We present Atacama Large Millimeter/submillimeter Array (ALMA) observations<br />
of the dust continuum emission at 1.3 mm and 12CO J=2-1 line emission of the<br />
transitional disk around DM Tau. DM Tau&#039;s disk is thought to possess a<br />
dust-free inner cavity inside a few au, from the absence of near-infrared<br />
excess on its spectral energy distribution (SED). Previous submillimeter<br />
observations were, however, unable to detect the cavity; instead, a dust ring<br />
~20 au in radius was seen. The excellent angular resolution achieved in the new<br />
ALMA observations, 43 x 31 mas, allows discovery of a 4 au radius inner dust<br />
ring, confirming previous SED modeling results. This inner ring is symmetric in<br />
continuum emission, but asymmetric in 12CO emission. The known (outer) dust<br />
ring at ~20 au is recovered and shows azimuthal asymmetry with a strong-weak<br />
side contrast of ~1.3. The gap between these two rings is depleted by a factor<br />
of ~40 in dust emission relative to the outer ring. An extended outer dust disk<br />
is revealed, separated from the outer ring by another gap. The location of the<br />
inner ring is comparable to that of the main asteroid belt in the solar system.<br />
As a disk with a &quot;proto-asteroid belt,&quot; the DM Tau system offers valuable clues<br />
to disk evolution and planet formation in the terrestrial planet forming<br />
region.