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 ${\mathrm{C}}_2\mathrm{H}$ and ${\mathrm{C}}_3{\mathrm{H}}_2$ have been identified as tracers of C/O, as they form abundantly under high C/O conditions. We present resolved ${\mathrm{C}}_3{\mathrm{H}}_2$ 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 ${\mathrm{C}}_3{\mathrm{H}}_2$ is consistent with 3 throughout extent of the emission, and the total abundance of both ${\mathrm{C}}_3{\mathrm{H}}_2$ isomers is $(7.5-10)\times 10^{-11}$ per H atom, or $1-10$% of the previously published ${\mathrm{C}}_2\mathrm{H}$ abundance in the same source. We find ${\mathrm{C}}_3{\mathrm{H}}_2$ 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 $\sim 30$ au. Even if we are not directly tracing the midplane, if planets accrete from the surface via, e.g., meridonial 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.

arXiv comment15 pages, 8 figures, Accepted in ApJ

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• Molecules with ALMA at Planet-forming Scales (MAPS) IV: Emission Surfaces and Vertical Distribution of Molecules
• Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC${}_3$N, CH${}_3$CN, and $c$-C${}_3$H${}_2$
• The chemical inventory of the planet-hosting disk PDS 70
• The diverse chemistry of protoplanetary disks as revealed by JWST
• C${}^{18}$O emission as an effective measure of gas masses of protoplanetary disks
• Investigating the asymmetric chemistry in the disk around the young star HD 142527
• Detection of HC18O+ in a protoplanetary disk: exploring oxygen isotope fractionation of CO
• ALMA high-resolution observations unveil planet formation shaping molecular emission in the PDS 70 disk
• Physical conditions for dust grain alignment in Class 0 protostellar cores I. Observations of dust polarization and molecular irradiation tracers
• Cold Deuterium Fractionation in the Nearest Planet-Forming Disk
• PRODIGE -- Planet-forming disks in Taurus with NOEMA. I. Overview and first results for 12CO, 13CO, and C18O
• First Detection of HC5N in a Class II Disk around TW Hya