Resolved Images of Large Cavities in Protoplanetary Transition Disks
  Circumstellar disks are thought to experience a rapid "transition" phase in
their evolution that can have a considerable impact on the formation and early
development of planetary systems. We present new and archival high angular
resolution (0.3" = 40-75 AU) Submillimeter Array (SMA) observations of the 880
micron dust continuum emission from 12 such transition disks in nearby
star-forming regions. In each case, we directly resolve a dust-depleted disk
cavity around the central star. Using radiative transfer calculations, we
interpret these dust disk structures in a homogeneous, parametric model
framework by reproducing their SMA visibilities and SEDs. The cavities in these
disks are large (R_cav = 15-73 AU) and substantially depleted of small
(~um-sized) dust grains, although their mass contents are still uncertain. The
structures of the remnant material at larger radii are comparable to normal
disks. We demonstrate that these large cavities are common among the
millimeter-bright disk population, comprising at least 20% of the disks in the
bright half of the millimeter luminosity (disk mass) distribution. Utilizing
these results, we assess some of the physical mechanisms proposed to account
for transition disk structures. As has been shown before, photoevaporation
models do not produce the large cavity sizes, accretion rates, and disk masses
representative of this sample. It would be difficult to achieve a sufficient
decrease of the dust optical depths in these cavities by particle growth alone:
substantial growth (to meter sizes or beyond) must occur in large (tens of AU)
regions of low turbulence without also producing an abundance of small
particles. Given those challenges, we suggest instead that the observations are
most commensurate with dynamical clearing due to tidal interactions with
low-mass companions --young brown dwarfs or giant planets on long-period
orbits.
