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A three-phase approach to grain surface chemistry in protoplanetary disks: Gas, ice surfaces and ice mantles of dust grains

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 Added by Maxime Ruaud
 Publication date 2019
  fields Physics
and research's language is English




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We study the effects of grain surface reactions on the chemistry of protoplanetary disks where gas, ice surface layers and icy mantles of dust grains are considered as three distinct phases. Gas phase and grain surface chemistry is found to be mainly driven by photo-reactions and dust temperature gradients. The icy disk interior has three distinct chemical regions: (i) the inner midplane with low FUV fluxes and warm dust ($gtrsim 15$K) that lead to the formation of complex organic molecules, (ii) the outer midplane with higher FUV from the ISM and cold dust where hydrogenation reactions dominate and, (iii) a molecular layer above the midplane but below the water condensation front where photodissociation of ices affects gas phase compositions. Some common radicals, e.g., CN and C$_2$H, exhibit a two-layered vertical structure and are abundant near the CO photodissociation front and near the water condensation front. The 3-phase approximation in general leads to lower vertical column densities than 2-phase models for many gas-phase molecules due to reduced desorption, e.g., H$_2$O, CO$_2$, HCN and HCOOH decrease by $sim$ two orders of magnitude. Finally, we find that many observed gas phase species originate near the water condensation front; photo-processes determine their column densities which do not vary significantly with key disk properties such as mass and dust/gas ratio.



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We compute the desorption rate of icy mantles on dust grains as a function of the size and composition of both the grain and the mantle. We combine existing models of cosmic ray (CR) related desorption phenomena with a model of CR transport to accurately calculate the desorption rates in dark regions of molecular clouds. We show that different desorption mechanisms dominate for grains of different sizes, and in different regions of the cloud. We then use these calculations to investigate a simple model of the growth of mantles, given a distribution of grain sizes. We find that modest variations of the desorption rate with grain size lead to a strong dependence of mantle thickness on grain size. Furthermore, we show that freeze-out is almost complete in the absence of an external UV field, even when photodesorption from CR produced UV is taken into consideration. Even at gas densities of $10^4$ ${rm cm^{-3}}$, less than 30% of the CO remains in the gas phase after $3times 10^5$ years for standard values of the CR ionization rate.
63 - E M Penteado 2017
Advanced telescopes, such as ALMA and JWST, are likely to show that the chemical universe may be even more complex than currently observed, requiring astrochemical modelers to improve their models to account for the impact of new data. However, essential input information for gas-grain models, such as binding energies of molecules to the surface, have been derived experimentally only for a handful of species, leaving hundreds of species with highly uncertain estimates. We present in this paper a systematic study of the effect of uncertainties in the binding energies on an astrochemical two-phase model of a dark molecular cloud, using the rate equations approach. A list of recommended binding energy values based on a literature search of published data is presented. Thousands of simulations of dark cloud models were run, and in each simulation a value for the binding energy of hundreds of species was randomly chosen from a normal distribution. Our results show that the binding energy of H$_{2}$ is critical for the surface chemistry. For high binding energy, H$_{2}$ freezes out on the grain forming an H$_{2}$ ice. This is not physically realistic and we suggest a change in the rate equations. The abundance ranges found are in reasonable agreement with astronomical ice observations. Pearson correlation coefficients revealed that the binding energy of HCO, HNO, CH$_{2}$, and C correlate most strongly with the abundance of dominant ice species. Finally, the formation route of complex organic molecules was found to be sensitive to the branching ratios of H$_{2}$CO hydrogenation.
We present a novel method for determining the surface density of protoplanetary disks through consideration of disk dust lines which indicate the observed disk radial scale at different observational wavelengths. This method relies on the assumption that the processes of particle growth and drift control the radial scale of the disk at late stages of disk evolution such that the lifetime of the disk is equal to both the drift timescale and growth timescale of the maximum particle size at a given dust line. We provide an initial proof of concept of our model through an application to the disk TW Hya and are able to estimate the disk dust-to-gas ratio, CO abundance, and accretion rate in addition to the total disk surface density. We find that our derived surface density profile and dust-to-gas ratio are consistent with the lower limits found through measurements of HD gas. The CO ice line also depends on surface density through grain adsorption rates and drift and we find that our theoretical CO ice line estimates have clear observational analogues. We further apply our model to a large parameter space of theoretical disks and find three observational diagnostics that may be used to test its validity. First we predict that the dust lines of disks other than TW Hya will be consistent with the normalized CO surface density profile shape for those disks. Second, surface density profiles that we derive from disk ice lines should match those derived from disk dust lines. Finally, we predict that disk dust and ice lines will scale oppositely, as a function of surface density, across a large sample of disks.
We present Submillimeter Array observations of H2CO and N2H+ emission in the disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296 at 2-6 resolution and discuss the distribution of these species with respect to CO freeze-out. The H2CO and N2H+ emission toward HD 163296 does not peak at the continuum emission center that marks the stellar position but is instead significantly offset. Using a previously developed model for the physical structure of this disk, we show that the H2CO observations are reproduced if H2CO is present predominantly in the cold outer disk regions. A model where H2CO is present only beyond the CO snow line (estimated at a radius of 160 AU) matches the observations well. We also show that the average H2CO excitation temperature, calculated from two transitions of H2CO observed in these two disks and a larger sample of disks around T Tauri stars in the DISCS (the Disk Imaging Survey of Chemistry with SMA) program, is consistent with the CO freeze-out temperature of 20 K. In addition, we show that N2H+ and H2CO line fluxes in disks are strongly correlated, indicative of co-formation of these species across the sample. Taken together, these results imply that H2CO and N2H+ are generally present in disks only at low temperatures where CO depletes onto grains, consistent with fast destruction of N2H+ by gas-phase CO, and in situ formation of H2CO through hydrogenation of CO ice. In this scenario H2CO, CH3OH and N2H+ emission in disks should appear as rings with the inner edge at the CO midplane snow line. This prediction can be tested directly using observations from ALMA with higher resolution and better sensitivity.
121 - W. F. Thi , S. Hocuk , I. Kamp 2018
The origin of the reservoirs of water on Earth is debated. The Earths crust may contain at least three times more water than the oceans. This crust water is found in the form of phyllosilicates, whose origin probably differs from that of the oceans. We test the possibility to form phyllosilicates in protoplanetary disks, which can be the building blocks of terrestrial planets. We developed an exploratory rate-based warm surface chemistry model where water from the gas-phase can chemisorb on dust grain surfaces and subsequently diffuse into the silicate cores. We apply the phyllosilicate formation model to a zero-dimensional chemical model and to a 2D protoplanetary disk model (ProDiMo). The disk model includes in addition to the cold and warm surface chemistry continuum and line radiative transfer, photoprocesses (photodissociation, photoionization, and photodesorption), gas-phase cold and warm chemistry including three-body reactions, and detailed thermal balance. Despite the high energy barrier for water chemisorption on silicate grain surfaces and for diffusion into the core, the chemisorption sites at the surfaces can be occupied by a hydroxyl bond (-OH) at all gas and dust temperatures from 80 to 700 K for a gas density of 2E4 cm^-3. The chemisorption sites in the silicate cores are occupied at temperatures between 250 and 700 K. At higher temperatures thermal desorption of chemisorbed water occurs. The occupation efficiency is only limited by the maximum water uptake of the silicate. The timescales for complete hydration are at most 1E5 years for 1 mm radius grains at a gas density of 1E8 cm^-3. Phyllosilicates can be formed on dust grains at the dust coagulation stage in protoplanetary disks within 1 Myr. It is however not clear whether the amount of phyllosilicate formed by warm surface chemistry is sufficient compared to that found in Solar System objects.
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