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