A Three-phase Approach to Grain Surface Chemistry in Protoplanetary Disks: Gas, Ice Surfaces, and Ice Mantles of Dust Grains

Abstract

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 ( 15K) 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 ∼ 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.

Reactivity of formic acid (HCOOH) with H atoms on cold surfaces of interstellar interest

“…However, the spectroscopy of the main ice constituents, such as carboxylic acids should be better understood, with the new infrared observations in the interstellar ices by James Webb Space Telescope (JWST). Our hydrogenation reactions of HCOOH on cold grain surfaces may help astrophysicists and astrochemists to further understand the chemistry in gas-grain interfaces of protoplanetary disk regions, where gaseous molecules CO, CH 3 OH, and HCOOH are expected to be abundant according to the recent atrochemical models of Ruaud & Gorti (2019). These species are likely to be formed on grain surfaces via hydrogenation reactions and then desorb into the gas phase, either through a chemical desorption process in the outer midplane disk cold regions (T < 15 K) located at higher vertical column densities (Av > 10 mag), or via the photoprocessing of ices in disk midplane regions at 3 < Av < 10 mag (Ruaud & Gorti 2019).…”

Molecular growth upon ionization of van der Waals clusters containing HCCH and HCN is a pathway to prebiotic molecules

“…A gas-grain model of protoplanetary disks predicted significant abundance of HCN and HCCH in the midplane beyond 100AU. 51 Another astrophysical environment suitable for large molecular cluster formation is Titan's upper atmosphere where neutral and ionic species are observed by the CASSINI mission. 52 If HCCH and HCN neutral cluster for in the ionosphere or Titan and get ionized by colliding with high-energy and oxidizing particles then the mechanism of carbon-nitrogen molecular growth proposed here will be very important in Titan's atmosphere.…”

Infrared Spectroscopy and Photochemistry of Anthracoronene in Cosmic Water Ice

“…Carbon dioxide emissions have been observed in protoplanetary disks and high-mass protostars. The CO 2 abundances in these environments are much less than what is seen in the ISM; however, an analysis 52 indicates that the amount of CO 2 in ice on grains increases as the distance from the disk center decreases. With strong modes, CO 2 presents a unique identifier with which to trace disk evolution.…”

“…Previous studies have discussed how the ice mantle surrounding a dust grain experiences adsorption and desorption yet the interface between the mantle and the dust’s core material can have different desorption and diffusion properties. 52 This work provides insights into the contribution of PAHs to the protoplanetary disk mantle chemistry. The rate constant, k , for the production of each AntCor:H 2 O photoproduct was determined from the growth of the integrated band area as a function of UV photolysis time using the 1:1280 ratio.…”

“…The production of CO 2 in AntCor:H 2 O increases until it reaches the 1:390 concentration, at which point there is a sharp decrease that coincides with an inflection point in the photochemistry as it goes from molecular to ion mediated. 52 In the high-PAH-concentration regime, AntCor molecules are clustered together with limited exposure to water molecules; therefore, the CO 2 production that occurs happens at the cluster edges. The dramatic change in the CO 2 production rate at 1:390 is due to the start of the low-PAH-concentration regime, where there are more AntCor monomers and thus there is greater opportunity for water molecules to access and react with AntCor.…”