On Cosmic Ray-Driven Grain Chemistry in Cold Core Models

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 non-thermal 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, NO$_2$, HC$_2$O, methyl formate (HCOOCH$_3$), and ethanol (CH$_3$CH$_2$OH). These model results imply that HOCO - and perhaps NO$_2$ - 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 three-body surface reactions.