Astrochemistry

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

By Keith Cowing
Press Release
February 5, 2025
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Tracking the Chemical Evolution of Hydrocarbons Through Carbon Grain Supply in Protoplanetary Disks
This is an artist’s impression of a young star surrounded by a protoplanetary disc in which planets are forming. An international team of astronomers have used the NASA/ESA/CSA James Webb Space Telescope to provide the first observation of water and other molecules in the inner, rocky-planet-forming regions of a disc in one of the most extreme environments in our galaxy. These results suggest that the conditions for rocky-planet  formation, typically found in the discs of low-mass star-forming regions, can also occur in massive-star-forming regions and possibly a broader range of environments. [Image description: At the centre of the image, a bright light source illuminates a surrounding disc, which transitions from colours of white, grey, to orange. The disc is slightly tilted from upper left to lower right, and has spiral features that are most prominent near the star. Small, rocky objects are scattered throughout the disc. At upper right, there is a gap through which background stars can be seen.]

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,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻