A Ring of C2H in the Molecular Disk Orbiting TW Hya

Abstract

We have used the Submillimeter Array to image, at ~1farcs5 resolution, C2H $N=3\to 2$ emission from the circumstellar disk orbiting the nearby (D = 54 pc), ~8 Myr-old, ~0.8 ${{M}_{\odot }}$ 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 $N=4\to 3$ emission indicates that the C2H molecules are subthermally excited and, hence, that the emission arises from the relatively warm ($T\gtrsim 40$ K), tenuous ($n\ll {{10}^{7}}$ cm−3) upper atmosphere of the disk. Based on these results and comparisons of the SMA C2H map with previous submillimeter and scattered-light imaging, 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.