Oral Presentation
From Young to Older Disks: JWST/MIRI Evidence for Fading Molecular Emission and Hints for Elevated C/O in Upper Scorpius
Presenter: Chengyan Xie (Lunar and Planetary Lab, University of Arizona)
Abstract: Disks around young stars are the birthplaces of planets and the chemical reservoirs from which their atmospheres, and potentially prebiotic inventories, are assembled. However, connections between disk composition and planetary outcomes are often drawn from static pictures of disk chemistry based on the well-studied young (∼1–3 Myr) disk population.
Here we present JWST/MIRI spectroscopy of 24 disks in the older (∼5–10 Myr) Upper Scorpius association, and compare them to young (∼1–3 Myr) disks analyzed in a consistent framework.
We find clear signatures of chemical evolution in inner disks during the epoch of planet atmosphere assembly. Older disks show lower detection rates and systematically weaker emission from common molecules (e.g., H2O, C2H2, HCN), alongside enhanced detections of complex hydrocarbons such as C4H2. At the same time, a subset of disks (particularly compact, millimeter-faint systems) exhibits elevated ratios of C- to O-bearing molecules compared to young disks.
These results indicate declining inner-disk gas mass and temperature, and a shift toward more carbon-rich chemistry over time, consistent with processes such as pebble drift. The evolving chemical environment implies that the composition of material accreted by forming planets depends on when accretion occurs, suggesting time-dependent pathways toward organic-rich atmospheres and the initial conditions for habitability.
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