Impact of size-dependent grain temperature on gas-grain chemistry in protoplanetary disks: The case of low-mass star disks

Abstract

Context. Grain surface chemistry is fundamental to the composition of protoplanetary disks around young stars. Aims. The temperature of grains depends on their size. We evaluate the impact of this temperature dependence on the disk chemistry. Methods. We modeled a moderately massive disk with 16 different grain sizes. We used the 3D Monte Carlo POLARIS code to calculate the dust grain temperatures and the local uv flux. We modeled the chemistry using the three-phase astrochemical code NAUTILUS. Photo processes were handled using frequency-dependent cross sections and a new method to account for self and mutual shielding. The multi-grain model outputs are compared to those of single-grain size models (0.1 μm); there are two different assumptions for their equivalent temperature. Results. We find that the Langmuir-Hinshelwood mechanism at equilibrium temperature is not efficient to form H2 at 3–4 scale heights (H), and we adopt a parametric fit to a stochastic method to model H2 formation instead. We find the molecular layer composition (1–3 H) to depend on the amount of remaining H atoms. Differences in molecular surface densities between single and multi-grain models are mostly due to what occurs above 1.5 H. At 100 au, models with colder grains produce H2O and CH4 ices in the midplane, and those with warmer grains produce more CO2 ices; both of these allow for an efficient depletion of C and O as soon as CO sticks on grain surfaces. Complex organic molecules production is enhanced by the presence of warmer grains in the multi-grain models. Using a single-grain model mimicking grain growth and dust settling fails to reproduce the complexity of gas-grain chemistry. Conclusions. Chemical models with a single-grain size are sensitive to the adopted grain temperature and cannot account for all expected effects. A spatial spread of the snowlines is expected to result from the ranges in grain temperature. The amplitude of the effects depends on the dust disk mass.