Cosmic-ray induced diffusion in interstellar ices

TL;DR

This study investigates how cosmic-ray heating enhances molecule mobility in interstellar ices, leading to increased formation of complex organic molecules through pore-assisted diffusion.

Contribution

It introduces a three-phase astrochemical model focusing on pore-assisted diffusion driven by cosmic-ray heating, revealing new pathways for molecule formation in interstellar ices.

Findings

Pores act as efficient transport routes for light species.

Radial diffusion enhances formation of complex organic molecules.

Higher abundances of radicals and molecules like OCS, H2O2, and cyanopolyynes.

Abstract

Cosmic rays are able to heat interstellar dust grains. This may enhance molecule mobility in icy mantles that have accumulated on the grains in dark cloud cores. A three-phase astrochemical model was used to investigate the molecule mobility in interstellar ices. Specifically, diffusion through pores in ice between the subsurface mantle and outer surface, assisted by whole-grain heating, was considered. It was found that the pores can serve as an efficient transport route for light species. The diffusion of chemical radicals from the mantle to the outer surface are most effective. These species accumulate in the mantle because of photodissociation by the cosmic-ray induced photons. The faster diffusion of hydrogen within the warm ice enhances the hydrogenation of radicals on pore surfaces. The overall result of the whole grain heating-induced radial diffusion in ice are higher abundances of the ice species whose synthesis involve light radicals. Examples of stable species synthesized this way include the complex organic molecules, OCS, H2O2 and cyanoplyynes.