A Ring of C2H in the Molecular Disk Orbiting TW Hya

Abstract

We have used the Submillimeter Array to image, at ~1.5" resolution, C2H (3-2) emission from the circumstellar disk orbiting the nearby (D = 54 pc), ~8 Myr-old, ~0.8 Msun classical T Tauri star TW Hya. The SMA imaging reveals that the C2H emission exhibits a ring-like morphology. Based on a model in which the C2H column density follows a truncated radial power-law distribution, we find that the inner edge of the ring lies at ~45 AU, and that the ring extends to at least ~120 AU. Comparison with previous (single-dish) observations of C2H (4-3) emission indicates that the C2H molecules are subthermally excited and, hence, that the emission arises from the relatively warm, tenuous upper atmosphere of the disk. We propose that the C2H emission most likely traces particularly efficient photo-destruction of small grains and/or photodesorption and photodissociation of hydrocarbons derived from grain ice mantles in the surface layers of the outer disk. The presence of a C2H ring in the TW Hya disk hence likely serves as a marker of dust grain processing and radial and vertical grain size segregation within the disk.

17 pages, 4 figures; to appear in The Astrophysical Journal

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• A Surface Density Perturbation in the TW Hydrae Disk at 95 au Traced by Molecular Emission
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• Desorption Kinetics and Binding Energies of Small Hydrocarbons
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• The TW Hya Rosetta Stone Project IV: A hydrocarbon rich disk atmosphere
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• ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT) III: The interplay between gas and dust in the protoplanetary disk of DG Tau
• Modeling Nitrogen Fractionation in the Protoplanetary Disk around TW Hya: Model Constraints on Grain Population and Carbon-to-Oxygen Elemental Abundance Ratio
• An SMA Survey of Chemistry in Disks around Herbig AeBe Stars