High-energy astrochemistry in the molecular interstellar medium

TL;DR

The review defines high-energy astrochemistry as the chemistry in the molecular interstellar medium driven by ionizing radiation that creates extensive secondary-electron cascades ($N_e \gg 1$), unifying X-ray and cosmic-ray effects in both gas and ices. It surveys the sources of high-energy radiation (AGN, SNRs, massive stars, protostars), the observational evidence of non-thermal chemistry (iCOMS, H$_3^+$, ArH$^+$, JWST detections), and the distinct transport and heating characteristics of X-rays versus cosmic rays. The paper details gas-phase ionization networks (oxygen, carbon, nitrogen chemistry and deuteration) and ice-processing mechanisms (radiolysis, secondary electrons, desorption, sputtering), supported by state-of-the-art modelling (PDR/XDR/grids, UCLCHEM), and laboratory experiments that illuminate chemical pathways and yields. It highlights how high-energy processes shape chemistries across environments from galactic nuclei to disks, and points to the need for more comprehensive X-ray radiolysis treatments, cosmic-ray gradients, and cross-section data in public astrochemical codes. The work emphasizes the synergy between observations (JWST, ALMA, future AtLAST), theory, and laboratory studies to advance a coherent picture of high-energy astrochemistry in space.

Abstract

In the past decade, there has been a significant shift in astrochemistry with a renewed focus on the role of non-thermal processes on the molecular interstellar medium, in particular energetic particles (such as cosmic ray particles and fast electrons) and X-ray radiation. This has been brought about in large part due to new observations of interstellar complex organic molecules (iCOMS) in environments that would inhibit their formation, such as cold, dense gas in prestellar cores or in the highly energetic environments in galactic centers. In parallel, there has been a plethora of new laboratory investigations on the role of high-energy radiation and electrons on the chemistry of astrophysical ices, demonstrating the ability of this radiation to induce complex chemistry. In recent years, theoretical models have also begun to include newer cosmic-ray-driven processes in both the gas and ice phases. In this review, we unify aspects of the chemistry driven by X-ray radiation and energetic particles into a ``high-energy astrochemistry'', defining this term and reviewing the underlying chemical processes. We conclude by examining various laboratories where high-energy astrochemistry is at play and identify future issues to be tackled.

Figures (13)

• Figure 1: The stopping column, or range, in units of cm$^{-2}$ as a function of energy, $E$, for protons, electrons, and photons moving through cold molecular gas. The gray shaded region denotes $N(H) > 10^{21}$ cm$^{-2}$. The colored boxes denote the minimum energy for that species such that the range exceeds $10^{21}$ cm$^{-2}$. Range functions are from Padovani2018.
• Figure 2: Top: Inelastic electron-impact cross sections for H2, from Padovani2022 and Scarlett2023. Secondary electron spectrum for the cosmic ray proton spectrum models Ivlev2015bPadovani2022$\mathcal{H}$ and $\mathcal{L}$ attenuated by a hydrogen nuclei column density of $N(H) = 10^{23}$ cm$^{-2}$ (black lines), from Padovani2022. Bottom: Inelastic electron-impact cross sections for CO, from Itikawa2015, and H2O, from Song2021.
• Figure 3: Tree highlighting key ion-neutral pathways initiated by cosmic ray or X-ray ionization. Adapted from Padovani2024.
• Figure 4: The normalized contributions of different heating (top) and cooling mechanisms (bottom) for three different densities, $n_H = 10, 10^3, 10^4$ cm$^{-3}$ (a, b, c, respectively) as a function of H2 cosmic-ray ionization rate.
• Figure 5: Left: Gas temperature, $T_{\rm gas}$ as a function of density and total H2 CRIR, $\zeta(\ce{H2})$. Center: Same as left, but showing the electron fraction, $x(\ce{e-})$. Right: Same as left, but showing a three-color image for the abundance of C+ (blue), C (green), and CO (red). The cyan dashed-dotted line denotes where $x(H) = 2x(H_2)$. The annotated white dashed and dotted lines show the cosmic ray attenuation models of Padovani2018, denoted $\mathcal{H}$ and $\mathcal{L}$.