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Protoplanetary disks are the target of many chemical studies (bothobservational and theoretical) as they contain the building material forplanets. Their large vertical and radial gradients in density and temperaturemake them challenging objects for chemical models. In the outer part of thesedisks, the large densities and low temperatures provide a particularenvironment where the binding of species onto the dust grains can be veryefficient and can affect the gas-phase chemical composition.  We attempt to quantify to what extent the vertical abundance profiles and theintegrated column densities of molecules predicted by a detailed gas-grain codeare affected by the treatment of the molecular hydrogen physisorption at thesurface of the grains. We performed three different models using the Nautilusgas-grain code. One model uses a H2 binding energy on the surface of water (440K) and produces strong sticking of H2. Another model uses a small bindingenergy of 23 K (as if there were already a monolayer of H2), and the stickingof H$_2$ is almost negligible. Finally, the remaining model is an intermediatesolution known as the encounter desorption mechanism. We show that theefficiency of molecular hydrogen binding (and thus its abundance at the surfaceof the grains) can have a quantitative effect on the predicted column densitiesin the gas phase of major species such as CO, CS, CN, and HCN.

Importance of the H2 abundance in protoplanetary disk ices for the molecular layer chemical composition