Hydrocarbon emission rings in protoplanetary disks induced by dust evolution

Abstract

We report observations of resolved C2H emission rings within the gas-rich protoplanetary disks of TW Hya and DM Tau using the Atacama Large Millimeter Array (ALMA). In each case the emission ring is found to arise at the edge of the observable disk of mm-sized grains (pebbles) traced by (sub)mm-wave continuum emission. In addition, we detect a C3H2 emission ring with an identical spatial distribution to C2H in the TW Hya disk. This suggests that these are hydrocarbon rings (i.e. not limited to C2H). Using a detailed thermo-chemical model we show that reproducing the emission from C2H requires a strong UV field and C/O > 1 in the upper disk atmosphere and outer disk, beyond the edge of the pebble disk. This naturally arises in a disk where the ice-coated dust mass is spatially stratified due to the combined effects of coagulation, gravitational settling and drift. This stratification causes the disk surface and outer disk to have a greater permeability to UV photons. Furthermore the concentration of ices that transport key volatile carriers of oxygen and carbon in the midplane, along with photochemical erosion of CO, leads to an elemental C/O ratio that exceeds unity in the UV-dominated disk. Thus the motions of the grains, and not the gas, lead to a rich hydrocarbon chemistry in disk surface layers and in the outer disk midplane.

Accepted by the Astrophysical Journal, 23 pages, 15 figures

References in corpus (18)

• Protoplanetary Disk Structures in Ophiuchus
• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• The stickiness of micrometer-sized water-ice particles
• Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate
• A Large Catalog of Accurate Distances to Molecular Clouds from PS1 Photometry
• Millimeter imaging of HD 163296: probing the disk structure and kinematics
• Reprocessing of Ices in Turbulent Protoplanetary Disks: Carbon and Nitrogen Chemistry
• Thermal desorption of circumstellar and cometary ice analogs
• Disks around CQ Tau and MWC 758: dense PDR or gas dispersal?
• Spatially extended PAHs in circumstellar disks around T Tauri and Herbig Ae stars
• PAH chemistry and IR emission from circumstellar disks
• Water vapor distribution in protoplanetary disks
• Exploring the Origins of Carbon in Terrestrial Worlds
• Chemistry in Protoplanetary Disks: the gas-phase CO/H2 ratio and the Carbon reservoir
• Unbiased mm-wave Line Surveys of TW Hya and V4046 Sgr: The Enhanced C2H and CN Abundances of Evolved Protoplanetary Disks
• C${}_2$H observations toward the Orion Bar
• A Ring of C2H in the Molecular Disk Orbiting TW Hya
• First Detection of [C I] ${}^3$P${}_1$-${}^3$P${}_0$ Emission from a Protoplanetary Disk

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