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The atmospheric composition of giant planets carries the information of theirformation history. Superstellar C/H ratios are seen in atmospheres of Jupiter,Saturn, and various giant exoplanets. Also, giant exoplanets show a wide rangeof C/O ratio. To explain these ratios, one hypothesis is that protoplanetsaccrete carbon-enriched gas when a large number of icy pebbles drift across theCO snowline. Here we report the first direct evidence of an elevated C/H ratioin 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 thatthe gas inside the CO snowline ($\sim$70 au) has a C/H ratio of 1-2 timeshigher than the stellar value. This ratio exceeds the expected valuesubstantially, as only 25-60% of the carbon should be in gas at these radii.Although we cannot rule out the case of a normal C/H ratio inside 70 au, themost probable solution is an elevated C/H ratio of 2-8 times higher than theexpectation. Our model also shows that the gas outside 70 au has a C/H ratio of0.1$\times$ the stellar value. This picture of enriched C/H gas at the innerregion and depleted gas at the outer region is consistent with numericalsimulations of icy pebble growth and drift in protoplanetary disks. Our resultsdemonstrate that the large-scale drift of icy pebble can occur in disks and maysignificantly change the disk gas composition for planet formation.

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