The role of atom tunneling in gas-phase reactions in planet-forming disks

Abstract

Context. Gas-phase chemical reactions of simple molecules have been recently revised to include atom tunneling at very low temperatures. This paper investigates the impact of the increased reaction rate constant due to tunneling effects on planet-forming disks. Aims. Our aim is to quantify the astrophysical implications of atom tunneling for simple molecules that are frequently used to infer disk structure information or to define the initial conditions for planet (atmosphere) formation. Methods. We quantify the tunneling effect on reaction rate constants by using H 2 + OH → H 2 O + H as a scholarly example in comparison to previous UMIST2012 rate constants. In a chemical network with 1299 reactions, we identify all chemical reactions that could show tunneling effects. We devise a simple formulation of reaction rate constants that overestimates tunneling and screen a standard T Tauri disk model for changes in species abundances. For those reactions found to be relevant, we find values of the most recent literature for the rate constants including tunneling and compare the resulting disk chemistry to the standard disk model(s), a T Tauri and a Herbig disk. Results. The rate constants in the UMIST2012 database in many cases already capture tunneling effects implicitly, as seen in the curvature of the Arrhenius plots of some reactions at low temperature. A rigorous screening procedure identified three neutral-neutral reactions where atom tunneling could change simple molecule abundances. However, by adopting recent values of the rate constants of these reactions and due to the layered structure of planet-forming disks, the effects are limited to a small region between the ion-molecule dominated regime and the ice reservoirs where cold (< 250 K) neutral-neutral chemistry dominates. Abundances of water close to the midplane snowline can increase by a factor of two at most compared to previous results with UMIST2012 rates. Observables from the disk surface, such as high excitation (> 500 K) water line fluxes, decrease by 60% at most when tunneling effects are explicitly excluded. On the other hand, disk midplane quantities relevant for planet formation such as the C-to-O ratio and also the ice-to-rock ratio are clearly affected by these gas-phase tunneling effects.

Gas-Phase vs. Grain-Surface Formation of Interstellar Complex Organic Molecules: A Comprehensive Quantum-Chemical Study

“…Consequently, the intermediate becomes stable on the surface and does not contain the same excess of internal energy as occurs in the gas phase, and accordingly substantial dissociation energy barriers emerge, which are hard to surmount in ISM conditions. However, when the intermediate dissociation is associated with the release of a H atom (as is the case of formamide, methyl formate, and formic acid formation), tunneling effects can become relevant, especially at very low temperatures [88,89]. Tunneling is more likely in symmetric processes than in nonsymmetric processes, that is, when the initial and final states present similar stabilities.…”