On Cosmic Ray-Driven Grain Chemistry in Cold Core Models

TL;DR

This study investigates how cosmic rays influence solid and gas-phase chemistry in cold interstellar clouds, showing they can produce complex molecules and increase observable species like methyl formate, with implications for astrochemical models.

Contribution

Introduces a new model incorporating cosmic ray-induced reactions into astrochemical simulations, highlighting their role in forming complex organic molecules in cold cores.

Findings

Cosmic ray-driven chemistry enhances complex organic molecule formation.

Increased gas-phase abundances of HOCO, NO$_2$, and methyl formate.

Potential observability of HOCO and NO$_2$ in TMC-1.

Abstract

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.