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.

Water Attachment onto Size-Selected Cationic Pyrene Clusters

“…Gas-grain interactions involving (very) small grains could thus play a key role in the evolution of physical and chemical conditions in the cold phases of star formation and in protoplanetary disks. [16][17][18][19] One of the topics in laboratory astrophysics concerns the characterization of gas-grain interactions for relevant laboratory analogues of interstellar dust. In a recent review, Potapov and McCoustra 20 emphasized the need for quantum chemical experiments and calculations to overcome the classical view of dust-ice mixing in which a refractory dust core is surrounded by a thick mantle of ice.…”

Modeling snowline locations in protostars: The impact of the structure of protostellar cloud cores

“…While we do not consider the vertical water snowline location, and Gavino et al (2021) cannot provide constraints on the radial water snowline location (<< 30 AU), both models agree on two aspects regarding the water snowline. The H 2 O snowline location is regulated by the penetration of UV and the amount of H 2 in the disk; and water in the gas phase is mainly present in the warm regions (upper layers) of the disk.…”

“…The Panić & Min (2017) models explore minimum dust grain sizes between 0.01 to 100 µm, and maximum dust grain sizes in the range of 1 mm to 1 km, with a power-law index of 3.5. The modeled region spans r = 0.24 -500 AU for an A-type star with 2 M , L star = 35 L and T e f f = 10 000 K. The Gavino et al (2021) models present a range of dust grains between 5 nm to 1 mm, with a power-law index of 3.5 within a region up to r . Peak radius of N 2 H + (left column), and HCO + (right column) simulated emission versus central protostellar luminosity for the fiducial models (black hatch) without disk (top row), and with disk (bottom row).…”

The Si + SO2 collision and an extended network of neutral–neutral reactions between silicon and sulphur bearing species

“…Along with radiative transfer models, those methods contribute to predict, e.g., molecular column densities and synthetic spectra in the sub-mm domain (e.g. Puzzarini & Barone 2020;Xie et al 2021;Gavino et al 2021;Keil et al 2022). In this work, we address specific questions on the chemistry of Si-and S-bearing molecules in the context of protostellar shock regions, taking L1157-B1 as a template source since molecules as SiS, SiO, HS and H 2 S were already detected towards it (Podio et al 2017b;Holdship et al 2019).…”