The role of OH in the chemical evolution of protoplanetary disks

Abstract

Context. Time-dependent gas-grain chemistry can help us understand the layered structure of species deposited onto the surface of grains during the lifetime of a protoplanetary disk. The history of trapping large quantities of carbon-and oxygen-bearing molecules onto the grains is especially significant for the formation of more complex (organic) molecules on the surface of grains. Aims. Among other processes, cosmic ray-induced UV photoprocesses can lead to the efficient formation of OH. Using a more accurate treatment of cosmic ray-gas interactions for disks, we obtain an increased cosmic ray-induced UV photon flux of 3.8 × 10 5 photons cm −2 s −1 for a cosmic-ray ionization rate of H 2 value of 5 × 10 −17 s −1 (compared to previous estimates of 10 4 photons cm −2 s −1 based on ISM dust properties). We explore the role of the enhanced OH abundance on the gas-grain chemistry in the midplane of the disk at 10 AU, which is a plausible location of comet formation. We focus on studying the formation/destruction pathways and timescales of the dominant chemical species. Methods. We solved the chemical rate equations based on a gas-grain chemical network and correcting for the enhanced cosmic ray-induced UV field. This field was estimated from an appropriate treatment of dust properties in a protoplanetary disk, as opposed to previous estimates that assume an ISM-like grain size distribution. We also explored the chemical effects of photodesorption of water ice into OH+H.

Reaction Pathways of Water Dimer Following Single Ionization

“…45 The D was chosen to have an exponential character based on the strongfield ionization approximation. 45 The unknown parameters of our model, i.e., F and the branching ratios of H + and OH + ions, were calibrated using the theoretical branching ratios and E A of each channel given by Svoboda et al 15 We then calculated the so far unknown E A of the less abundant ions from (8), see the Supporting Information for further details. These estimated E A values are shown in Table 1 including the reported ones.…”

Vacuum-UV Photodesorption from Compact Amorphous Solid Water: Photon Energy Dependence, Isotopic and Temperature Effects

“…This finding is paramount for the chemistry in the comet-formation region of protoplanetary disks. Indeed, using time-dependent gas-grain chemical model in a region of a T-Tauri disk corresponding likely to comet-formation, Chaparro Molano and Kamp have shown that secondary VUV (induced by cosmic rays) photodesorption from water ice delivers highly reactive OH radicals into the gas phase. This pathway opens new gas phase chemical routes that change the chemical balance and the final abundances, enabling in particular oxygen to be distributed in O 2 , SiO, and atomic oxygen, and carbon to be stored in CO and CO 2 .…”

“…The desorption directly induced by cosmic ray impact onto icy grain mantles also strongly contributes to these nonthermal phenomena. , Nonthermal desorption are believed to play an important role in protoplanetary environments by maintaining molecules into the gas phase in the surface or intermediate regions of the disk. , Dupuy et al recently highlighted the efficient role played by soft X-ray photons to induce the nonthermal desorption of neutral water from water ice. In the comet formation region of the disk, VUV photodesorption from cold grains is supposed to be a source of reactive species, such as OH, which alter drastically the gas phase chemical composition . Knowing the nature and photodesorption yields of the desorbing photofragments is therefore of primary importance.…”

Direct ab initio molecular dynamics study on the reactions of multi-valence ionized states of water dimer

“…The ionization of water clusters is an important phenomenon in astrophysics and astrochemistry. For example, solid water in a comet is predicted to be amorphous ice [1][2][3][4][5], similar to aggregated water clusters. When the comet approaches the Sun, the clusters are ionized upon irradiation with cosmic rays from the Sun [6][7][8][9][10][11][12][13].…”