The role of atom tunneling in gas-phase reactions in planet-forming
  disks
  This paper investigates the impact of the increased reaction rate constant
due to tunneling effects on planet-forming disks. 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. We explain the tunneling effect
on reaction rates by using a scholarly example in comparison to previous
UMIST2012 rate constants. In a chemical network with 1299 reactions, we
identify all reactions that could show atom tunneling. 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 models. The rate constants in the UMIST2012
database in many cases already capture tunneling effects implicitly. 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
where cold 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.
