Excess C/H in Protoplanetary Disk Gas from Icy Pebble Drift across the CO Snowline

Abstract

The atmospheric composition of giant planets carries the information of their formation history. Superstellar C/H ratios are seen in atmospheres of Jupiter, Saturn, and various giant exoplanets. Also, giant exoplanets show a wide range of C/O ratio. To explain these ratios, one hypothesis is that protoplanets accrete carbon-enriched gas when a large number of icy pebbles drift across the CO snowline. Here we report the first direct evidence of an elevated C/H ratio in disk gas. We use two thermo-chemical codes to model the ${}^{13}$C${}^{18}$O, C${}^{17}$O, and C${}^{18}$O (2-1) line spectra of the HD 163296 disk. We show that the gas inside the CO snowline ($\sim$70 au) has a C/H ratio of 1-2 times higher than the stellar value. This ratio exceeds the expected value substantially, as only 25-60% of the carbon should be in gas at these radii. Although we cannot rule out the case of a normal C/H ratio inside 70 au, the most probable solution is an elevated C/H ratio of 2-8 times higher than the expectation. Our model also shows that the gas outside 70 au has a C/H ratio of 0.1$\times$ the stellar value. This picture of enriched C/H gas at the inner region and depleted gas at the outer region is consistent with numerical simulations of icy pebble growth and drift in protoplanetary disks. Our results demonstrate that the large-scale drift of icy pebble can occur in disks and may significantly change the disk gas composition for planet formation.

9 pages, 4 figures, and 1 table. Accepted for publication in ApJL

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• CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing
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• How drifting and evaporating pebbles shape giant planets II: Volatiles and refractories in atmospheres
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• Depletion of gaseous CO in protoplanetary disks by surface-energy-regulated ice formation
• Enriching inner discs and giant planets with heavy elements
• Measuring the atomic composition of planetary building blocks
• Investigating the asymmetric chemistry in the disk around the young star HD 142527
• On the stickiness of CO${}_2$ and H${}_2$O ice particles
• Disentangling protoplanetary disk gas mass and carbon depletion through combined atomic and molecular tracers
• Locked In Ice: how Pebble Drift and Volatile Entrapment can Significantly Impact Carbon and Oxygen Ratios in Evolving Protoplanetary Discs
• Global Modeling of Nebulae With Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles
• Zodiacal Exoplanets in Time (ZEIT) XII: A Directly-Imaged Planetary-Mass Companion to a Young Taurus M Dwarf Star
• Molecules with ALMA at Planet-forming Scales (MAPS). XV. Tracing protoplanetary disk structure within 20 au
• Linking Outer Disk Pebble Dynamics and Gaps to Inner Disk Water Enrichment