A Three-phase Approach To Grain Surface Chemistry In Protoplanetary Disks: Gas, ice Surfaces And Ice Mantles Of Dust Grains

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Protoplanetary disc

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 C2H, 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., H2O, CO2, 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.

Maxime Ruaud, Uma Gorti
(Submitted on 2 Oct 2019)
Comments: 36 pages, 14 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:1910.01097 [astro-ph.GA] (or arXiv:1910.01097v1 [astro-ph.GA] for this version)
Submission history
From: Maxime Ruaud
[v1] Wed, 2 Oct 2019 17:29:17 UTC (9,923 KB)
https://arxiv.org/abs/1910.01097
Astrobiology, Astrochemistry

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