Chemical Evolution In Planet-forming Regions With Growing Grains

By Keith Cowing
July 27, 2022
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Chemical Evolution In Planet-forming Regions With Growing Grains
Middle and bottom panels: time -and grainchoice-dependent abundances, relative to abundances assuming Rgrain = 0.1µm.. The y-axes are linear for the middle panels, and logarithmic for the bottom ones. Vertical dotted lines in top panel indicate evolution times (0.1Myr, 0.5Myr, 1Myr, 2Myr and 5Myr) associated with markers in middle and bottom panels. The blue shaded area for H2O ice in middle panel has extra annotation, to guide the reader: first vertical category is assuming an evolving grain size, growing with time (as annotated above the plot). The second category assumes the final (constant) grain size from the grain growth models, which here, at 1.5AU, is 34.3µm (see Table 1). The following four categories represent log-spaced grain size increases (as also indicated with the black arrow, indicating increasing grain sizes going from categories three through six). This sequence of x-axis-categories is identical across all middle and bottom panels in Figs. 2-9 in this paper.

Planets and their atmospheres are built from gas and solid material in protoplanetary disks. This solid material grows from smaller, micron-sized grains to larger sizes in the disks, during the process of planet formation.

Our goal is to model the compositional evolution of volatile ices on grains of different sizes, assuming both time-dependent grain growth and several constant grain sizes. The state-of-the-art Walsh chemical kinetics code is utilised for modeling chemical evolution. This code has been upgraded to account for the time-evolving sizes of solids. Chemical evolution is modelled locally at four different radii in a protoplanetary disk midplane for up to 10Myr.

The evolution is modelled for five different constant grain sizes, and one model where the grain size changes with time according to a grain growth model appropriate for the disk midplane. Local grain growth, with conservation of total grain mass and the assumption of spherical grains, acts to reduced the total grain-surface area that is available for ice-phase reactions. This reduces these reactions efficiency compared to a chemical scenario with a conventional grain-size choice of 0.1μm.

The modelled chemical evolution with grain growth leads to increased abundances of H2O ice. For carbon in the inner disk, grain growth leads CO gas to overtake CO2 ice as dominant carrier, and in the outer disk, CH4 ice to become the dominant carrier. Overall, a constant grain size adopted from a grain evolution model leads to almost identical chemical evolution, when compared with chemical evolution with evolving grain sizes. A constant grain size choice, albeit larger than 0.1μm, may therefore be an appropriate simplification when approximating the impact of grain growth on chemical evolution.

Christian Eistrup, L. Ilsedore Cleeves, Sebastiaan Krijt

Comments: Accepted by Astronomy & Astrophysics. 21 pages
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2207.13158 [astro-ph.EP] (or arXiv:2207.13158v1 [astro-ph.EP] for this version)
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Submission history
From: Christian Eistrup
[v1] Tue, 26 Jul 2022 19:27:30 UTC (1,458 KB)
Astrobiology, Astrochemistry

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