-temperatures ices can result in the formation of complex - even prebiotic - molecules. Even so, modeling the chemical effects of cosmic ray collisions with interstellar dust grain ice mantles has proven challenging due to the complexity and variety of the underlying physical processes. In this talk, recent work on this subject by us is reviewed and the possible applications to better understanding the chemistry in star and planet-forming regions is highlighted.
Originally Given 6 May 2020
The Effects of Cosmic Rays on Carbon Chemistry
Universität zu Köln
Cosmic rays (CRs) are energetic charged particles accelerated in extreme environments. Galactic CRs are accelerated primarily through supernovae. However, it has been recently proposed that protostars may be able to accelerate CRs. These cosmic rays will substantially alter the carbon chemistry in molecular gas. I will present astrochemical calculations of the impact of different CR populations and physics on carbon chemistry. The calculations were performed on a modified version of the publicly available astrochemistry code, 3D-PDR, in which CR attenation has been included in-situ. As such, we're to directly study the impact of different external and internal CR spectra. We find embedded sources of CRs significantly impact the carbon chemistry in relation to the HI-to-Hw transition. The dense gas is heated above 50 K and bright in [CII] emission. Embedded CR sources allow for atomic carbon to exist in dense gas where traditional models have carbon locked into carbon monoxide. Finally, I will show that the CO-to-H2 conversion factor used in extragalactic studies is relatively stable to significant enhancements of the CR ionization rate. However, the CI-to-H2 conversion factor is sensitive to the CR ionization rate and external environment.
Originally Given 6 May 2020
Probing Galactic Cosmic Rays with Small Molecules
Johns Hopkins University
In the century following their discovery by Victor Hess in 1912, cosmic rays have been recognized as an important constituent of the Galaxy. With a total energy density somewhat larger than that of starlight, cosmic rays are the dominant source of ionization for the cold neutral medium (CNM) within the Galactic ISM. In starless molecular cloud cores, they are also the dominant source of heating. Thus, cosmic rays play a central role in astrochemistry by initiating a rich ion-neutral chemistry that operates within the CNM, and the cosmic-ray ionization rate (CRIR) is a key parameter in models for the chemistry of the ISM. In this talk, I will discuss recent estimates for the cosmic-ray ionization rate in the Galactic disk, obtained by using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously-published measurements of the abundance of four molecular ions: ArH+, OH+, H2O+ and H3+. The CRIR estimates thereby obtained show a remarkably small dispersion from one interstellar cloud to another. At the Galactocentric distance of the Sun, the primary CRIR per H nucleus is ~ 2x10-16 s-1 in both diffuse atomic clouds and diffuse molecular clouds. I will also discuss a recently-selected SOFIA Joint Legacy Program, HyGAL, which (among other things) will greatly expand the number of sight-lines on which ArH+, OH+, and H2O+ have been observed.
Originally Given 6 May 2020
From Clouds to Planets, The Astrochemical Link
The Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics
All ingredients to make stars like our Sun and planets like our Earth are present in the dense (~100,000 H2 molecules per cc) and cold (~10 K) interstellar clouds. In these "stellar-system precursors" an active chemistry is already at work, as demonstrated by the presence of a rich variety of organic molecules in the gas phase and icy mantles encapsulating the sub-micrometer dust grains, the building blocks of planets. Here, I’ll present a journey from the earliest phases of star formation to protoplanetary disks, with links to our Solar System, highlighting the crucial role of astrochemistry as powerful diagnostic tool of the various steps present in the journey.
Originally Given 22 April 2020