Radiolysis of ammonia-containing ices by energetic, heavy and highly charged ions inside dense astrophysical environments

TL;DR

This study investigates how heavy, highly charged cosmic ray ions interact with ammonia-containing ices in dense space environments, revealing chemical dissociation, compaction, and potential amino acid formation, which impacts astrochemical models.

Contribution

It provides experimental data on the effects of heavy cosmic ray analogs on ammonia ices, including dissociation cross sections and evidence of amino acid formation, which was previously unquantified.

Findings

Dissociation cross sections for water, ammonia, and CO are approximately 2x10^{-13}, 1.4x10^{-13}, and 1.9x10^{-13} cm^2.

Half-life of ammonia and related molecules under cosmic ray exposure is estimated at 2-3 million years.

Heavy cosmic rays cause significantly more ice compaction than protons, by at least three orders of magnitude.

Abstract

Deeply inside dense molecular clouds and protostellar disks, the interstellar ices are protected from stellar energetic UV photons. However, X-rays and energetic cosmic rays can penetrate inside these regions triggering chemical reactions, molecular dissociation and evaporation processes. We present experimental studies on the interaction of heavy, highly charged and energetic ions (46 MeV Ni^13+) with ammonia-containing ices in an attempt to simulate the physical chemistry induced by heavy ion cosmic rays inside dense astrophysical environments. The measurements were performed inside a high vacuum chamber coupled to the heavy ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France.\textit{In-situ} analysis is performed by a Fourier transform infrared spectrometer (FTIR) at different fluences. The averaged values for the dissociation cross section of water, ammonia and carbon monoxide due to heavy cosmic ray ion analogs are ~2x10^{-13}, 1.4x10^{-13} and 1.9x10^{-13} cm$^2$, respectively. In the presence of a typical heavy cosmic ray field, the estimated half life for the studied species is 2-3x10^6 years. The ice compaction (micropore collapse) due to heavy cosmic rays seems to be at least 3 orders of magnitude higher than the one promoted by (0.8 MeV) protons . In the case of the irradiated H2O:NH3:CO ice, the infrared spectrum at room temperature reveals five bands that were tentatively assigned to vibration modes of the zwitterionic glycine (+NH3CH2COO-).