On Cosmic Rays in Astrochemical Models

Cosmic rays are known to have significant physicochemical impact on interstellar regions (Indriolo and McCall, 2013). For example, even in the very first astrochemical modeling work of Herbst and Klemperer (1973), cosmic rays were shown to drive the gas-phase chemistry of molecular clouds through the ionization of H2 and subsequent formation of H3+. Later, Prasad and Tarafdar (1983) demonstrated that cosmic rays - and the secondary electrons they generate - could collisionally excite the Lyman and Werner bands of H2, leading to the formation of internal UV photons in dense regions where the external interstellar UV radiation field is quickly attenuated. However, despite the central role that the ionization and excitation of H2 currently plays in astrochemical models, extending the treatment of these types of interactions to other species has proven challenging thanks to both the complexity and variety of underlying microscopic processes, which has stymied the development of suitable theoretical and computational techniques. In spite of these difficulties, radiation chemistry remains astrochemically attractive since, as demonstrated by an extensive body of work in laboratory astrophysics, non-thermal reactions triggered ice irradiation can lead to the formation of even complex organic molecules (Holtom et al., 2005; Hudson and Moore, 2001; Hudson et al., 2008; Rothard et al., 2017). Therefore, we have developed new techniques for modeling solid-phase cosmic ray-induced radiation chemistry in both detailed microscopic Monte Carlo models and more general models utilizing rate equations. Where possible, results from our new models are compared with previous experimental data and found to be in good agreement. Finally, we have incorporated solid-phase radiation chemistry into an existing chemical network in order to examine the impact of such reactions on the abundances of species in cold cores. Our results suggest that cosmic ray-driven chemistry is indeed a powerful formation mechanism which both increases the abundance of complex organic molecules and improves the agreement between models and observations. Thus, given the ubiquity of cosmic rays in the interstellar medium and their substantial physicochemical impact, as shown in part by the new computational results presented here, we conclude that, similar to photochemistry, radiation chemistry - particularly in interstellar ices - should become a standard component of future astrochemical models.