Destruction of Refractory Carbon Grains Drives the Final Stage of Protoplanetary Disk Chemistry

• Published in: The Astrophysical journal Vol. 910; no. 1; p. 3
• Main Authors: Bosman, Arthur D., Alarcón, Felipe, Zhang, Ke, Bergin, Edwin A.
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 01-03-2021
• Subjects: Abundance; Astrophysics; Carbon; Chemistry; Equilibrium; Grains; Oxygen; Planet formation; Protoplanetary disks; Vapor phases
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abe127

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.