Compact Disks in a High Resolution ALMA Survey of Dust Structures in the
  Taurus Molecular Cloud
  We present a high-resolution ($\sim0.''12$, $\sim16$ au, mean sensitivity of
$50~\mu$Jy~beam$^{-1}$ at 225 GHz) snapshot survey of 32 protoplanetary disks
around young stars with spectral type earlier than M3 in the Taurus
star-forming region using Atacama Large Millimeter Array (ALMA). This sample
includes most mid-infrared excess members that were not previously imaged at
high spatial resolution, excluding close binaries and highly extincted objects,
thereby providing a more representative look at disk properties at 1--2 Myr.
Our 1.3 mm continuum maps reveal 12 disks with prominent dust gaps and rings, 2
of which are around primary stars in wide binaries, and 20 disks with no
resolved features at the observed resolution (hereafter smooth disks), 8 of
which are around the primary star in wide binaries. The smooth disks were
classified based on their lack of resolved substructures, but their most
prominent property is that they are all compact with small effective emission
radii ($R_{\rm eff,95\%} \lesssim 50$ au). In contrast, all disks with $R_{\rm
eff,95\%}$ of at least 55 au in our sample show detectable substructures.
Nevertheless, their inner emission cores (inside the resolved gaps) have
similar peak brightness, power law profiles, and transition radii to the
compact smooth disks, so the primary difference between these two categories is
the lack of outer substructures in the latter. These compact disks may lose
their outer disk through fast radial drift without dust trapping, or they might
be born with small sizes. The compact dust disks, as well as the inner disk
cores of extended ring disks, that look smooth at the current resolution will
likely show small-scale or low-contrast substructures at higher resolution. The
correlation between disk size and disk luminosity correlation demonstrates that
some of the compact disks are optically thick at millimeter wavelengths.
