Videos

August 2021 Extravaganza - Virtual Lab Tours!

Sims Lab Tour

Join Ian Sims and his group for a tour of his laboratory and their CRESU-CHIRP instrumentation at the University of Rennes 1!

You can find out more about their work at:
cresuchirp.wordpress.com

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

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

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

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.

Phosphorus Carriers in the Protosolar Analog B1-a

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.

PO and PN in Orion-KL: Insights to the Phosphorus Chemistry of Molecular Clouds

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

The Interstellar Journey of Phosphorus

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.

Vibrational and rotational characterization of phenylpropiolonitrile to facilitate astrochemical surveys

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.

The 130 – 370 GHz Rotational Spectra of Four Astronomically Relevant Cyanobutadiene Isomers (C5H5N)

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

Finding Unknown Unknowns for Astrochemistry with Broadband Rotational Spectroscopy

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.

The Search for New Molecular Species in the Atmosphere of Titan

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.

Photochemistry in Exoplanet Atmospheres: Insight from Lab Experiments

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.


High-Temperature Non-LTE and LTE Cavity Ringdown Spectroscopy using SMAUG experimental setup

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 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

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.

The elemental composition of planet feeding gas

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.

Connecting astrochemistry and planet formation: A C/O main sequence for warm and hot giant planets

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.

From Hot-corinos to our chemical heritage; the case of NGC1333 IRAS 4A

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.


Molecular spectroscopic line lists

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.


New solid-state environments for the formation of complex organics

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.


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.

August Extravaganza!!

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
Univeristät Bern

Benzene on Ice and in the ISM

David Benoit
University of Hull

Matrix-Isolation FT-IR Spectroscopy of Nitrogen Based Heterocyclic Radicals

Mayank Saraswat
Indian Institute of Science Education and Research (IISER) Mohali, India

VUV Spectroscopy for Laboratory Astrophysics and Astrochemistry

Yu-Jung Chen
National Central University, Taiwan

Sketch Your Science!

Olivia Harper Wilkins
California Institute of Technology

Submit your art to Olivia's Sketch Your Science Contest at https://forms.gle/9P2BGKrG4chFbp9g6!

Pointing the Green Bank Telescope

Ellie White
Marshall University

My 15 Years of Astrochemistry

Zainab Awad
Cairo University

Laboratory Measurements of Gas-Phase Reaction Kinetics with CN radical at Low Temperatures

Divita Gupta
Univ. Rennes

Did Photons or Electrons Create Life?

Ella Mullikin
Wellesley College

Photodissociation of CS via Highly Excited Electronic States: Ab Initio and Experimental Study

Zhongxing Xu
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

Mark Blitz
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

Johannes Kästner
Universität Stuttgart

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

Sergio Ioppolo
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

Martin Cordiner
NASA Goddard

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

Christopher Shingledecker
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

Brandt Gaches
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

David Neufeld
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

Paola Caselli
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