Volume 627, July 2019
|Number of page(s)||16|
|Section||Interstellar and circumstellar matter|
|Published online||02 July 2019|
The role of atom tunneling in gas-phase reactions in planet-forming disks
Institute for Theoretical Chemistry, University of Stuttgart,
2 Department of Chemistry and The PULSE Institute, Stanford University, Stanford, California 94305, USA
3 Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
4 Max Planck Institute for Extraterrestrial Physics, Gießenbachstrasse 1, 85741 Garching, Germany
Accepted: 25 March 2019
Context. Chemical Gas-phase 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 H2 + OH → H2O + 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.
Key words: astrochemistry / protoplanetary disks / infrared: planetary systems / stars: low-mass
© ESO 2019
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