We present a high-resolution ( \sim 0 \farcs 12 , \sim 16 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 _ { eff, 95 \% } \lesssim 50 au ) .
In contrast , all disks with R _ { 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 .