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Register a new userImpact of Size-dependent Grain Temperature on Gas-Grain Chemistry in Protoplanetary Disks: the case of low mass star disks
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Grain surface chemistry is key to the composition of protoplanetary disks around young stars. The temperature of grains depends on their size. We evaluate the impact of this temperature dependence on the disk chemistry. We model a moderately massive disk with 16 different grain sizes. We use POLARIS to calculate the dust grain temperatures and the local UV flux. We model the chemistry using the 3-phase astrochemical code NAUTILUS. Photoprocesses are 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 $mu$m), with two different assumptions for their equivalent temperature. We find that the Langmuir-Hinshelwood (LH) mechanism at equilibrium temperature is not efficient to form H$_2$ at 3-4 scale heights ($H$), and adopt a parametric fit to a stochastic method to model H$_2$ 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 H$_2$O and CH$_4$ ices in the midplane, and warmer ones produce more CO$_2$ ices, both allowing efficient depletion of C and O as soon as CO sticks on grain surfaces. Complex organic molecules (COMs) 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. 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 will depend on the dust disk mass.
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Turbulence is the dominant source of collisional velocities for grains with a wide range of sizes in protoplanetary disks. So far, only Kolmogorov turbulence has been considered for calculating grain collisional velocities, despite the evidence that turbulence in protoplanetary disks may be non-Kolmogorov. In this work, we present calculations of grain collisional velocities for arbitrary turbulence models characterized by power-law spectra and determined by three dimensionless parameters: the slope of the kinetic energy spectrum, the slope of the auto-correlation time, and the Reynolds number. The implications of our results are illustrated by numerical simulations of the grain size evolution for different turbulence models. We find that for the modeled cases of the Iroshnikov-Kraichnan turbulence and the turbulence induced by the magneto-rotational instabilities, collisional velocities of small grains are much larger than those for the standard Kolmogorov turbulence. This leads to faster grain coagulation in the outer regions of protoplanetary disks, resulting in rapid increase of dust opacity in mm-wavelength and possibly promoting planet formation in very young disks.
We study the impact of dust evolution in a protoplanetary disk around a T Tauri star on the disk chemical composition. For the first time we utilize a comprehensive model of dust evolution which includes growth, fragmentation and sedimentation. Specific attention is paid to the influence of grain evolution on the penetration of the UV field in the disk. A chemical model that includes a comprehensive set of gas phase and grain surface chemical reactions is used to simulate the chemical structure of the disk. The main effect of the grain evolution on the disk chemical composition comes from sedimentation, and, to a lesser degree, from the reduction of the total grain surface area. The net effect of grain growth is suppressed by the fragmentation process which maintains a population of small grains, dominating the total grain surface area. We consider three models of dust properties. In model GS both growth and sedimentation are taken into account. In models A5 and A4 all grains are assumed to have the same size (10(-5) cm and 10(-4) cm, respectively) with constant gas-to-dust mass ratio of 100. Like in previous studies, the three-layer pattern (midplane, molecular layer, hot atmosphere) in the disk chemical structure is preserved in all models, but shifted closer to the midplane in models with increased grain size (GS and A4). Unlike other similar studies, we find that in models GS and A4 column densities of most gas-phase species are enhanced by 1-3 orders of magnitude relative to those in a model with pristine dust (A5), while column densities of their surface counterparts are decreased. We show that column densities of certain species, like C2H, HC(2n+1)N (n=0-3), H2O and some other molecules, as well as the C2H2/HCN abundance ratio which are accessible with Herschel and ALMA can be used as observational tracers of early stages of the grain evolution process in protoplanetary disks.
Abridged: We detail and benchmark two sophisticated chemical models developed by the Heidelberg and Bordeaux astrochemistry groups. The main goal of this study is to elaborate on a few well-described tests for state-of-the-art astrochemical codes covering a range of physical conditions and chemical processes, in particular those aimed at constraining current and future interferometric observations of protoplanetary disks. We consider three physical models: a cold molecular cloud core, a hot core, and an outer region of a T Tauri disk. Our chemical network (for both models) is based on the original gas-phase osu_03_2008 ratefile and includes gas-grain interactions and a set of surface reactions for the H-, O-, C-, S-, and N-bearing molecules. The benchmarking is performed with the increasing complexity of the considered processes: (1) the pure gas-phase chemistry, (2) the gas-phase chemistry with accretion and desorption, and (3) the full gas-grain model with surface reactions. Using atomic initial abundances with heavily depleted metals and hydrogen in its molecular form, the chemical evolution is modeled within 10^9 years. The time-dependent abundances calculated with the two chemical models are essentially the same for all considered physical cases and for all species, including the most complex polyatomic ions and organic molecules. This result however required a lot of efforts to make all necessary details consistent through the model runs, e.g. definition of the gas particle density, density of grain surface sites, the strength and shape of the UV radiation field, etc. The reference models and the benchmark setup, along with the two chemical codes and resulting time-dependent abundances are made publicly available in the Internet: http://www.mpia.de/homes/semenov/Chemistry_benchmark/home.html
Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. The gas in protoplanetary disks should influence the vertical distribution of small dust grains ($sim$1 $mu m$) in the disk.Utilizing archival near-infrared polarized light and millimeter observations, we can measure the scale height and the flare parameter $beta$ of the small dust grain scattering surface and $^{12}$CO gas emission surface for three protoplanetary disks IM Lup, HD 163296, and HD 97048 (CU Cha). For two systems, IM Lup and HD 163296, the $^{12}$CO gas and small dust grains at small radii from the star have similar heights but at larger radii ($>$100 au) the dust grain scattering surface height is lower than the $^{12}$CO gas emission surface height. In the case of HD 97048, the small dust grain scattering surface has similar heights to the $^{12}$CO gas emission surface at all radii. We ran a protoplanetary disk radiative transfer model of a generic protoplanetary disk with TORUS and showed that there is no difference between the observed scattering surface and $^{12}$CO emission surface. We also performed analytical modeling of the system and found that gas-to-dust ratios larger than 100 could explain the observed difference in IM Lup and HD 163296. This is the first direct comparison of observations of gas and small dust grain heights distribution in protoplanetary disks. Future observations of gas emission and near-infrared scattered light instruments are needed to look for similar trends in other protoplanetary disks.
Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $gtrsim10{,rm Myr}$. However, previous models predict $lesssim 10{,rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the present study, we investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled as those where radiation force is sufficiently strong to continuously blow out small grains ($lesssim 4 {,rm mu m}$), which are an essential component for driving photoevaporation via photoelectric heating induced by stellar far-ultraviolet (FUV). We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of $2$--$5times10^{-10}(Phi_{rm EUV}/10^{41}{,rm s}^{-1})^{1/2},M_odot,{rm yr}^{-1}$, where $Phi_{rm EUV}$ is the EUV emission rate. The estimated lifetimes of the gas component are $sim 50 (M_{rm disk}/10^{-2},M_odot)(Phi_{rm EUV}/10^{41},{rm s}^{-1})^{1/2},{rm Myr}$ and depend on the ``initial disk mass at the point small grains have been depleted in the system. With an order estimation, we show that the gas component can survive for a much longer time around A-type stars than lower-mass stars. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.
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