2021ApJ...910....3B

Here we aim to explore the origin of the strong C₂H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C₂H is detected in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C₂H is rapidly destroyed, within <1000 yr. These two statements hint that the chemistry responsible for C₂H 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 C₂H abundance. Most of the C₂H resides in gas with an F_UV/n_gas ∼ 10^–7 G₀ cm³. 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 C₂H 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 C₂H observations.