  We report the first detection of a gap and a ring in 336 GHz dust continuum
emission from the protoplanetary disk around TW Hya, using the Atacama Large
Millimeter/Submillimeter Array (ALMA). The gap and ring are located at around
25 and 41 au from the central star, respectively, and are associated with the
CO snow line at ~30 au. The gap has a radial width of less than 15 au and a
mass deficit of more than 23%, taking into account that the observations are
limited to an angular resolution of ~15 au. In addition, the 13CO and C18O J =
3 - 2 lines show a decrement in CO line emission throughout the disk, down to
~10 au, indicating a freeze-out of gas-phase CO onto grain surfaces and
possible subsequent surface reactions to form larger molecules. The observed
gap could be caused by gravitational interaction between the disk gas and a
planet with a mass less than super-Neptune (2M_{Neptune}), or could be the
result of the destruction of large dust aggregates due to the sintering of CO
ice.
