Modeling the processing of interstellar ices by energetic particles

Abstract

Context. Interstellar ice is the main form of metal species in dark molecular clouds. Experiments and observations have shown that the ice is significantly processed after the freeze-out of molecules onto grains. The processing is caused by cosmic-ray particles and cosmic-ray-induced UV photons. These transformations are included in current astrochemical models only to a very limited degree. Aims. We aim to establish a model of the "cold" chemistry in interstellar ices and to evaluate its general impact on the composition of interstellar ices. Methods. The ice was treated as consisting of two layers -the surface and the mantle (or subsurface) layer. Subsurface chemical processes are described with photodissociation of ice species and binary reactions on the surfaces of cavities inside the mantle. Hydrogen atoms and molecules can diffuse between the layers. We also included deuterium chemistry.Results. The modeling results show that the content of chemically bound H is reduced in subsurface molecules by about 30% on average. This promotes the formation of more hydrogen-poor species in the ice. The enrichment of ice molecules with deuterium is significantly reduced by the subsurface processes. On average, it follows the gas-phase atomic D/H abundance ratio, with a delay. The delay produced by the model is on the order of several Myr. Conclusions. The processing of ice may place new constraints on the production of deuterated species on grains. In a mantle with a two-layer structure the upper layer (CO) should be processed substantially more intensively than the lower layer (H 2 O). Chemical explosions in interstellar ice might not be an important process. They destroy the structure of the mantle, which forms over long timescales. Besides, ices may lack the high radical content needed for the explosions.

Cosmic-Ray Induced Diffusion in Interstellar Ices

“…Elemental abundances were taken the same as in Kalvāns & Shmeld (2013). Deuterium chemistry was not included.…”

“…The extinction of the interstellar UV radiation field A V was taken to be 20 mag, more consistent for a large molecular cloud than an isolated dense core. This is the same value employed by Kalvāns & Shmeld (2013). The grains were assumed to be of uniform size, with the radius a = 10 −5 cm.…”

“…Accretion and desorption have been retained from the earlier paper (Kalvāns & Shmeld 2013), except that grains with a size of a + b have been considered. Grain growth usually does not influence ice chemistry (Acharyya et al 2011;Taquet et al 2012).…”

Chemical fractionation of deuterium in the protosolar nebula

“…The modeling work by Taquet et al (2013b) and Kalvāns & Shmeld (2013) indicates that deuterium-rich ice molecules mostly are concentrated on the outer surface of ice mantles residing on interstellar or circumstellar grains. To put it another way, ice molecules that are formed early in the evolution of a contracting cloud core (e.g., H2O) typically have low RD, while species that are synthesized in the later stages of prestellar cores typically, and are abundant on the outer surface, have a high D content (e.g., CH3OH).…”

“…With the help of numerical simulations, such processes have been studied by, e.g., Kalvāns & Shmeld (2010); Garrod (2013), and Chang & Herbst (2014). Chemical processing of bulk ice may introduce changes in the D/H ratio for icy species (Kalvāns & Shmeld 2013). Therefore, the specific aim of the present study is to investigate the deuterium fractionation of circumstellar molecules by using a model that considers active subsurface ice chemistry.…”