A Ring of C2H in the Molecular Disk Orbiting TW Hya

TL;DR

This study images C2H emission in the TW Hya disk, revealing a ring structure that indicates active photochemistry and dust grain processing in the disk's outer regions.

Contribution

First high-resolution imaging of C2H in TW Hya's disk, linking molecular emission to dust grain processing and disk chemistry.

Findings

C2H forms a ring at ~45-120 AU in the disk.

C2H emission is subthermally excited, indicating a warm, tenuous upper disk atmosphere.

The C2H ring traces photochemical processes affecting dust and hydrocarbons.

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.