Brett A. McGuire

AS · TRO · CHEM · IS · TRY

/ astrō'kemestrē /
(n) the study of molecules in space - where they are, how they got there, and what they are doing

Research

Lagoon Nebula near the Galactic Center

NASA missions have found some of the most chemically diverse organic materials ever detected in astronomical environments yet there is no agreed upon chemical pathway as to their formation. We know from meteorites and, more recently, cometary samples returned by the STARDUST mission that amino acids, the building blocks of life as we understand it, are present in extraterrestrial sources (Elsila et al. 2009). In the last decade, complex gas-grain chemical models have become widely used tools in the attempt to understand the chemical pathways that can result in the species observed and their abundances in interstellar environments. A key goal of these models is to attempt to predict the most likely chemical pathways for the formation of life-essential molecules, such as amino acids. While such methods are valuable, they suffer both from a lack of laboratory data and a lack of observational data with which to constrain them. Thus, laboratory studies and observational follow-ups are vital.

Legend Elite femtosecond laser used to generate high-power, broadband THz pulses

For my PhD work, I constructed a spectrometer to measure the far-infrared (THz) absorption spectra of interstellar ice analogs. Such ices may be the dominant source of complex molecule formation in the ISM, and yet their compositions are largely unknown due to the difficulty of characterizing them using known infrared spectra. The THz region of the spectrum, which overlaps well with the new SOFIA observatory’s capabilities, provides the opportunity for unambiguous observation and characterization of these ices once laboratory data are known.

As part of my postdoc, I've transitioned into the microwave/mm/sub-mm region of the spectrum with high-resolution, gas-phase rotational spectroscopy of reactive or transient species. Rotational spectra are, in principle, completely defined by the three moments of inertia of a molecule. Thus, in addition to providing the spectral signatures needed for identifications in the ISM, the exact geometries of these species can be determined as well. In turn, when coupled with high-level ab initio calculations, these geometries provide the precise energetics of a system needed to robustly understand the formation pathways and mechanisms used in models of formation chemistry in the ISM.

The Milky Way stretches over the Ancient Bristlecone Pine Forest. In the distance are the glows of the Combined Array for Research in Millimeter-wave Astronomy and Big Pine, CA.

I am also a member of a team of astronomers working to expand our knowledge of the gas-phase chemical inventories in the ISM through the Prebiotic Molecular Survey of the Sgr B2(N) star-forming region. As a result, I have published the first detection of propylene oxide (CH₃CHCH₂O), the first chiral molecule found outside our solar system, and carbodiimide (HNCNH) a new astronomical maser. Through observational programs such as this, which rely heavily on complimentary laboratory efforts, I hope to shed light on the processes which can give rise to species such as glycine in the ISM.

The Taurus Molecular Cloud, a collection of gas and dust that blocks out the starlight from behind, hangs in the sky alongside the Pleiades Cluster and the bright star Aldebaran, in the constellation of Taurus.

Most recently, I led an effort which resulted in the first detection of an aromatic, benzene-ring containing molecule in the interstellar medium: benzonitrile (c-C₆H₅CN). It's thought that a class of interstellar aromatic benzene-ring molecules known as the polycyclic aromatic hydrocarbons (PAHs) contain ~10% of all the carbon in the universe. Due to their unique structural properties, however, it is extraordinarily challenging to observe them. Our detection of benzonitrile changes the game on this, and offers us the ability to study these types of molecules routinely, and without the need for the infrared space-based telescopes we have had to use (with limited success) until now. As part of my microwave spectroscopy work, I have undertaken a comprehensive follow-up program to study the formation of these ring structures from non-cyclic precursor molecules in the laboratory.

Selected Publications

For a full list of publications, please see my CV.

First Results of an ALMA Band 10 Spectral Line Survey of NGC 6334I: Detections of Glycolaldehyde (HC(O)CH₂OH) and a New Compact Bipolar Outflow in HDO and CS

Brett A. McGuire, Crystal L. Brogan, Todd R. Hunter, Anthony J. Remijan, Geoffrey A. Blake, Andrew M. Burkhardt, P. Brandon Carroll, Ewine van Dishoeck, Robin T. Garrod, Harold Linnartz, Christopher N. Shingledecker, and Eric R. Willis.

Astrophysical Journal Letters 2018, 863, 35.

We present the first results of a pilot program to conduct an Atacama Large Millimeter Array (ALMA) band 10 spectral line survey of the high-mass star-forming region NGC 6334I. The observations were taken in exceptional weather conditions (0.19 mm precipitable water) with typical system temperatures T_sys < 950 K at ∼890 GHz. A bright, bipolar north-south outflow is seen in HDO and CS emission, driven by the embedded massive protostar MM1B. This has allowed, for the first time, a direct comparison of the thermal water in this outflow to the location of water maser emission from prior 22 GHz Very Large Array observations. The maser locations are shown to correspond to the sites along the outflow cavity walls, where high-velocity gas impacts the surrounding material. We also compare our new observations to prior Herschel Heterodyne Instrument for the Far-infrared (HIFI) spectral line survey data of this field, detecting an order of magnitude more spectral lines (695 versus 65) in the Atacama Large Millimeter/submillimeter Array (ALMA) data. We focus on the strong detections of the complex organic molecule glycolaldehyde (HC(O)CH₂OH) in the ALMA data that is not detected in the heavily beam-diluted HIFI spectra. Finally, we stress the need for dedicated THz laboratory spectroscopy to support and exploit future high-frequency molecular line observations with ALMA. (Full Paper)

Detection of the Aromatic Molecule Benzonitrile (c-C₆H₅CN) in the Interstellar Medium

Brett A. McGuire, Andrew M. Burkhartd, Sergei Kalenskii, Christopher N. Shingledecker, Anthony J. Remijan, Eric Herbst, and Michael C. McCarthy

Science 2018, 359, 202.

Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are thought to be widespread throughout the Universe, because these classes of molecules are probably responsible for the unidentified infrared bands, a set of emission features seen in numerous Galactic and extragalactic sources. Despite their expected ubiquity, astronomical identification of specific aromatic molecules has proven elusive. We present the discovery of benzonitrile (c-C₆H₅CN), one of the simplest nitrogen-bearing aromatic molecules, in the interstellar medium. We observed hyperfine-resolved transitions of benzonitrile in emission from the molecular cloud TMC-1. Simple aromatic molecules such as benzonitrile may be precursors for polycyclic aromatic hydrocarbon formation, providing a chemical link to the carriers of the unidentified infrared bands. (Full Paper)

Electron Donor-Acceptor Nature of the Ethanol-CO₂ Dimer

Brett A. McGuire, Marie-Aline Martin-Drumel, and Michael C. McCarthy

J. Phys. Chem. A 2017, 121, 6283.

Supercritical CO₂ is an appealing nontoxic, environmentally friendly solvent for the industrial extraction of many classes of compounds, from caffeine to natural product drug precursors to petrochemical impurities. Apolar in isolation, the ability of supercritical CO₂ to dissolve polar species has been empirically shown to be greatly enhanced by the addition of a small molar percentage of a polar cosolvent, often ethanol. Computational work predicts that the isolated ethanol–CO₂ complex can exist either in an electron-donor configuration or through a hydrogen-bonding one; yet, neither has been previously experimentally observed. Here, we demonstrate by rotational spectroscopy that the isolated, gas-phase ethanol–CO₂ dimer is an electron donor–acceptor complex. (Full Paper)

Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH₃CHCH₂OH)

Brett A. McGuire and P. Brandon Carroll, Ryan A. Loomis, Ian A. Finneran, Philip R. Jewell, Anthony J. Remijan, and Geoffrey A. Blake

Science 2016, 352, 1449.

We report the first detection of propylene oxide (CH₃CHCH₂OH) in the interstellar medium. This is the first detection of a chiral molecule outside of our solar system. Life relies on chiral molecules, and, because it enables the building of complex chiral structures like helices, often uses exclusively one enantiomer of a given molecule across the entire biosphere. The origin of this homochirality may be able to be traced back to the formation of chiral species in the interstellar medium. A number of proposals for the generation of a slight enantiomeric excess in these regions have been proposed, but there has not been a chiral target for testing these hypotheses until now. (Full Paper)

Molecular Polymorphism: Microwave Spectra, Equilibrium Structures, and an Astronomical Investigation of the HNCS Isomeric Family

Brett A. McGuire, Marie-Aline Martin-Drumel, Sven Thorwirth, Sandra Brünken, Valerio Lattanzi, Justin L. Neill, Silvia Spezzano, Zhenhong Yu, Daniel P. Zaleski, Anthony J. Remijan, Brooks H. Pate, and Michael C. McCarthy

Physical Chemistry Chemical Physics 2016, 18, 22693.

We present the laboratory rotational spectra of the [H,N,C,S] isomeric family: HNCS, HSCN, HCNS, and HSNC at high spectral resolution using a combination of chirped-pulse and cavity-enhanced Fourier-transform microwave spectroscopy. Relative stabilities and formation pathways are discussed. Semi-experimental equilibrium structures are derived based on isotopic substitution measurements and high-level quantum-chemical calculations. An astronomical search in Sgr B2(N) shows a strong detection of HSCN, weak evidence for HNCS, and a non-detection of HCNS and HSNC. Weak masing may be occurring in some transitions of HSCN and HNCS. (Full Paper)

CSO and CARMA Observations of L1157. I. A Deep Search for Hydroxylamine (NH₂OH)

Brett A. McGuire, P. Brandon Carroll, Niklaus M. Dollhopf, Nathan R. Crockett, Joanna F. Corby, Ryan A. Loomis, Andrew M. Burkhardt, Christopher Shingledecker, Geoffrey A. Blake, and Anthony J. Remijan

Astrophysical Journal 2015, 812, 76.

A deep search for the potential glycine precursor hydroxylamine (NH₂OH) using the Caltech Submillimeter Observatory (CSO) at λ = 1.3 mm and the Combined Array for Research in Millimeter-wave Astronomy at λ = 3 mm is presented toward the molecular outflow L1157, targeting the B1 and B2 shocked regions. We report non-detections of NH₂OH in both sources. We perform a non-LTE analysis of CH3OH observed in our CSO spectra to derive the kinetic temperatures and densities in the shocked regions. Using these parameters, we derive upper limit column densities of NH₂OH of ≤1.4 x 10¹³ cm⁻² and ≤1.5 x 10¹³ cm⁻² toward the B1 and B2 shocks, respectively, and upper limit relative abundances of N_NH₂OH/N_H₂ ≤1.4 x 10^-8 and ≤1.5 x 10⁻⁸ , respectively. (Full Paper)

Ignition of Thermite Using the Potassium Chlorate "Rocket" Reaction: A Systematic Demonstration of Reaction Chemistry

Brett A. McGuire, P. Brandon Carroll, Adam N. Boynton, Jeffrey M. Mendez, and Geoffrey A. Blake

Journal of Chemical Education 2015, 92, 1117

Presented here is a set of demonstrations that are used as visual tools for engaging students in a discussion of reaction chemistry and thermodynamics. Students are first shown a series of simple exothermic reactions: (1) the reaction of H₂SO₄ with sugar, (2) the decomposition of KClO₃, and (3) the reaction of H₂SO₄ with KClO₃. These three basic reactions are then combined as a pyrotechnic chlorate demonstration in which H₂SO₄ is used to ignite a KClO₃ + sugar mixture. It is finally shown that this latter reaction is very effective for igniting the highly exothermic thermite redox reaction. (Full Paper)

An Observational Investigation of the Identity of B11244 (l-C₃H⁺/C₃H^-)

Brett A. McGuire, P. Brandon Carroll, Pierre Gratier, Viviana Guzman, Jerome Pety, Evelyne Roueff, Maryvonne Gerin, Geoffrey A. Blake, and Anthony J. Remijan

Astrophysical Journal 2014, 783, 36

Pety et al. have reported the detection of eight transitions of a closed-shell, linear molecule (B11244) in observations toward the Horsehead photodissociation region (PDR), which they attribute to the l-C₃H⁺ cation. Recent high-level ab initio calculations have called this assignment into question; the anionic C₃H^- molecule has been suggested as a more likely candidate. Here, we examine observations of the Horsehead PDR, Sgr B2(N), TMC-1, and IRC+10216 in the context of both l-C₃H⁺ and C₃H^-. We find no observational evidence of K_a = 1 lines, which should be present were the carrier indeed C₃H^-. Additionally, we find a strong anticorrelation between the presence of known molecular anions and B11244 in these regions. Finally, we discuss the formation and destruction chemistry of C₃H^- in the context of the physical conditions in the regions. Based on these results, we conclude there is little evidence to support the claim that the carrier is C₃H^-. (Full Paper)

Photography

In the last few years I've taken an interest in amateur nature photography and astrophotography. I haven't quite gotten to the point of investing in a dedicated telescope for imaging, but I will give in soon enough. A small selection of my photos are below.

View of Lake Wanaka in the south island of New Zealand from the summit of Rocky Mountain on the Diamond Lake trail. Colors are not enhanced - it really is that blue!

Comet Wirtanen as seen from Champaign, IL on December 16th, 2018 very near its closest approach to Earth. About 5 minutes of total integration in 3 second increments at 50 mm.

August 21st, 2017 total Solar Eclipse as seen from just outside Shawnee National Forest, IL.

A New Zealand sheep questions my presence. The Rob Roy Glacier can be seen in the background.

Andromeda as seen from Red Rocks Canyon State Park, CA. 10 minutes of integration at 300 mm (20 x 30s subs). Nikon D5100.

The Galactic Center as seen from Red Rocks Canyon State Park, CA. 15 minutes of integration at 16 mm (3 x 300s subs). Nikon D5100.

Sunset over the Owens Valley Radio Observatory. In the distance, smoke rises from fires in Yosemite National Forest.

The Milky Way rises over Lake Waiau on Mauna Kea.

An osteospermum "whirlygig" flower on Caltech's campus.

Sunset at the Asilomar Conference Grounds after the Pacific Conference on Spectroscopy and Dynamics.

Always try to fail better the next time

E-mail: bmcguire@nrao.edu