A Search for Heterocycles in GOTHAM Observations of TMC-1

Abstract

We have conducted an extensive search for nitrogen-, oxygen-, and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.

Discovery of Interstellar 2-Cyanoindene (2-C9H7CN) in GOTHAM Observations of TMC-1

“…The fourth data reduction of the GOTHAM collaboration survey, hereafter referred to as DR4, includes new observations taken between 2020 June and 2022 May on the Robert C. Byrd 100 m Green Bank Telescope (GBT) in Green Bank, West Virginia. These new observations include the remainder of project code AGBT19B-047 not included in previous reductions, as well as AGBT20A-516, AGBT21A-414, and AGBT21B-210, which contributed an additional 953 hr to those presented in DR3 (Barnum et al 2022). All previous inclusions of 19B-047 and 18B-007 were reduced again to maintain a consistent method across the new data set, while the archived data from project codes AGBT18A-333, AGBT17A-164, and AGBT17A-43 were folded in for the final spectra.…”

The reactivity of pyridine in cold interstellar environments: The reaction of pyridine with the CN radical

“…From the last wave of detections numerous questions arise, for example, whether the molecules are formed from a bottom-up approach from simpler precursors or are produced from the fragmentation of, for example, graphene (Merino et al, 2014). Another fundamental question to be resolved is the apparent absence of heterocyclic species (molecules whose skeleton backbone contains other elements apart from carbon), for example pyridine, in relation with the bottom-up formation of cyclic structures and the possible conversion routes of these molecules once formed (Barnum et al, 2022).…”

“…These bands, in combination with others attributed to pure polycyclic aromatic hydrocarbon (PAH) ones, i.e., 3.3, 7.7, 8.6, 11.3, and 12.7 μm contribute to the so-called "Unidentified Infrared Emission" owing to the high blending of bands and the impossibility of disentangle the complex spectra. Conversely, such a limitation is not in place for radio astronomical observations, and specially considering that PANHs do possess a significant dipole moment which make them susceptible of detection (Mattioda et al, 2003;Malloci et al, 2007), and yet neither Nbearing heterocycles nor PANHs have been conclusively detected to date (Barnum et al, 2022).…”

10 Years of the ACS PHYS Astrochemistry Subdivision

“…One is an extension of the GOTHAM project where a search for heterocyclic species containing nitrogen, oxygen, and sulfur does not return any new detections in the Taurus Molecular Cloud. 1 Similarly, a radioastronomical search for methoxyacetone and methyl methoxyacetate does not find any correlating spectral features in Sagittarius B2(N), a giant molecular cloud found close to the center of the Milky Way Galaxy, even though new laboratory data are produced for comparison. 2 In spite of the negative results of these searches, these studies are providing new spectral data and exploring the possible natural formation of molecules with little previous impetus to examine.…”

“…To start, two manuscripts in this work focus, at least in part, on the observational portion of astrochemistry. One is an extension of the GOTHAM project where a search for heterocyclic species containing nitrogen, oxygen, and sulfur does not return any new detections in the Taurus Molecular Cloud . Similarly, a radioastronomical search for methoxyacetone and methyl methoxyacetate does not find any correlating spectral features in Sagittarius B2­(N), a giant molecular cloud found close to the center of the Milky Way Galaxy, even though new laboratory data are produced for comparison .…”