The point at which a protoplanetary disk becomes a debris disk is difficult to identify .
To better understand this , here we study the \sim 40Â AU separation binary TÂ 54 in the Chamaeleon I cloud .
We derive a K5 spectral type for TÂ 54Â A ( which dominates the emission of the system ) and an age of \sim 2Â Myr .
However , the dust disk properties of TÂ 54 are consistent with those of debris disks seen around older and earlier-type stars .
At the same time , TÂ 54 has evidence of gas remaining in the disk as indicated by [ Ne ii ] , [ Ne iii ] , and [ O i ] line detections .
We model the spectral energy distribution of TÂ 54 and estimate that \sim 3 \times 10 ^ { -3 } M _ { \earth } of small dust grains ( < 0.25Â \mu m ) are present in an optically thin circumbinary disk along with at least \sim 3 \times 10 ^ { -7 } M _ { \earth } of larger ( > 10Â \mu m ) grains within a circumprimary disk .
Assuming a solar-like mixture , we use Ne line luminosities to place a minimum limit on the gas mass of the disk ( \sim 3 \times 10 ^ { -4 } M _ { \earth } ) and derive a gas-to-dust mass ratio of \sim 0.1 .
We do not detect substantial accretion , but we do see H \alpha in emission in one epoch , suggestive that there may be intermittent dumping of small amounts of matter onto the star .
Considering the low dust mass , the presence of gas , and young age of TÂ 54 , we conclude that this system is on the bridge between the protoplanetary and debris disk stages .