Modeling Irradiation of Interstellar Ices by Cosmic Rays

Throughout the interstellar medium (ISM), there are multiple ongoing questions on the role of gaseous, dust surface, ice surface, and ice bulk chemistry in the formation of rotationally observed molecules. Both experiments and models suggest the importance of ices upon the grain surface in complex chemical formation, especially at low temperatures(Garrod et al. 2008; Herbst & van Dishoeck 2009). Many recent discoveries of complex organic molecules (COMs) have occurred toward cold dark clouds, which were thought to have less chemical diversity compared to warmer star forming regions (SFRs), as the lack of higher temperature gas phase chemistry limited chemical models (McGuire 2022). To explain the chemical diversity at 10 K in cold dark clouds, efficient low temperature chemical routes and desorption methods are needed (Herbst 2021). Vast chemical networks, with over 15000 reactions, are utilized by computer models solving systems of rate equations to model the chemical evolution of environments in the ISM.
As a way around the limitations of cold chemistry, bombardment of interstellar ices is a common source of energy throughout the interstellar medium. Irradiation caused by cosmic rays, atomic nuclei traveling at significant fractions of the speed of light, have previously been shown to increase chemical diversity of interstellar grain ices (Abplanalp et al. 2016; Blasi 2013; Shingledecker et al. 2018). This thesis expands upon the chemistry of cosmic ray collisions with grain ices, by addressing the formation of C2H4O2 isomers, and precursor chemistry leading to the formation and destruction of PAHs. The C2H4O2 isomers are shown to have increased abundances with the inclusion of radiolysis chemistry reactions, which is notable because glycolaldehyde is a sugar-adjacent molecule. Additionally, the added destruction pathways of these molecules do not reduce the predicted abundance, suggesting that the production is more efficient than destruction.
The chemical model is enhanced with a new non-thermal desorption method resulting from cosmic ray collisions with ices, called sputtering (Behrisch & Eckstein 2007; Johnson 1990). Experiments and other models have shown sputtering via swift heavy ions to be effective at removing material from the surface of amorphous solid water, thought to be analogous to interstellar ices(Dartois et al. 2018; Wakelam et al. 2021). Models with sputtering enable significant desorption of all molecules on and within ices, though we do not see uniform increases in gas phase abundances for all species, partially because of efficient thermal gas-phase destruction pathways. The effects of sputtering on COMs and precursor species to PAHs is also addressed; while the inclusion of sputtering, radiolysis, and the expansion of the chemical network to include speculative grain reactions does increase the abundance of molecules to match with observation in the case of some molecules, not every molecule is enhanced enough with the inclusion of sputtering and radiolysis to match observations. We discuss the lack of thermal reactions as a possible reason, and possible remedies to the problem of three-phase models not reproducing observed abundances for PAH-related molecules and precursors.

Abplanalp, M. J., Gozem, S., Krylov, A. I., et al. 2016, Proc Natl Acad Sci, 113, 7727, https://www.pnas.org/content/113/28/7727

Behrisch, R., & Eckstein, W. 2007, Sputtering by Particle Bombardment: Experiments and Computer Calculations From Threshold to MeV Energies (Berlin: Springer)

Blasi, P. 2013, Astron Astrophys Rev, 21, 70, https://doi.org/10.1007/s00159-013-0070-7

Dartois, E., Chabot, M., Barkach, T. I., et al. 2018, Astron Astrophys, 618, A173

Garrod, R. T., Weaver, S. L. W., & Herbst, E. 2008, Astrophys J, 682, 283, https://doi.org/10.1086%2F588035

Herbst, E. 2021, Front Astron Space Sci, 8, https://www.frontiersin.org/articles/10.3389/fspas.2021.776942

Herbst, E., & van Dishoeck, E. F. 2009, Annu Rev Astron Astrophys, 47, 427, https://www.annualreviews.org/doi/10.1146/annurev-astro-082708-101654

Johnson, R. E. 1990, Energ Charg-Part Interact Atmospheres Surf X 232 Pp 84 Figs 28 Tabs

Springer-Verl Berl Heidelb N Y Also Phys Chem Space Vol 19, http://adsabs.harvard.edu/abs/1990ecpi.book.....J

McGuire, B. A. 2022, Astrophys J Suppl Ser, 259 (American Astronomical Society), 30, https://doi.org/10.3847/1538-4365/ac2a48

Shingledecker, C. N., Tennis, J., Gal, R. L., & Herbst, E. 2018, Astrophys J, 861, 20, http://stacks.iop.org/0004-637X/861/i=1/a=20

Wakelam, V., Dartois, E., Chabot, M., et al. 2021, ArXiv210608621 Astro-Ph, http://arxiv.org/abs/2106.08621