On Cosmic-Ray-driven Grain Chemistry in Cold Core Models

Christopher N. Shingledecker1, Jessica Tennis1, Romane Le Gal1,2, and Eric Herbst1,3
The Astrophysical Journal 861, 20 Link to Article [https://doi.org/10.3847/1538-4357/aac5ee]
1Department of Chemistry, University of Virginia Charlottesville, VA 22904, USA
2Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
3Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA

In this paper, we present preliminary results illustrating the effect of cosmic rays on solid-phase chemistry in models of both TMC-1 and several sources with physical conditions identical to TMC-1 except for hypothetically enhanced ionization rates. Using a recent theory for the addition of cosmic-ray-induced reactions to astrochemical models, we calculated the radiochemical yields, called G values, for the primary dust grain ice-mantle constituents. We show that the inclusion of this nonthermal chemistry can lead to the formation of complex organic molecules from simpler ice-mantle constituents, even under cold core conditions. In addition to enriching ice mantles, we find that these new radiation-chemical processes can lead to increased gas-phase abundances as well, particularly for HOCO, NO2, HC2O, methyl formate (HCOOCH3), and ethanol (CH3CH2OH). These model results imply that HOCO—and perhaps NO2—might be observable in TMC-1. Future detections of either of these two species in cold interstellar environments could provide strong support for the importance of cosmic-ray-driven radiation chemistry. The increased gas-phase abundance of methyl formate can be compared with abundances achieved through other formation mechanisms such as pure gas-phase chemistry and surface reactions.

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