Destruction of Refractory Carbon Grains Drives the Final Stage of Protoplanetary Disk Chemistry
Arthur D. Bosman, Felipe Alarcón, Ke Zhang, Edwin A. Bergin
Abstract
Abstract Here we aim to explore the origin of the strong C 2 H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C 2 H is detected in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C 2 H is rapidly destroyed, within <1000 yr. These two statements hint that the chemistry responsible for C 2 H emission must be predominantly in the gas phase and must be in equilibrium. Combining new and published chemical models, we find that elevating the total volatile (gas and ice) C/O ratio is the only natural way to create a long-lived, high C 2 H abundance. Most of the C 2 H resides in gas with an F UV / n gas ∼ 10 −7 G 0 cm 3 . To elevate the volatile C/O ratio, additional carbon has to be released into the gas to enable equilibrium chemistry under oxygen-poor conditions. Photoablation of carbon-rich grains seems the most straightforward way to elevate the C/O ratio above 1.5, powering a long-lived equilibrium cycle. The regions at which the conditions are optimal for the presence of high C/O ratio and elevated C 2 H abundances in the gas disk set by the F UV / n gas condition lie just outside the pebble disk as well as possibly in disk gaps. This process can thus also explain the (hints of) structure seen in C 2 H observations.