The TW Hya Rosetta Stone Project IV: A Hydrocarbon-rich Disk Atmosphere

Abstract

Connecting the composition of planet-forming disks with that of gas giant exoplanet atmospheres, in particular through C/O ratios, is one of the key goals of disk chemistry. Small hydrocarbons like C₂H and C₃H₂ have been identified as tracers of C/O, as they form abundantly under high C/O conditions. We present resolved c–C₃H₂ observations from the TW Hya Rosetta Stone Project, a program designed to map the chemistry of common molecules at 15–20 au resolution in the TW Hya disk. Augmented by archival data, these observations comprise the most extensive multi-line set for disks of both ortho and para spin isomers spanning a wide range of energies, E_u = 29–97 K. We find the ortho-to-para ratio of c–C₃H₂ is consistent with 3 throughout extent of the emission, and the total abundance of both c–C₃H₂ isomers is (7.5–10) × 10⁻¹¹ per H atom, or 1%–10% of the previously published C₂H abundance in the same source. We find c–C₃H₂ comes from a layer near the surface that extends no deeper than z/r = 0.25. Our observations are consistent with substantial radial variation in gas-phase C/O in TW Hya, with a sharp increase outside ~30 au. Even if we are not directly tracing the midplane, if planets accrete from the surface via, e.g., meridional flows, then such a change should be imprinted on forming planets. Perhaps interestingly, the HR 8799 planetary system also shows an increasing gradient in its giant planets' atmospheric C/O ratios. While these stars are quite different, hydrocarbon rings in disks are common, and therefore our results are consistent with the young planets of HR 8799 still bearing the imprint of their parent disk's volatile chemistry.