Aromatic chemistry is a key component of prebiotic chemistry and the formation of dust and planets. A major question about these molecules is whether they form from the destruction of large carbon clusters in evolved stars (top-down) or from the build-up of small organics in dark clouds (bottom-up). Up until recently, much of this chemistry was either unconstrained or only understood in aggregate by infrared vibrational transitions of multiple-ringed species known as Polycyclic Aromatic Hydrocarbons (PAHs). During my Ph.D, we discovered the first aromatic molecule with radio astronomy, benzonitrile (c-C6H5CN) toward the dark cloud, TMC-1, which has opened the door to a whole new regime of interstellar chemistry (McGuire, Burkhardt et al. 2018). As Co-PI of the GBT Large Project GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) and PI of the ARKHAM (A Rigorous K/Ka-Band Hunt for Aromatic Molecules) survey, I am working to study the formation of carbon chemistry, both rings and chains, at the earliest stages of star formation by leading the observations, data reduction, and chemical modeling efforts. Totaling over 700 hours upon completion, these surveys have already published/submitted the detection of unique 16 molecules, with many more on the way!
Rich Reservoir of Rings and Chains
Through a deep spectral survey of TMC-1, we are continually detecting plentiful new aromatics and complex carbon-chain chemistry. This includes the individually detected first polycyclic aromatic hydrocarbons (cyanonaphthalene, c-C10H7CN; McGuire, Loomis, Burkhardt et al. 2021), the first purely hydrocarbon PAH (indene, c-C9H8; Burkhardt et al. 2021b), and the longest carbon chain yet (HC11N; Loomis, Burkhardt et al. 2021). With many many more to come!
Ubiquitous Aromatics with ARKHAM
TMC-1 is not the only interesting source to study aromatic chemistry! In order to determine how and where these molecules form, we need to study how it changes at different ages and conditions. Recently, we successfully detected benzonitrile (c-C6H5CN) toward the first 5 sources we looked for it and found a potential correlation between ring/chain ratios and parent cloud (Burkhardt et al. 2021a). We are currently working to search for aromatics towards many more environments.
Chemical Modeling of Aromatics
As we detect many new exciting molecules, we can now begin to constrain the underlying physical and chemical processes that produce them and where it could occur. Excitingly, the aromatic abundances we measure far exceed what our chemical models would predict, indicating that this regime over chemistry is far more extensive than one would predict. We are actively working on the inclusion of new physical and chemical processes to our chemical models to explain what GOTHAM is revealing.
Studying the isotope-substituted species of molecules (isotopologues) can help constrain the underlying chemical pathways that produce them. Alongside the detection of benzonitrile, we also detected many such species for HC5N and HC7N, which provides insight to possible key precursors to this family of molecules (Burkhardt et al. 2018). As part of GOTHAM, we will substantially increase the number of known isotopologues. As such, we are also developing chemical models to study this type of chemistry in detail.