  Based on Atacama Large Millimeter/submillimeter Array (ALMA) observations,
compact protoplanetary disks with dust radii of $r\lesssim 20-40$ au were found
to be dominant in nearby low-mass star formation regions. However, their
substructures have not been investigated because of the limited spatial
resolution achieved so far. We apply a newly developed super-resolution imaging
technique utilizing sparse modeling (SpM) to explore several au-scale
structures in such compact disks. SpM imaging can directly solve for the
incomplete sampling of visibilities in the spatial frequency and potentially
improve the fidelity and effective spatial resolution of ALMA images. Here, we
present the results of the application to the T Tau system. We use the ALMA 1.3
mm continuum data and achieve an effective spatial resolution of $\sim 30\%$ (5
au) compared with the conventional CLEAN beam size at a resolution of 17 au.
The reconstructed image reveals a new annular gap structure at $r= 12$ au in
the T Tau N compact disk with a dust radius of 24 au, and resolves the T Tau Sa
and Sb binary into two sources. If the observed gap structure in the T Tau N
disk is caused by an embedded planet, we estimate a Saturn-mass planet when the
viscous parameter of the disk is $10^{-3}$. Ultimately, ALMA observations with
enough angular resolution and sensitivity should be able to verify the
consistency of the super-resolution imaging and definitely confirm the
existence of this disk substructure.
