Probing Isolated Ice and Gas-Phase Chemistry in Astrophysical Environments

Author: ORCID icon
Burkhardt, Andrew, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Remijan, Anthony, As-Astronomy, University of Virginia

With the advent of modern astrophysical observatories such as the Atacama Large Millimeter/submillimeter Array (ALMA), the Karl J. Jansky Very Large Array (VLA), and the Robert C. Byrd Green Bank Telescope (GBT), the field of astrochemistry is now capable of probing into the heart of many of the most chemically-rich and complex astrophysical sources, such as massive star-forming regions, protoplanetary disks, and evolved stars. However, as new sensitivities and spatial resolutions are achieved, the synthesis of many competing physical and chemical processes across the dynamic history of these sources make it difficult to disentangle the underlying chemistry. It is a worthwhile endeavor, then, to study the earlier stages of star formation where many of these processes, instead, occur in relative isolation. Here, we perform two experiments to probe the gas-phase and solid-phase chemistry in prototypical environments.

To start, we examine how carbon-chain gas-phase chemistry evolves in isolation within the dark cloud Taurus Molecular Cloud (TMC-1). Here, we performed a deep spectral survey of TMC-1 with the GBT across 1.8 GHz of bandwidth between 18 and 24 GHz to find new, exotic carbon-chain molecules. First, we report the detection of 7 new isotopologues of the large cyanopolyynes, HC5N and HC7N, and, by comparing the relative isotopic ratios, conclude that cyanopolyynes may not have a consistent formation route across the molecular family. Next, we discuss the first radio detection of an aromatic molecule, benzonitrile (c-C6H5CN), which may prove to both be a key proxy for benzene and a crucial chemical link between small carbon chain and the ubiquitous polycyclic aromatic hydrocarbons (PAHs). Finally, we describe the detection of HC5O and the tentative detection of HC7O, whose previously undiscovered molecular family appears to have a formation chemistry that varies significantly from the cyanopolyynes.

In the second half of this manuscript, we study how shocks can impact the chemistry in astrophysical environments and be used as a probe for the relatively unconstrained interstellar solid-phase chemical reservoir. First, we discuss the results from interferometric observations of key shock tracing molecules within the prototypical chemically-active outflow, L1157. Here, we find that shocks affect the abundance of molecules by either primarily sputtering ice-produced species into the gas phase, inducing post-shock gas-phase chemistry, or disrupting the abundance of gas-phase species through the extreme physical conditions in shocks. Second, we reproduce the majority of the predictions made from these observations by adapting the three-phase gas-grain chemical network model, NAUTILUS, to include relevant shock physical conditions and processes. Furthermore, this model can make relevant, testable predictions on the temporal evolution of numerous shock-relevant molecules in molecular outflows. And so, with work presented here, we have shown that the underlying chemistry in the interstellar medium can be efficiently and effectively constrained by studying relatively simple and isolated astrophysical environments in great detail.

PHD (Doctor of Philosophy)
Astrochemistry, Interstellar Medium, Outflows, Dark Clouds, Molecules
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