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Ubiquitous aromatic carbon chemistry at the earliest stages of star formation

Abstract

Benzonitrile (c-C6H5CN, where ‘c’ indicates a cyclic structure), a polar proxy for benzene (c-C6H6), has the potential to serve as a highly convenient radio probe for aromatic chemistry, provided that this ring can be found in other astronomical sources beyond the molecule-rich prestellar cloud TMC-1. Here we present radio astronomical evidence of benzonitrile in four other prestellar, and possibly protostellar, sources: Serpens 1A, Serpens 1B, Serpens 2 and MC27/L1521F. These detections establish that benzonitrile is not unique to TMC-1; rather, aromatic chemistry appears to be widespread throughout the earliest stages of star formation, probably persisting at least until the initial formation of a protostar. The abundance of benzonitrile far exceeds predictions from models that well reproduce the abundances of carbon chains such as HC7N, a cyanpolyyne with the same heavy atoms, indicating that the chemistry responsible for planar carbon structures (as opposed to linear ones) in primordial sources is favourable but not well understood. The abundance of benzonitrile relative to carbon chain molecules displays sizable variations between sources within the Taurus and Serpens clouds, implying the importance of physical conditions and initial elemental reservoirs of the clouds themselves.

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Fig. 1: Velocity-stacked spectra of C6H5CN and the impulse response function of the stacked spectra in the five sources observed here.
Fig. 2: Simulated abundances with respect to hydrogen from NAUTILUS chemical models for C6H6 and C6H5CN in comparison to that derived for C6H5CN from observations in the five sources studied here.
Fig. 3: Derived abundance ratios between HC7N, HC9N and C6H5CN for each of the five sources studied here.
Fig. 4: Simulated abundances and abundance ratios from NAUTILUS chemical models over a range of gas and grain temperatures, gas densities and initial elemental oxygen abundances.

Data availability

The datasets analysed during the current study are available in the Green Bank Telescope archive (https://archive.nrao.edu/archive/advquery.jsp). A user manual for their reduction and analysis is also available (https://go.nature.com/3npRxW5). For the ARKHAM survey, the complete, reduced survey data are available in the Harvard Dataverse Archive50. For the GOTHAM survey, the complete, reduced survey data in the X band are available as supplementary information in ref. 37. The individual portions of the reduced spectra used in the analysis of the individual species presented here are available in the Harvard Dataverse Archive49.

Code availability

All codes used in the MCMC fitting and stacking analysis presented in this paper are open source and publicly available at https://github.com/ryanaloomis/TMC1_mcmc_fitting.

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Acknowledgements

A.M.B. acknowledges support from the Smithsonian Institution as a Submillimeter Array Fellow. C.N.S. thanks the Alexander von Humboldt Stiftung/Foundation for their support. A.M.B. and C.N.S. also thank V. Wakelam for the use of the NAUTILUS v1.1 code. M.C.M and K.L.K.L. acknowledge financial support from NSF grant numbers AST-1908576 and AST-1615847, and NASA grant number 80NSSC18K0396. Support for B.A.M. was provided by NASA through Hubble Fellowship grant number HST-HF2-51396 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract number NAS5-26555. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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All authors contributed to the design of the GOTHAM and ARKHAM survey and helped to revise the manuscript. A.M.B and B.A.M. performed the astronomical observations and subsequent analysis. R.A.L., K.L.K.L. and B.A.M. performed the spectral fitting analyses. A.M.B. and C.N.S. contributed to or undertook the astronomical modelling and simulations. A.M.B., M.C.M. and B.A.M. wrote the manuscript with the help of C.N.S.

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Correspondence to Andrew M. Burkhardt or Brett A. McGuire.

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Supplementary Figs. 1–11, Tables 1–12 and Discussion.

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Burkhardt, A.M., Loomis, R.A., Shingledecker, C.N. et al. Ubiquitous aromatic carbon chemistry at the earliest stages of star formation. Nat Astron 5, 181–187 (2021). https://doi.org/10.1038/s41550-020-01253-4

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