Tracking the Chemical Evolution of Hydrocarbons Through Carbon Grain Supply in Protoplanetary Disks

The gas present in planet-forming disks typically exhibits strong emission features of abundant carbon and oxygen molecular carriers.
In some instances, protoplanetary disks show an elevated C/O ratio above interstellar values, which leads to a rich hydrocarbon chemistry evidenced in the mid-infrared spectra. The origin of this strengthened C/O ratio may stem from the release of less complex hydrocarbons from the chemical processing of carbonaceous grains.
We have explored a set of 42 single-cell models in which we match the physical conditions to the inner regions of planet-forming disks, while varying the C/O ratio by exploring different levels of CH4, C, H2O, and CO to the gas-phase chemistry, which we evaluate in both the cosmic/X-ray and UV-driven limit.
We find that the carbon-bearing species in our models exhibit high dependencies on the driver of the chemistry, where both CO and long chain hydrocarbons act as carbon sinks in the cosmic/X-ray-driven chemistry limit, while the vast majority ends up in atomic carbon and CO in the UV-driven limit. We also find moderate dependencies upon the C/O ratio, where this and the ionization rate/UV field determines the point of peak production of a species as well as its equilibrium abundance.
We also find that the production of several hydrocarbons, specifically C2H2, is strongly dependent up to an order of magnitude on the initial water abundance. We lastly find that in the X-ray-driven limit, both CH4 and C serve as highly transient donor species to the carbon chemistry.
Eshan Raul, Felipe Alarcón, Edwin A. Bergin
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2502.01765 [astro-ph.EP] (or arXiv:2502.01765v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2502.01765
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Related DOI:
https://doi.org/10.3847/1538-4357/adaeae
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Submission history
From: Eshan Raul
[v1] Mon, 3 Feb 2025 19:09:49 UTC (490 KB)
https://arxiv.org/abs/2502.01765
Astrobiology,