Quantifying the gas inside dust cavities in transitional disks:
  implications for young planets
  ALMA observations of a small sample of transitional disks with large dust
cavities observed in Cycle 0 and 1 are summarized. The gas and dust surface
density structures are inferred from the continuum and 12CO, 13CO and C18O line
data using the DALI physical-chemical code. Thanks to its ability to
self-shield, CO can survive inside dust cavities in spite of being exposed to
intense UV radiation and can thus be used as a probe of the gas structure.
Modeling of the existing data shows that gas is present inside the dust
cavities in all cases, but at a reduced level compared with the gas surface
density profile of the outer disk. The gas density decrease inside the dust
cavity radius by factors of up to 10^4 suggests clearing by one or more
planetary-mass companions. The accompanying pressure bumps naturally lead to
trapping of the mm-sized dust grains observed in the ALMA images.
